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GAS PLANT FAMILIARISATION
Power Management Institute Noida
CONTENTS S. No.
Description
Page No.
1.
Introduction to Gas Power Plant
1
2.
Gas Turbine Starting System
10
3.
Fuel System
12
4.
Fire Detection and Protection System
17
5.
Salient Features NTPC Gas Based Power Projects
22
6.
Modern large size Gas Turbine
25
7.
Gas Turbine Compressor
50
8.
Waste Heat Recovery Boiler
67
9.
Gas Turbine Generator
82
10.
Excitation System
92
11.
Gas Plant Operation
103
12.
Protection & Interlock
156
13.
Spare Part Management
177
14.
Data Sheet for Gas Projects
194
1. Introduction To Gas Power Plants Introduction The development of the sector in the country, since independence has been predominantly through the State Electricity Boards. In order to supplement the effects of the states in accelerating power development and to promote power development on a regional basis to enable the optimum utilisation of energy resources, the Government of India decided to take up a programme of establishment of large hydro and thermal power stations in the central sector on a regional basis. With this in view, the Government set up the National Thermal Power Corporation Ltd., in November 1975 with the objective of planning, construction, commissioning, operation and maintenance of Super Thermal and Gas Based Power projects in the country. The availability of gas in a large quantity in western offshore region has opened an opportunity to use the gas for power generation, which is an economical way and quicker method of augmenting power generating capacity by natural gas as fuel in combined cycle power plant in a power deficit country like ours. With this intention in mind the Government has asked NTPC to take up the construction of Kawas, Auraiya, Anta, Dadri and Gandhar Gas Power Project along the HBJ Gas pipe line. The power plant will consist of gas turbine generating units waste heat recovery boilers, steam turbo generator, ancillary electrical and mechanical equipments. The power generated at this power station will be fed over 220 KV AC transmission system associated with this project to distribute the power in the various Regions.
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In the Power Sector, the use of gas turbine drive generators has just started and will go a long way in meeting the country’s requirements. Gas turbines range in size from less than 100 KW up to about 140.000 KW. The gas turbine has found increasing application due to the following potential advantages over competive equipment. •
Small size and weight per horsepower
•
Rapid loading capability
•
Self-contained packaged unit
•
Moderate first cost
•
No cooling water required
•
Easy maintenance
•
High reliability
•
Waste heat available for combined cycle application.
•
Low Gestation Period
•
Low Pollution Hazards
The function of a gas turbine in a combined cycle power plant is to drive a generator which produce electricity and to provide input heat for the steam cycle. Power for driving the compressor is also derived from gas turbine. Combined Cycle Combined Cycle power plant integrates two power conversion cycles namely. Brayton Cycle Gas Turbines) and Rankin Cycle (Conventional steam power plant) with the principal objective of increasing overall plant efficiency.
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Brayton Cycle Gas Turbine plants-operate on Brayton Cycle in which air is compressed (process 1-2, in P-V diagram of Figure-1), this compressed air is heated in the combustor by burning fuel combustion produced is allowed to expand In the turbine (process 3-4) and the turbine is coupled with the generator. Without losses the theoretical cycle process is represented by 1’ 2’ 3’ 4’ In the actual process losses do occur. Deviation from the theoretical process, results from the fact that compression and expansion are not performed isentropically but polytropically which is conditioned by heat dissipation (expansion) and heat supply (Compression) caused by various flow and fraction by losses. In the combined cycle mode, the Brayton Cycle is chosen as the topping cycle due to the high temperature of the exhaust of the gas turbine (point 4 in the P.V diagram). In modern gas turbines the temperature of the exhaust gas is in the range of 500 to 550oC. Reference to the T.S. diagram may indicate the amount of heat that is produced, converted into mechanical energy and extracted from this process. For the evaluation of the cyclic process, two parameters are of greatest importance; 1)
Thermal efficiency
2)
Process working capacity
Thermal efficiency is obtained from chemical binding energy of the fuel and mechanical energy available at the shaft of the gas turbine. Thermal efficiency (th) as follows:
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th = Energy at GT shaft Chemical Energy of fuel =
Q Input. Q output
1 — Q Output
Qinput
Q Input
Working capacity is also obtained from the difference between the amounts of heat supplied and removed. This is achieved by increasing P2 that is increasing gas inlet temperature T3.
BRAYTON CYCLE FIG-1 Rankine Cycle The conversion of heat energy to mechanical energy with the aid of steam is carried out through this cycle. In its simplest form the cycle works as follows (fig.2). The initial state of the working fluid is water (point-3) which, at a certain temperature is compressed by a pump (process 3-4) and fed to the boiler. In the
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boiler the compressed water is heated at constant pressure (process 4-5-6-1). Modern steam power plants have steam temperature in the range of 500oC to 550oC at the inlet of the turbine.
COMBINE CYCLE FIG-2 Combining two Cycles to Improve Efficiency We have seen in the above two cycles that gas turbine exhaust is at a temperature of 500 –550oC and in Rankine Cycle heat is required to generate steam at the temperature of 500-550oC. so, why not use the gas-turbine exhaust to generate steam in the Rankine cycle and save the fuel required to heat the water ? Combined Cycle does just the same. © PMI,NTPC
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The efficiency of Gas Turbine cycle alone is 30% and the efficiency of Rankine Cycle is 35%. The overall efficiency of combined cycle comes to 48%. Types of Combined Cycles It is basically of two types, namely Unfired Combined cycle and Fully Fired combined cycle. Unfired combined Cycle The basic system is shown in figure- 3. in this system the exhaust gas is used only for raising steam to be fed to the steam turbine for power generation. The conventional fossil fuel fired boiler of the steam power plant is replaced with a ‘Heat Recovery Steam Generator’ (HRSG). Exhaust gas from the gas turbine is led to the HRSG where heat of exhaust gas is utilised to produce steam at desired parameters as required by the steam turbine.
UNFIRED COMBINED CYCLE FIG-3
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However, non-reheat steam turbine is the preferred choice for adopting this type of system as usually the live steam temperature for HRSG will be solely controlled by the gas turbine exhaust temperature which is usually around 500oC. In recent development, with the introduction of Dual Pressure Cycles more heat is recovered in the HRSG and steam with higher pressure and temperature can be generated. But higher capital investment and sometimes necessity of supplemental firing system makes the system complex and costly. Fully Fired Combined Cycle Fig – 4 shows the basic schematic of this cycle. In this system the heat of exhaust gas from gas turbine is used for two purposes as described below: Heat contained in exhaust gas is used to heat feed water to a desired temperature at the inlet to the boiler. This leads to the reduction or elimination of the extraction steam requirement from the steam turbine. In case, the steam turbine has a larger steam swallowing capacity to generate more power the amount of steam which is being extracted from steam turbine for regenerative feed heating could be made to expand in the turbine to increase its base load capacity and improve the overall efficiency. In case the steam turbine does not have the capacity to swallow extra steam available due to cutting down of extraction, the fuel being fired in the boiler can be cut down to generate less steam by an amount equivalent to steam required for extractions and thus improving the overall efficiency due to less consumption of fuel. Gas turbine exhaust contains about 14 to 16 % oxygen (by weight) and can be used as hot secondary air in the conventional fossil fired furnaces. So the heat required to heat the secondary air will be saved and can be used for other purposes. FD fan power consumption will also be reduced to a great extent.
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BOILER REPOWERING SYSTEM EXHAUST HEAT EXCHANGER Fig – 4 Fuels Gas turbines are capable of burning a range of fuels including naptha, distillates, crude oils and natural gas. Selection of fuel (s) depends on
several factors
including fuel availability, fuel cost and cleanliness of fuel. Natural gas is an ideal fuel because it provides high thermal efficiency and reliability with a low operation and maintenance cost. Liquid fuels, particularly heavy oils, usually contain contaminants, which cause corrosion and fouling in the gas turbine. Contaminants, which cannot be removed from the fuel, may
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leave deposits in the gas turbine, which reduce performance and add maintenance costs. Dual fuel systems are commonly used, enabling the gas turbine to burn back-up fuels when the primary fuel source is not available. Dual fuel systems can also be designed to fire both fuels simultaneously. Advantages of Combined Cycle Plants Apart from the higher overall efficiency, the combined cycle power plants have following advantages: Low installation Cost: power from a combined cycle power plant is approximately 70% of a conventional coal based power plant of same capacity. Low Gestation Period: Power from a combined cycle power plant can be obtained in two phases, i.e. two third power, obtainable from gas turbines, is available within 16-18 months and the balance in next 1 month. This is much lower than the gestation period for conventional thermal power plants, which is around 48-60 months. Better Reliability: Combined cycle power plants are considered to be highly reliable with the reliability factor of 85-90% as compared to 60-65% for conventional coal Fired Power Plants.
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2. Gas Turbine Starting System The function of the starting system is to crank the gas turbine upto the required speed until : it becomes self sustaining. One method of starting large gas turbine is by using a motor driven hydraulic starting system. Alternatively, the GTG can be started by using a frequency converter to rotate the generator which drives the turbine for starting. A typical hydraulic starting system for each gas turbine consist of the following: •
Starting motor, electric AC induction motor
•
Hydraulic torque converter
•
Auxiliary Gear
•
Couplings
The electric starting motor drives the hydraulic torque generator through a coupling. The hydraulic torque converter consists of an impeller, which forces the fluid against hydraulic starting motor. The hydraulic torque converter is coupled to the accessory gear, which is connected to the gas turbine shaft. The torque converter receives hydraulic fluid from hydraulic and lube oil reservoir during operation. When gas turbine reaches self-sustaining speed the starting device is disconnected and shut down. To break the inertia of the starting motor and reduce the starting current a pony motor is provided. Gas turbines of GE and WH designs are provided with starting motor system for cranking purpose.
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The static frequency converter serves the same function of starting, accelerating running at preset speed by starting the generator as a synchronous motor by feeding variable frequency current drawn from the connected grid. Gas turbines of ABB and SIEMENS make are provided with frequency converter s for the GT cranking and normally 2 * 100% static converters are provided for the power station. Black Start System To start a gas turbine in the event of AC-power failure an emergency black start system is provided. It also helps in safe coasting down of the gas turbine and its auxiliaries following a ‘trip’ in the event of grid collapse. The black start system consist of a separate diesel engine or a gas turbine driven synchronous generator connected to station switch gear bus. It can be operated manually from local or remote and also it automatically comes into operation following a black out condition. Capacity of the black start unit should be such that it can supply the total auxiliary power required to start a gas turbine from standstill condition. The NTPC’s Auraiya project gas turbine is provided for emergency black-start purpose and all other projects are provided with diesel generator set for the same duty.
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3. Fuel System Fuel System of Gas Turbine The function of the fuel system is to deliver fuel to the combustion chamber(s) of a gas turbine at quantity and pressure as required by the control system Liquid fuel system The liquid fuel system consists of the liquid fuel storage and handling system. The liquid fuel storage and handling system provides means for unloading , storage and distribution of the fuel oil within the plant and typically composed of the following major components: •
Fuel oil unloading pumps
•
Fuel oil transfer pumps
•
Fuel oil storage tanks
•
Flow meter
•
Strainers
•
Pressure and Level control stations.
•
Distribution piping
The number and size of equipments is site dependant. The storage facility is dependant on such factors as the location of the site, proximity of the oil supply and the reliability of the oil supply. Fuel oil may be supplied to the plant by a pipeline, oil barriages, oil tankers, rail/road or high way trucks. Fuel oil unloading pumps may not be required if the oil arrives by a pipeline, an oil barge, or an oil tanker since they have their own pumps.
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Naphtha Pressurising System By the forwarding pumps,Naptha is pumped upto the GTs and kept under recirculation. For firing the naphtha is the GT,separate pressurising system with high pressure pump, allied filters and measurements and recirculation system is used. This pressurising is required because in the naphtha burner this fuel is mechanically atomised. There is no other medium like air, etc are used for atomisation. Naphtha drain system Since the flash/fire point of this fuel is very low it is designed to drain the oil from the piping burner etc when the system is stopped. Separate drain tanks are kept at pumping stations, which will collect these naphtha and pump back to the main storage tanks. Mode of firing It is provided to fire natural gas or naphtha or in mixed mode of the fuel in the GT Combustion Chamber. FUEL GAS SYSTEM The purpose of the fuel gas system is to take gas fuel from the custody transfer point, process it to the quality and pressure conditions required by the gas turbine and to distribute it and monitor its use. Fuel gas system consists of the off base system and the on base system.
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Off base fuel gas System The off-base fuel gas system is typically composed of the following major components: •
An emergency stop valve at the gas inlet to the plant knock out drum
•
Filters
•
Pressure-control station
•
Distribution piping
Optional Components are •
Gas metering station
•
Gas Heaters
The need for any of the optional components is dictated by the conditions specific to each individual application. The final supply pressure required at gas turbine is a function of the gas turbine compression ratio and the control valve and nozzle pressure drops. Large gas turbines typically require a gas supply pressure of about Kg/cm2 with pressure regulated with in +10 percent of set point. The emergency stop valve at the in let to the station is provided to completely shut off the gas supply in an emergency. The gas fuel then goes through a knockout drum. A knock out drum will remove the free liquid and some solids from the gas stream. However, it will not separate out much of the entrained liquids. The liquids separation from gas stream takes place by decreasing the gas velocity in its transit through the drum. Impurities collect at the drum bottom and are periodically automatically purged A level controller opens a drain valve which allows the waste to Adrian tank vented to the atmosphere.
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The filter separators must have minimum of two stages of separation. The initial stage being utilised for removal of large liquid droplets and the final stage for mist separation . Any solid particles carried with the gas stream are separated in the first stage . The first stage elements consist of hollow tubes of glass fibers through which gas flows radially from outside. The separation of carried solids and liquid particles is by interception, diffusion and/or inertial impaction of the droplets on the fibers. The liquid droplets coalesced on the surface drain on to the shell and from there to the drain pot. The second stage mist separator is preferably of the vans type in which liquid separation takes place by subjecting the gas path to multiple changes of direction. The gas analysis is carefully checked from the stand point these point of hydrate formation or icing at the pressure reducing station under the worst ambient temperatures prevalent at each site. If there is any tendency for the above, a suitably, rated gas fired indirect heater is provided ahead of the pressure reducing station. The gas heater ensures a gas temperature sufficiently high such that the after pressure reduction the gas temperature is at least 20” C higher than hydrate forming temperature or the gas dew point whichever is higher. The heater normally utilises natural gas for firing. Heat from the fire tube be utilised to heat a water bath in which the U tubes for carrying the gases to be heated are immersed. The control system ensures constant temperature of the outlet gas for gas flows ranging from zero to full gas flow. A full capacity by is provided so as to enable operation of the fuel gas system is designed to deliver gas fuel to the gas turbine combustion chamber(s) at the proper pressure and flow rates to meet all the starting, acceleration and loading requirements of gas turbine operation.
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Naphtha forwarding system The forwarding system is mainly remote controlled from the GT LCR and CCR. Although there is a possibility to control the main devices locally by a switching the selector switch on local position in the local . By means of three way valves a tank selection is possible. To ensure the standby position of the lines keep the value open. The pumps are protected by the automatic recirculating valves. The condition of the strainers as well as of the filters is monitored by the differential pressure indicators and switches. The safety valves protect the line against the line against over pressure.
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4. Fire Detection And Protection System The combined cycle power station represents a large capital investment in high technology plant and equipment, the loss of which would entail disruption to the power system and a larger reduction revenue. It is necessary to protect this investment against fire damage and to this effect an elaborate fire detection and protection system is being provided throughout the power plant. The areas to be protected by fire detection and protection system in a combined cycle gas project are 1.
Power house building consisting of gas turbine generator and its auxiliaries, steam turbine and its auxiliaries and waste heat recovery boiler equipment and its auxiliaries. •
Gas regulating station
•
Liquid fuel storage and unloading area
•
Cable galleries/spreader room
•
Transformer switchyard and other transformers located in the plant area
•
Various control rooms
•
istrative office and other auxiliary equipment buildings
•
Covered and uncovered stores
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The various fire detection and protection system are : Hydrant System The Hydrant System essentially consists of larger network of pipes both underground and over ground which feeds pressurized water to number of hydrant valves which are located throughout the power station at strategic locations. In addition, monitors are provided to protect gas reducing station and WHRB. Hydrant system is designed considering the plant as “ORDINARY HAZARD” High Pressure Water Spray System The high pressure water spray system applies water in the form of a conical spray consisting of droplets of water travelling at high velocity. The automatic operation of flow control/deluge valve can be through “wet” Detection or “Dry” detection-Initiation System. Separate 2x100% capacity pumps and header is provided for H.V.W. spray system. This system is provided to protect transformer and gas reducing station (for indoor installations) and lub oil storage tanks. Medium velocity Water spray system The medium velocity water system applies water in finely divided droplets travelling at medium velocity to control the fire and the same time renders adjacent areas safe by cooling action. This system is operated manually and protect tanks containing flammable/combustible liquid such as Naptha. The discharge density shall not be less than 10.2 litres per minutes per sq. metre.
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Sprinkler System The sprinkler system consists of a pipe work array filled with water having sprinkler needs at laid down intervals which operates at a predetermined temperature. Conventional cable galleries are not provided in the combined cycle power plant, however the spreader rooms where power cable are is protected by sprinkler system. Detection of fire in cable spreader room is through combination of smoke detectors i.e. “Ionisation” type and “optical” type detectors fully cross-zoned. Beside this, linear thermal sensors are also used. The discharge density for this system shall not be less than 12.2.Ipm/sq.M Foam Injection System To extinguish the fire inside tanks containing flammable/combustive liquids, foam is produced by mixing foam concentrate with water in the required proportion and aerating the resultant solution. Low expansion floroprotein in foam concentrate is used as foam concentrate. Foam injection is carried out manually only. Foam extinguishing system is provided for floating roof tanks storing Naptha, tank car unloading manifold and the dyke area surrounding the tanks. Halon System In NTPC’s earlier gas project, automatic inert gas extinguishing system using Halon 1301 was used to protect electrical and electronic equipments. However, recent scientific researches have indicated that man chemicals such as Halon and Chlorofluorocarbon deplete stratospheric ozone layer. In view of adverse Environmental impact, NTPC has stopped using Halon flooding as well as Halon extinguishers for all future projects.
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CO2 Extinguishing System After deflected of halon extinguishing system for all future NTPC projects, modular CO2 extinguishing system is envisaged in control s, fuel gas control block of Gas turbine (under sound proof enclosure), GT sound proof enclosure, bearing housing within exhaust diff and the cable space below electronic equipment room. This system is designed to achieve 34% concentration by volume in the areas to be protected and are equipped with optical type and ionisation type smoke detectors. Temperatures detectors are installed in the bearing housing within exhaust diff. Fire Alarm System An automatic fire detection system is provided for an early warning throughout the power plant area. This system detects the outbreak of fire at the inception stage and facilities to take remedial action at the initial stage to prevent/reduce damage of costly equipments. Since Combined cycle plant operation is simpler as compared to thermal power plant and the manpower required is less, the computerized early warning system is preferred than the conventional fire alarm system. Computerized addressable type early warring system which have following advantages: i)
The system has provision for automatic sensitivity compensation
ii)
Each detector is individually addressed
iii)
The system has facility for complete data logging.
iv)
The system is on modular basis and can be expanded at a later date.
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v)
The interface units are provided for connecting open type alarm initiating devices such as pressure switches, potentials free contracts etc.
vi)
Exact location of fire can be known from the fire alarm control .
Potable Extinguishers Portable pressurised water type and CO2 type extinguishers are provided at strategic locations inside the entire power station area. For 600 MW combines cycle power plant, approx. 75 nos. water type and 100nos. of CO2 type portable extinguishers are provided.
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5. Salient Features Of NTPC Gas Projects A.
ANTA GBCP STAGE I (413 MW) The plant is located near Anta Town of District Kota in Rajasthan. The nearest railway station is that of Anta which is approximately 1 km.from plant site. The branch offline of HBJ pipeline is approximately 15 kms.from the site, from where a tap offline is taken. The Kota right main canal which is the source of plant water requirements, runs along the plant boundary. In the main plant block, the gas-turbines and the steam-turbine are housed in separate buildings. There are 3 GTs in are one hall and one ST in another building, with GT hall towards the west. In the extension stage of the project, the mirror image of stage I is anticipated and after completion it shall have 2 GT, halls with 3 GTs on each side of the ST hall having 2 STs. The extension is planned towards the east of stage I. The GTs are placed transversally. The control building is adjacent to ST building. Each GT is having one local control centre located towards south of GT hall. The transformer yard is towards south of turbine hall and switchyard is further south.
B.
KAWAS GBCP.STAGE I ( 640 MW ) The plant is located near village Kawas in District Surat of Gujarat. The nearest railway station is that of Surat which is approximately 19 kms. From site. In the main plant block two modules each consisting of 2 GTs and ST have been provided. The turbine building houses two pairs of GTs at the east and west ends with 2 ST in the middle. Here all GTs and STs are
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placed longitudinally. The common control building is located adjacent to the ST hall towards north. Each WHRB is located towards south of the respective gas turbine. Each GTG is having one local control room which is located towards north, below the inlet air filter. The transformer yards is in north of turbine hall and switch yard further north. C.
AURAIYA GBCP-STAGE I (650MW) The plant is located in Etawah District of Uttar Pradesh. The plant site is approximately 2.5 kms. from the nearest railway station of phaphund. In the main plant block, two no of STGs are located in between 2 pairs of GTs. GTGs are placed transversally. The central control room with 220 KV switchyard control room is located close to STG-I. Local control of each GTS is located near the GT pedestal itself. The water pre-treatment plant and DM plant are located adjacent to both the power house and CW pump house. Induced draft cooling towers have been located considering the proper flow of cooling water, nearness to main power house & convenient routing of open return channel to CW pump house. The 200/400 KV switchyard has been located in front of the power station.
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D
DADRI GBCP-STAGE I (817MW) General Layout Plan In the main plant block two modules, each consisting of two GTGs placed on each side of 2 STGs. The central control room is located towards west of the ST hall. The transformer yard Is on the western side of the turbine hall, with switchyard further down west. Induced draft cooling towers have been located considering the proper flow of cooling water. Nearer to main power house & convenient routing of open return channel to CW pump house. The 220/400 KV switchyard has been located in front of the power station. The
220 KV switchyard control room is accommodated in the central
control room itself. Space has been kept for liquid oil installation and oil unloading facilities. The GAIL terminal for receiving gas is located within boundary of plant site.
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6. The Modern Large Size Gas Turbine, Type GTI3 The main feature of this gas turbine is the simple design, one compressor, one combustion chamber with one burner, a 5- stage turbine arranged on a common shaft with the 17- stage compressor, the shaft being mounted on two journal bearings, the axial thrust absorbed by one thrust bearing as well as an exhaust gas diff which permits an easy connection to a waste heat boiler. With the increasing operational experience the coupling output of this type could be considerably raised by gradually increasing the gas turbine inlet, temperature. This was made possible by cooling the blades of the 1st and 2nd stationary and rotating blade rows or total, with the typical gas turbine, by cooling the first three stages and by an intensified cooling of the rotor and the gas turbine blade carrier. The positive results expected by these measures such as higher power, higher efficiency and lower specific cost have been fully confirmed by the experience gained in operation. The single combustion chamber was mounted at the side of the gas turbine, and, via a U-shaped double piping, connected to the gas turbine. In the course of the further development of this type it was possible, in spite of the higher gas turbine inlet temperature, to reduce the size of the combustion chamber so that it is now arranged directly above the gas turbine. A dual fuel burner makes it possible, simultaneously or alternately, to use gaseous or liquid fuels at improved combustion conditions.
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The separation of flow surging, during partial speeds, may be prevented also with the aid of several adjustable stationary blade rows. Today,, economic reflections lead to the blow –off solution or to combined system with blade readjustment and blow off facilities in the case of industrial gas turbines. INTAKE AND EXHAUST SYSTEM A typical intake & exhaust system for an industrial gas turbine, comprise the following components: a) Intake system
b) Exhaust system
(i)
Inlet Air filter
(ii)
Inlet air silencer
(iii)
Inlet Duct
(i)
Exhaust duct
(ii)
Expansion t
(iii)
Dampers
(iv)
Exhaust silencer
(v)
By stack
Design, construction and operating principle of the above components are described in the following paragraphs. Inlet Air Filtration The need for filtration on gas turbine inlet arises because of four phenomena: foreign object damage erosion, fouling and corrosion. Foreign object damage is usually defined as the resultant blade damage caused by the ingestion of an object of suitable mass and hardness to create physical destruction.
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Erosion is typically defined as the actual removal of material by collision with a moving object. It is often observed on the compressor blades of a gas turbine due to the exposure to larger sized contaminant particles. Fouling is defined as the deposition of smaller particles on the compressor blades, there by changing the blade shape and degrading the performance of the compressor. It is usually not a permanent situation and can be corrected by cleaning of the compressor section. Corrosion is the loss of material in the hot sections due to chemical reaction. It is typically observed in off shore and near shore environments and is attributable to small salt practices and heavy metals. Design/ Construction and operating principles various type of filtration systems are described as follows: No Filtration In the early days of stationary turbine application, it was widely believed that inlet filtration was not needed, that the turbine could ingest almost anything as long as it was not a large object. Many early units were only provided with a simple screen on the inlet to prevent entry of animals, birds, and any other large object. Moving Media The first major problem undertaken was to deal with the larger particles which caused erosion of the compressor blades. By experience and past history, it is generally accepted that a 10 micro metre particle is a size of sufficient mass and inertia to cause actual removal of the metal in a compressor blade. Two of the earliest designs selected were the roll-type and the travelling screen filters.
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The roll-type filter utilizes a roll of dry viscous treated filter material inserted at the top, fed down through a ageway to take-up reel on the bottom. This material is advanced either on demand via the increasing pressure drop across it or automatically on a time basis. The travelling screen filter utilizes an endless belt of metal elements which es through a through of viscous oil, removing the accumulated contaminants and replenishing the viscous treatment. The visors treatment itself is the medium for tapping the airborne particle. The travelling screen type has many of the same operating parameters as the roll-type (efficiency, pressure drop) with one unique potential disadvantage, that of oil carry-over contamination. Both the roll-type with viscous impingement and the travelling screen type depend upon one primary mechanism to trap the airborne contaminants, an oily surface to which the particle will adhere. If the oil is not present or has been covered over with particles, the removal efficiency will drop off markedly. Both of these types have been used throughout the world with limited success. Today they are both basically obsolete. Inertial Separators Because of the shortcomings of the moving media types, in of large particle protection, the search turned to the inertial separator as a likely candidate for dealing with the larger particles. Inertial separators today are in basically two forms, the vane type and the spin-tube types. The vane type operates in two dimensions and the spin-tube type operates in three. Both use the principle of accelerating the particle and using its own inertia to remove it from the main air stream.
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The primary advantage with an inertial separator is its low maintenance cost and relatively trouble free operation, making it very suitable for remote locations. The need for a secondary power source for the scavenge blower is occasionally a disadvantage, however, the restriction across the inertial separator does not increase with operating time and remains constant under normal conditions. The inertial separator does have proven quite successful in of dealing with the larger particles and have gone a long way in reducing significantly the typical erosion problem with turbines. One major shortcoming has been in areas of extremely heavy dust conditions such as desert where the percentage of particles that do the inertial separator can still be harmful. High Efficiency Media Filter While the previously described inertial separators proved quite effective in dealing with the larger particles sizes, they lack the necessary separation efficiency when confronted with the smaller particles (under 10 micrometers). The advances in design and materials of gas turbines necessitated that fouling and corrosion effects be considered; hence the need arose for the high efficiency media filter. The majority of high efficiency media filters in use today are composed of randomly oriented small filters, either of cellulose or a synthetic material. Such filters depend upon the contaminant particle ing a fiber by one or a combination of three mechanisms: Inertia Impaction, Interception and Diffusion. Multi-Stage Filters While the high efficiency filters do an excellent job of protecting the turbine, they typically have a rather low dust holding capacity and are subject to frequent
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servicing. The solution to this has been to use some form of filtration ahead of the high efficiency filter to extend its useful life. In multistage filters a number of different concepts have been applied. All operate on the basic principle of having one or more stages of some type of device ahead of the high efficiency filter elements to help extend the useful life of the high efficiency elements. Self-Cleaning Filters The self-cleaning or Pulse-Cleaning Filter represents the latest technology in gas turbine filtration. The need for it arose when experience indicated shortcoming in multi-stage systems, namely their finite dust holding capacity. Regions such as the Middle East experienced the problems of excessive filter servicing, turbine damage, lack of proper maintenance, etc., all due to the fixed limit every conventional system has with regard to its dust capacity. The Pulse Cleanings system effectively eliminates the finite limit. The concept of the Pulse-Cleaning filter is relatively straight-forward, a high efficiency filter which, after loading with dust, is automatically back flushed with compressed air and effectively renewed. The Pulse-Cleaning system operation is described here with reference to a basic “module” or collection of high efficiency filters, cylindrical cartridges. The high efficiency filter cartridges are attached to the bottom of a horizontal plate and hang vertically downward: Above each filter cartridge, in the clean air plenum, is a venture flow nozzle. During normal operation, ambient air flows upward, radically inward through the cartridges and then upward through the venture and finally to the turbine. Up to this point the device is operating basically as just a high efficiency filter. Each cartridge is rated for 500-600 cfm at an initial
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pressure drop of 1.2-1.8 inches of water, hence the total number of cartridges required for any installation is controlled by this operating range. Skirts are suspended around the exposed sides of the cartridges to protect them from damage and to force the incoming air to travel upward. As the contaminants build up on the surface of the cylindrical cartridges, the pressure drop across the cartridges will continually increase. When the pressure drop reaches a preset limit (typically 2.2-2.8 inches of water above the “new and clean” initial pressure drop) the automatic Pulse-Cleaning sequence is activated. A pressure switch senses that this upper limit has been reached and activates the timer/sequencer. The timer/sequencer signals the solenoid value to open, releasing the pressure holding the pneumatic air valve. The release of the air valve (typically for only 50-100 milliseconds) allows a “blast” of compressed air (80-100 psig) to leave the air manifold and proceed out through the blow pipe. Above each venture is an outlet hole in the blowpipe. This “blast” of compressed air exist through the hole, enters the venturi, induces a secondary flow into the venturi and effectively “closes off” the flow through the cartridge for a fraction of a second. The compressed air pressure wave continues down into the cartridge and then radically outward, blowing the accumulated dust off the cartridge. In another fraction of second, the cartridge is back in service. A row of cartridges is pulsed at one time. Then 10-20 seconds later the timer/sequencer chooses another row of cartridges and the Pulse-Cleaning procedure is repeated. The timer/sequences continues to sequence the cleaning operation through out the elements until the overall pressure drop reaches a preset lower limit, at which time the cleaning process ceases until called upon again by a high-pressure drop reading. The number of elements cleaned at any instant and the time interval between are chosen so that no significant distortion of the airflow is presented to the turbine.
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Since the pulse-cleaning filter renews it self as needed, the old concept of how much dust a filter can hold no longer applies. The system will maintain the average pressure within its upper and lower limits virtually independently of the ambient dust concentration. Augmented Self clearing system American air filter company has introduced an augmented self cleaning air filtration system (ASC) In which self-cleaning principle is combined with inertial separation, and the separated dust is taken out from the operational area by an exhaust system. Dust laden air enters the ASC intake system and is cleaned by ing through the pack filter elements with in the housing. The clean filtered air exit through the ventures in to the clean air
plenum and on to the turbine in take.
However, by means of the integral secondary air cleaning circuit about 75% by weight of the dust is removed from the incoming air by inertial separation before it reached the pack filter elements. The inertial separated dust together with a small amount of scavenge air is conveyed into the secondary cleaning air circuit where the secondary air fan exhausts air fan exhausts it to the atmosphere at any desired point of discharge. The reverse air cleaning cycle is controlled by a solid state printed circuit timer which is housed in a control assembly. The timer sequentially energizes the solenoid valves that activate air valves to it sharp bursts of compressed air through vertical blowpipes located in the clean air plenum. Orifices located in the blowpipes direct the compressed air horizontally through the ventures into the filter chamber. The momentary pressurization and reversal of air flow dislodges the accumulated particulate from the filter elements, automatically reconditioning the filter for continued uninterrupted services. The dislodged dust is immediately
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sucked into the secondary cleaning air circuit together with the scavenge air. In this manner there is no re entrainment of the dislodged dust back into the filter elements. Entrance to the secondary air circuit is through a strategically located slot at the apex of the “V” formed by two ading pack
filter elements. This
secondary air slot leads into a vertical metal secondary air duct that conveys the particulate to a horizontal dust chute and exhaust fan. Pulse-Cleaning Filters The pulse-cleaning concepts represents the “heavy-weight” in the air filtration industry and is the closest idea so far to a universal filtration system. They are presently in use in the arctic, heavy deserts, off-shore, rural areas, industrial areas, major cities, etc. If there is one thought to keep in mind on turbine filtration, it is to add in favour of “better” filtration, not ‘lesser”,. Considering the cost of a modern gas turbine, the cost of ownership of a more expansive filtration system can be an excellent investment. Silencer A silencer is needed to attenuate the noise while allowing the medium to continue to flow in a duct pipe with least possible hindrance. Silencers work either in absorption principle or in resonance principle In a absorption silencer sound energy when comes in direct with acoustically absorbent material, the air molecules oscillate freely in the medium and thus sound energy is transformed into heat energy by molecular friction. The abortive material is therefore covered with acoustically transparent perforated
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sheeting. If the surface of absorption material is covered with an impermeable layer of dirt, the effectiveness of the silencer reduces. Thus the draw back of these type of silencers is their susceptibility to soiling. Resonator silencer attenuate noise by principle of resonance/interference. These silencers are not affected by loose non sticky dust unlike absorption type silencers. However, because of their effectiveness is limited to a narrow bank of frequency, much larger silencer volume is required attenuate same noise levels as compared to absorbent type silencers. This entails in increase pressure drop & increased cost of material. In gas turbine inlet and exhaust system adsorption silencers with splitter or baffle type arrangement are provided. To optimise pressure drop, and to attenuate noise levels over a range of frequency baffle/splitter are arranged in staggered form. Normally baffles are made of perforated stainless sheets filled with mineral wool felts and pads. Inlet & Exhaust Dust The GT engine, by its very nature, requires a considerable mass flow of air per KW delivered than other conventional engines. Thus, the duct sizes will be larger, if conventional power plant criteria for ducts, gas velocities are used. Of course smaller duct sizes require higher velocities & resulting greater pressure loss. Design consideration or pressure loss. Design considerations or pressure loss influences depends on Fluid dynamic and geometric consideration. The detailed producers for calculation of pressure loss in inlet and exhaust system is not presented here & standard literature on the subject may be referred in case it is required.
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Normally, when a GT unit is supplied on a turnkey basis, GT manufacture optimises the pressure loss in the inlet & exhaust system to maximize output & heat rate of the engine. However, in case GT inlet & exhaust system are ordered are ordered separately, the GT manufacturer specifies corresponding maximum losses in inlet & exhaust system respectively, at which output & heat rate of the machine are guaranteed. Accordingly allowable pressure drop in the dusting system is decided, taking into pressure losses in other system components. COMBUSTION CHAMBER Introduction In the combustion chamber the air delivered and compressed by the compressor is brought to the necessary process temperature by the combustion of liquid fuel (e.g. fuel oil) and/or gaseous fuel (e.g. natural gas). The simple combustion chamber is vertically arranged on the thermal block. Cooling air required for cooling of the hot gas turbine parts is byed. Design Features The combustio9n chamber is an all-welded sheet structure fig-5 and essentially consists of these outer
acket with combustion chamber cover, the bottom and
top combustion chamber liners. The complete finned segments insert and the burner with swirl insert fixed in the burner cover.
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GAS TURBINE UNIT Fig . 5 The combustion chamber outer jacket consists of a creep resistant, low-alloy ferrite sheet steel. A specific BBC structure (patented) is the ribbed body linear with the welded finned segments which has proved a successful design for more than 20 years. The finned segments insert encloses the hottest zone of the combustion chamber, the actual combustion zone, and consists of five rows of welded metal finned segment also called brick, the carrier, the lifting device and the intermediate jacket. Owing to the high thermal stresses of these parts, a special design and carefully selected material are required to ensure a long operating life. They have proven very successful for a long time. The finned segments (“bricks”) made of a highly heat resistant material are, on the fire side, connected by a cladding and protected from the hot flame.
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FINNED SEGMENTS figure. 6 Convective cooling of the finned segments is carried out on the rear side but cooling air flowing between the ribs, and open the fire side by a cooling air film which escapes on from the bottom side on the end of the bricks and is ed over the row located beneath. The finned segments can be easily replaced with the aid of a special lifting device. The upper combustion chamber inert consist of the outside unit and the doublewall interior unit is made of heat-resistant sheet metal welded together. The lower combustion chamber insert is a double-jacket structure made of crimped, non-scaling sheet metal, and is composed of an inner part and an outer part. It is connected to the finned segment insert. By means of the gimbal suspension the lower combustion chamber insert can expand freely . Outer and inner parts are centred against each other by a three-point mounting. For the observation of the combustion zone during operation, several sight glass units are mounted on the combustion chamber. Moreover, there is an automatic flame monitoring devices on the combustion chamber cover. The whole
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combustion chamber is fitted with a thermal- acoust insulation and encapsulated with galvanized sheet steel. For maintenance and checking purpose, the combustion chamber is equipped with a platform, which the access is ensured via a ladder or spiral staircase. Advantages of the Combustion Chamber 1.
Burner -
A burner for the gas turbine in Contrast to gas turbines with many combustion chamber, no measure for an accurate distribution of fuel and air are required to obtain a good temperature distribution.
-
Can be applied for any liquid or gaseous fuels.
-
Standard burner for oil and/or natural gas, consequently suited also for mixed operation.
-
Special burners are available in this field.
-
For liquid low-calorific value fuels such as ethanol or methanol standard burners can be used which only differ the oil burners by the size of their jet diameters.
-
The burner itself can be removed and checked which ensures easy maintenance.
-
The design ensure a good ensures a good atomising of the fuel and consequently, optimum evaporation and mixing with the air which is
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essential to good combustion. The combustion efficiency amounts to > 99.58, under full load > 99.98. -
Since there is only one fuel jet, it has relatively larger openings so that it is less prone to contamination.
-
One ignition for the whole system easier, i.e. higher ignore on reliability than with multiple combustion chamber systems.
Combustion Space The whole dimensioning of the combustion space results in the following advantages. •
No flame with the wall, which means that no combustion takes places near the wall and, consequently, the film cooling air cannot be misused as combustion air (this danger exists with small – size chamber chambers).
•
No oil spraying on the flame tube possible.
•
Combined cooling of convective cooling and film cooling by special cooling elements provided with ribs (ribbed bodies ), which form the flame tube.
•
Thus the flame tube does not consist of one ring, but is formed by individual elements. Therefore, no stability problems (dents) of the combustion wall as is the case with flame tubes.
•
Long dwell time ensure burning out of the fuel (the emission of CO and non-burnt hydrocarbons is practically zero).
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•
Flame radiation independent of the fuel since E of the flame (desire of emission) is nearly 1.0 (black body).
•
The combustion chamber is laid out for easy maintenance since it is possible, after removal of the further including swirl insert, to enter the combustion chamber, so that it can be checked in a very simple manner. Also the hot-gas housing and the first gas turbine stage are easily accessible. (See fig.7)
COMBUSTION CHAMBER Fig.7
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Burner Combustion a)
General description of the duel burner.
The dual burner with its auxiliary facilities is used for the supply of gaseous and liquid fuels to the combustion chamber. Simultaneously, mixing of the fuel with the “primary air “ is achieved with this burner, while the flame is stabilized by an appropriate air supply. b)
Constructional features (fig.8) -
Burner cover
-
Swirl inset
-
Fuel supply with fuel nozzle
-
Servomotor for liquid fuel
-
Gas burner jacket with inner jacket For the supply of the gaseous fuel
c)
-
Nozzle cap
-
Igniter
Combustion The air arriving from the compressor enters the combustion chamber from the bottom and flows upwards. Approximately 30% of the air between combustion chamber outer jacket and other part of the lower combustion chamber liner flow as secondary air via the mixing jets to the combustion chamber inner space while the remainder flows further upwards between inner and outer part. Further distribution takes places at the funnel shaped outlet of the lower combustion chamber outlet. Via an orifice on the lower end of the intermediate jacket another 30% of the air are branched off for cooling the finned segments. This air then is
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dispersed on the partition walls and flows between the cooling ribs downwards into the combustion space, while a layer of protective cooling air is formed on the flame side of the ribs which protects the ribbed bodies from adverse heating. The remaining 40% flow as “primary air” i.e. as oxidizing agent for the combustion, into the swirl insert in the air enters the combustion space in a turbulent state. This ensures a very good mixing with the fuel. Moreover, the swirl insert is used for a good stabilization of the flame. Another portion of the “primary air” flows through four orifices in the gas burner jacket into the annuls between inner jacket and inner wall of the gas burner jacket and enters the combustion space directly at the fuel nozzle with a turbulence caused by the inner swirl insert. By this type of air guide a very intensive mixing with the fuel is attained which ensures a complete and rapid combustion. The ignition of the fuels during starting is effected by the igniter with a propane flame.
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After ignition of the fuel, the hot combustion gases flow downwards. Before they enter the turbine, these gases are well mixed with the secondary air, coming from the mixing orifices and brought of a temperature issible for the turbine. Figure-9 shows a schematic diagram of the streamlines.
The air enters with swirl to the combustion space through the primary air inlet (swirl insert). Due to this flow scheme (cyclone principle), a back flow to the burner jet is produced. This reduces the flame front speed, which ensures a stable flame. The cyclone effect is terminated by the inflowing cooling air at the mixing orifices (D). Fig-10 shows a fuel jet liquid fuel is injected at a very high pressure and finely atomized.
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FIG-10 TURBINE In the gas turbine, the power for driving both the compressor and the generator is produced. The energy is supplied to the turbine in the form of the hot gases as they come from the combustion chamber. The kinetic energy of the gases is transferred to the rotor in the turbine by means of the rotating blades. In the following this process is explained with the example of a gas turbine stage. The combustion gases, coming from the combustion chamber or from the preceding stage, are accelerated in the stator blade to the speed C while, © PMI,NTPC
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simultaneously, the inlet pressure is reduced. By vectorial addition with the circumferential speed the relative speed w is obtained. Depending on the degree of reaction this speed is accelerated to W2 in the rotating blades row (fig.11). On
the other hand the absolute speed C2 is
obtained by the addition of W2 and U2: The pulse change of m.W1(mass by speed) into m.W2 generates the driving power acting on the blade.
TURBINE BLADES FIG.-11 PT = PV+PN = mR. ∆ ht . Pt
=
Gas turbine power in KW
Pv
=
Compressor driving power in KW
Pn
=
Generator effective power in KW
Ms
=
mass flow in kg/sec
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∆ht
=
Isentropic turbine gradient in kj / kg
=
effective efficiency,
it
=
internal turbine efficiency, approximately 0.80 to 0.87
it
=
mechanical efficiency, approx. 0.99
e
=
it
.
m
Example with partial data of the GT 13 D2 m
=
372kg/sec;
P1
=
11.6bar t1
=
990oC
P2
=
1 bar t2
=
490oC
∆ht
=
680 kj/kg for fuel oil
Hu
=
42000kj/kg
PT
= 372 kg/sec; 680 kj/kg; 0.83
=
212.5 MW
PN
= PT - PV
=
212.5-124.5=88.0MW.
DESIGN FEATURES Talking into its thermal load the turbine housing made of creep-resistant ferrite mater raise as welded structure or steel casting. On the exhaust gas side the inner cone of the turbine housing is connected to the outer wall by ribs. Inner cone and outer wall form in this area the annuals for the escaping exhaust gas. Because of the relatively high exhaust gas temperatures the walls and ribs of this duct in the gas turbine housing are cladding with a sheet metal lining. The
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sheet metal lining is made of a temperature resistant material and fastened in the turbine outer housing allowing for thermal motion. The bearing
housing
made of cast steel for the reception of the journal bearing on the exhaust gas side is suspended allowing for thermal motion in the inner cone of the outer housing bottom part. The stationary blade carrier made of alloyed ferrite cast steel is cooled with the aid of compressor air which enters through appropriate holes and air inlets. To keep the thermal flux and the temperature stresses in the stationary blade carrier(fig.12) low also during start-up and shutdown of the machine the side exposed to hot gas is provided with heat shields which are radically suspended.
BLADE CARRIER Fig-12 The turbine blading consists of five stationary and five rotating blades. Because of the continuously increasing turbine inlet temperature the number of cooled
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stages is increased(from a stage at the beginning with 13D2stages on 13D2to 3stages on the 13Etype). The cooled blades are manufactured of nickel-base alloys in a precision casting process. Different types of blade cooling methods (fig.13)
BLADE COOLING Fig-13 In convection cooling air flows through holes smooth or ribbed ducts (figure 14) and absorbs part of the quantity of heat transferred by the hot gas into the blades. Depending on the design, the heated cooling air is mixed to the hot gas on the blade trailing edge, on the blade tip and or an the blade.
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GUIDE VANE CONNECTING COOLING Fig-14 DEFLECTOR COOLING- which also called jet cooling is also type of convection cooling. However, the cooling air is ed through the sheet metal deflector plates. These deflector inserts are provided with holes so that the cooling air bounces as jets on the blade wall to be cooled.
TURBINE COOLING SYSTEM FIG-15
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7. Gas Turbine Compressor General The characteristic feature of this turbine generator set is the use of a single, composite welded shaft for the compressor and turbine gas rotor, carried in only two bearings. This enables the complete compressor/turbine assembly to be of very compact construction and ensures good shaft stability with a simple bearing arrangement of two journal bearings and one thrust bearing. The welded rotor concept has been in use for many years in the steam turbine field and this design has also proved superior for the gas turbine. In comparison to a rotor comprising several disc held together by a tie rod. The welded rotor has several important advantages: the welded discs have o holes in highly stressed areas, especially in the center, and dangerous stress concentrations are therefore avoided. The welding s the discs together into a single shaft without mechanical s in which misalignments caused by centrifugal force or thermal expansion cannot occur. This is particularly important with large shafts. The commonly- observed heavy vibrations caused by rapid acceleration and load application are completely avoided. The welded designs avoids all the problems which can otherwise occur due to the above- mentioned mechanical s. In addition, this design guarantees maintenance free service of the rotor. The compressor and turbine casing consist of several sections which are ed together to form a single unit. Design and function of Individual Components of the Compressor/Turbine Assembly
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Air Intake connection The air intake connection has a horizontal air inlet, and is situated axially in front compressor. It is a welded, solid steel plate construction split horizontally and held together with hexagon bolts and nuts. The attachment to the compressor casing is effected by two guide flanges welded on to the connection which locate in the appropriate slots turned in the compressor casing. Sealing tubes made of synthetic material are laid in these slots and form a seal against the environment. The air intake connection has four feet which are bolted with adjusting belts to steel plates embedded in the foundations. These bolts allows the height to be adjusted to that of the rest of the compressor/turbine assembly. Compressor casing The compressor casing, which is horizontally spilt as axis height is made of spheroidal graphite cast iron. This material possesses high tensile strength and good expansion qualities and for this reason is well suited to the stresses and high temperatures in the pressure and caused by the compression process. The upper and lower halves of the compressor casing are provided with robust flags and are held together by expansion studs with sockets head nuts. To bearing housing for the intake end journal bearing and thrust bearing is integrally cast with the
help of the compressor casing. The bearing cover
is bolted on to the bearing housing. The oil glass for the thrust bearing and the axial shaft position monitor are mounted on the. The flanges of the bearing cover and housing are sealed with an oil resistant ting The bearing cover and housing incorporate all the necessary grilling for lube oil purpose, for the sealing air of the intake end shaft seal and for temperature monitoring.
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The lower half of the compressor casing is also fitted with paws for the fixed s at the intake end and for the pin-ended s at the pressure end, as well as the flange for the king pin
which is situated at the intake end and
determines the fixed point form the complete compressor/turbine assembly. The compressor casing has three integrally- cast circular ducts, which are connected to the blade duct behind the fourth, seventh, and tenth rows of fixed blades. These ducts are closed to the outside by four blow-off values flanged directly to the upper half of the compressor casing. the blow-off values are controlled pneumatically and are open at lower speeds,i.e.up to approx 90%of the rated speed, so that a certain amount of the compressor output can be blown off. the air blown off flows into the machine room through the blow-off valve bonnet arranged around the valves and a silencers in that order. these blow-off valves enable a consideration reduction to be made in the external power input requirements during start-up, and they also avoid undesirable and dangerous flow conditions in the compressor which result in rotating stall and pumping effects. Turbine outer casing The outer casing is split into two halves the same as the compressor casing. the turbine and compressor casing are bolted together at a radial flange with expansion bolt. the casing parts are centered by means of eight round wedges in the circumference. The turbine casing is made of heat-resisting ferrite steel in order to withstand the thermal stresses, and is of welded construction. the upper and lower halves are bolted together at the horizontal flanges with expansion studs, expansion sleeves and socket head nuts. The horizontal flanges are of solid construction and are slotted at the drilled holes so as to avoid high thermal stresses. Horizontal and radial flanges of the turbine and compressor are metal-to- metal ts.
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The inner cone and outer wall of the turbine casing are ed together by ten ribs at the exhaust end, and the resulting ring duct between the two constitutes the flow age for the exhaust gas from the turbine. Due to the relatively exhaust gas temperature, the walls and ribs of the ring duct are lined with a special plating to prevent the temperature differences between various parts of the casing walls and therefore, the thermals stresses during transitional operating phase (e.g. starting up and shutting down); from exceeding the allowable limits. The planting is manufactured from temperature resistant materials and is attached to the turbine outer casing in such a way as to accommodate thermal expansion. A cast steel bearing housing is mounted in the inner cone of the lower half of the outer casing and provides the for the exhaust end journal bearing. The housing is free to thermally expand. The bearing housing is retained in the turbine outer casing by means of integrally cast guide at axis height and by a key collar bolts are provided to prevent lifting. The reaction forces of the journal bearing are transmitted through the lower half of the inner cone and the above-mentioned ribs to the turbine outer casing. From here are transmitted through the pin-ended s to the unit . The trip and mechanism for the two over speed trips, which are mounted at the rotor are installed on the bearing housing. The oil supply line mounted on a cover on the hearing housing enables the over speed trips to be checked while the turbine is operating by supplying oil pressure to the underside of the over speed bolts. Ventilation and oil wiper rings are inserted in the bearing cover and bearing housing and also in the intake end bearing housing and cover to prevent the escape of oil. Two air deflector’ plates are welded in to the lower half of the outer casing to stabilize the flow. In addition two paws are cast integrally with the lower half of the turbine outer casing, and the pin-ended s of the exhaust end are bolted to these paws.
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The vertically arranged combustion chamber is mounted on a horizontal flange welded on to the upper half of the outer casing. In addition, the outer casing has a circumferential coller for the blade carrier. Blade carrier The blade carrier for the turbine fixed blade is a ferrite steel alloy casting and is axially spilt the same as the compressor/turbine casing. It is bolted together with expansion bolts, expansion sleeves and hexagon socket head nuts. The blade carrier is fluxed axially by a collar in the turbine outer casing, which fits in to a salt around the blade carrier circumference. Paws are integrally cast with the parting flange of the lower half of the blade carrier at the same level as the outer circumferential salt. These paws have hardened pads fixed to them and together with the corresponding surfaces in the turbine outer casing they form the vertical for the blade carrier. An axial salt, is machined in the lower half of the blade carrier at the same level as the circumferential slot , and the guide pin fitted in the lower half of the turbine outer casing locates in this axial slot hold the blade carrier in its lateral position. The external pressure of the compressed combustion air on the blade carrier causes it to be pressed against the collar and forms a metal-to-metal seal to the exhaust end. This air also cools the blade carrier at the same time. A more intensive cooling of the blade carrier is achieved by allowing a small amount of air to flow through the appropriate holes and circular ducts. This air flow results fro the relatively small differential pressure between the compressor exhaust and turbine Intel and is sufficient to maintain the temperatures within the allowable limits. This air stream reenters the hot gas flow channel before the first row of fixed blades. Radically ed heat storage segments are fitted between the rows of fixed blades in the areas exposed to the gas flow, similar to the heat shield of the turbine end of the rotor. These segments contain the heat flow and thermal stresses within the blade carrier at a low level during start-up and shut-down.
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Owing the absence of centrifugal stresses these segments can be mounted in the blade carrier more simply than on the rotor. The cooling air deflectors fitted to the upper half of the blade carrier are to improve the cooling of the corresponding inner casing walls. Combined compressor- turbine rotor The combined compressor and turbine rotor comprises the individual discs which are welder together to form the compressor and turbine blading. The eleven discs constituting the body of the shaft are centralized with one another by circumferential recesses and connected together by a special welding process. The complete shaft comprises seven compressor discs, and a center drum connecting the two section made of a ferritic forged steel alloy. The end discs have forged journals, and the one at the compressor end is fitted with a coupling flange and thrust bearing caller. The complete shaft is stress-relieved after welding. The blade rotor is then over speeded to 20% above the rated speed and balanced. The compressor –turbine rotor is carried by two journal bearings which are force-lubricated and cooled. The bearings are each held by four fitted plates and are each prevented from rotating by a solid pin. The bearing are lined with white metal and the radial clearance is approx 1.5%. A lubricating oil film is formed by the pressurized oil, which enters the bearing from both ends and is forced in to two lube oil groves. The lower halves of the journal bearing are connected to jacking oil pumps which deliver high pressure oil to the bearing to lift the compressor- turbine rotor from the lower shells thereby reducing the initial breakaway torque.
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COMPRESSOR TURBINE ASSEMBLY Fig-16 The axial thrust of the rotor is taken up by the thrust bearing situated at the compressor end. This bearing is of the pad type and is also lubricated and cooled by pressurized oil. Since the thrust on the turbine balding is in opposition to that compressor balding the resulting differential axial thrust remains small. Moreover, this thrust always acts in the same direction, i.e. against the gas flow and
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towards the generator. The thrust bearing also fixes the axial position of the rotor in the casing. Both expand in the same direction since the fixed point is given by the king pin for the complete casing and is situated in the area of the thrust bearing. A shaft monitor is provide which measures the axial displacement. This gives an alarm in the event of abnormal displacement and then gives a trip signals for the unit when a maximum value is reached. The axial clearance required can be adjusted at the bearings. The surface of the rotor between the rows of turbine blades is protected against excessively high temperatures by heat storages segments. These subdivided the hot upper surface of the rotor into small rectangular zones. They can expand freely in all directions independently of the exhaust end of the shaft. In this way the major part of the heat which enters the rotor from outside is conducted away just below the surface of the rotor, so that the amount of heat reaches the actual body of the shaft remains small. This results in a relatively slow warm-up of the shaft body and therefore high thermal stresses cannot occur. A similar effect is experienced during shutting down when the flame is extinguished and a relatively cold air stream flows through the machine while it slows down. The lifespan of the rotor is extremely good with this method or construction. The cooling air required for the turbine part of the rotor, the feet of the turbine rotor blades and the first and second rows or rotor blades is taken directly from the compressor flow at the diff after the last row of fixed blades. The air flows through the channels between the compressor diff and the shaft cover to a ring nozzle, and then to the turbine part of the rotor. Here it es through drilling in the projecting shoulder of the rotor and flows through the cooling air ducting system between shaft and heat storage segments and also around the blade feet. After the fourth rotor blade row the cooling air exhausts into the gas steam from the rotor. It then flows over the feet of the last row of rotor blades and leaves with
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the exhaust gases. Part of this cooling air flow is used to cool the rotor blades in the first and second rows. The cooling air flow is monitored by measuring the pressure difference across the ring nozzle of the nozzle of the compressor diff. The sealing
air for the shaft seal cools the end of the rotor at the
exhaust end. Shaft seals are formed by thin sheet metal strips caulked into grooves in the rotor by means of caulking wire, operating at a small running clearance with either the shaft seal shell or the corresponding part of the casing, respectively relatively few sealing strips are required to give an adequate seal since the pressure difference at the shaft seal is small. The exhaust end shaft seals prevent the gas from escaping to atmosphere. Those at the intake end prevent the ingress of oil laden or otherwise contaminated air from the bearing regions into the flow duct/ both these shaft seals have a sealing air supply taken from the blow off duct after the fourth row of fixed blades in the compressor. At the exhaust end draft seal the sealing air supply comes through the shaft seal shell, thus cooling the rotor end as well as explained above. The shaft seal at the air intake end is divided into two groups of sealing strips. Here the sealing air enters the annular space between two groups via the appropriate ducts and holes in the compressor casing. The shaft seals situated between the compressor and turbine prevent the leakage of too much cooling air. Pressure tapping are located before and after the sealing strips of the shaft seal directly below the first row of turbine fixed blades. These are used to measure the pressure difference, which is necessary to monitor the correct functioning of the shaft sealing. Compressor balding The compressor balding is designed as reaction balding and consists of eighteen stages with one additional inlet guides row. The inlet guide row serves to correct the flow direction for the first row of rotor blades. The blades of the eighteen rotor and nineteen fixed rows are manufactured from high tensile ferric chrome steel.
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They are either precision forged or machined, and fit into slots which are turned in the shaft and compressor casing. The blades are separated from one another by spacers. The blades of the first seven rows are rooted directly in the shaft, those of the remaining rows are inserted in intermediate pieces. Turbine Blading The turbine blading is also of the reaction type, and consists of five rotor and five fixed rows of blades. Due to the high temperature of the incoming gas, the first and second rows of the rotor and fixed blading are cooled with air from the compressor. The cooling air is fed to the first and second rows of fixed blades through holes drilled in the blade carrier and to the first and second rows of rotor blades through drilling in the shaft and through the age underneath the heat storage segments. After entering the blade foot, the air es along several parallel ducts into the blade proper. Each air stream changes direction several times inside the blades in the first and second fixed blade rows and before finally blowing out through a number of slits in the trailing edge of the blade. Part of the cooling air in the first row of rotor blades and the second row of fixed blades and the entire cooling air flow in the second row of rotor blades is blown out through the blade tips. The first row of fixed blades is film- cooled as well, whereby the air leaves the blades through numerous small holes and forms a protective film over the blades surface. This method of cooling ensures that the leading and trailing edges of the blades in particular are thoroughly cooled, thereby avoiding cracks induced by thermal stresses. These cooled blades and the fixed and rotor blades of the other rows are precision cast in a nickel –based alloy. The fourth and fifth rows of rotor blades and the fifth row of fixed blades are precision drop-forged.
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The first row of fixed blades sits in an axial slot in the carrier segments, which are in turn fitted into a slot in the blade carrier. The specially-formed ends of these blades are ed in the shroud segments which are mounted in a radial slot in the compressor diff, which extends as far as this point. The remaining fixed blades have rhombic shaped feet which fit into the appropriate slots of the blades carrier. The first row of rotor blades are fitted into radial double. “T” slots in the rotor while the fifth row fits into angled fir tree slots. The so-called “super materials: and high alloy steels used in the turbine blading are all resistant to extremely high temperatures and to scale. Their anticipated service life is calculated on the respective prevailing temperature and resulting stresses. An additional protective measure has been taken to counteract corrosion in the form of a specially applied chrome layer on the first rows of blade. This increases the service life of the blades considerably. Compressor Diff The main function compressor diff is that of flow device, namely reducing the relatively high velocity of the combustion air from the compressor blading while keeping losses to a minimum, and ensuring a favorable flow into the turbine outer casing and combustion chamber connection. The compressor diff is branched into a radial and an axial section, so giving a favourable flow distribution which produces uniformly intensive cooling of the entire inner casing surface. The shaft cover and the inner shell of the compressor diff together form the annular duct for the cooling air for the turbine section of the rotor. The nozzle segments are fitted in circumferential slots in the diff at the outlet of this annular duct. These nozzle direct the cooling air flow with
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minimum losses to the rotor. At the inlet to the turbine blading, the end shroud segments of the first row of fixed blades and the inlet segments together form a heat shield for the wall of the compressor diff. Both segment types are inserted in circumferential slots in the compressor diff. The compressor diff and the shaft cover are heat resistant spheroid graphite iron castings. They are both splits horizontally, the same as the rest of the casings in the compressor/turbine assembly. The top and bottom section of the compressor diff are bolted together at the parting flange with expansion bolts. The complete compressor diff is flanged at the front end to the lower half only of the compressor casing, and is cantered by a collar on the front flange which fits into a corresponding recess turned in the compressor casing. Inner Casing The inner casing guides the hot gases coming from the combustion chamber to the turbine blading. It is a relatively thin walled welled construction made of heat resistant chrome nickel austenitic alloy. The outside of the inner casing is cooled by the combustion air coming from the compressor diff. It rests on s in the turbine outer casing is split axially, and is held together by expansion bolts made of high temperature resistant materials. All flanges bolts of the inner casing are secured by special locking devices so that they cannot become loose during operation. The inner casing is ed by two flange rings at its outlet which fit into corresponding recesses turned in the blade carrier and compressor diff. This prevents axial movement and at the same time guarantee good separation between the hot gas and compressed combustion air in the turbine casing. A drain connection is arranged in the lower half of the inner casing which is used
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for draining fuel which has not burnt due to ignition failure (only with oil or dual fuel operation). Special equipment and the associated guide bolts must be used when fitting or removing the inner casing, and the instructions must be followed implicitly. Exhaust Diff The exhaust diff is a single unit of welded steel plate construction to the turbine outer casing by two radial flanges forming metal to metal ts, and guides the relatively hot exhaust gases into the exhaust gas system. Expansion bolts with hexagon head nuts are used to bolt the diff to the turbine outer casing. The inner part of the exhaust diff, which is annular in shape, is essentially held by three oval- shaped, hollow plate ribs. These serve at the same time to permit access to the diff up the exhaust end bearing, for the installation of pipe- work, and also for ventilation purposes. The sheet metal cover of the thermal installation on the inside of the diff is welded over a third of its circumference to make it oil tight and so prevent smouldering which might occur as a result of oil leakage. A leakage drain is fitted to the lowest point of the diff unit. A traveling hoist can be fitted to one of the rails inside the exhaust diff and this assists in the assembly and disassembly of the exhaust end bearing during erection and overhauls so that bearing parts can be lifted in and out through the upper hollow rib. The exhaust diff is ed on the foundations through two adjustable spring s. These take up small relative movements between the exhaust gas system, exhaust gas diff, and machine foundations.
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The complete compressor/turbine assembly is ed on both sides at axis height by four pin-ended s and two fixed s: These rest on the unit s that are bolted to the anchor plates which are in turn cast into the machine foundations. The casing rests on two fixed s at the intake end between which is a solid king pin which fixes the position of the compressor/turbine assembly laterally and longitudinally. The king pin, which is situated below the machine in line with the central axis, takes up the forces which are transmitted by the exhaust gas system. Longitudinal movement of the compressor/turbine assembly is guided by the guide key below the turbine flange. In general, all s are principally of welded construction. Insulation The complete outer surface of the compressor/turbine assembly is thermally insulated by two layers of lagging mats. These are made of mineral fibre cloth with a sewn in mineral wool insult ant and are laid on the casings in a staggered pattern attached to one another by books and eyes. The method allows easy and non-destructive removal during overhauls. The exhaust gas diff is thermally insulated and soundproofed with the special insulating material consisting of several layers. This is covered with the galvanized steel sheet. Insulation of the air intake connection is similar to the exhaust gas diff. The enclosure over the compressor/turbine assembly, ie. the gas turbine unit, is free-standing on the machine foundation and can be
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walked on for access purposes. It reduces the noise level and protects the insulation from damage. It also improves the appearance of the unit. Compressor Section Compressors for gas turbines may be centrifugal or axial flow type, but all large gas turbine use axial flow compressors. Gas turbine compressors are multistage design and consists of individual discs or wheels mounted on a shaft. Each disc holds the compressor blades which extend rapidly. This complete assembly is called the compressor rotor and is enclosed in the compressing casing. The casing holds stationary blades which direct the air flow against the rotating blades. The stationary assembly is called the stators. As the blades force air through the compressor, the casing area decreases and air is compressed through each successive stage. Early axial compressor designs were almost all based upon constant axial velocity over the length of the compressor. This constraint simplified some flow analysis and together with other arbitrary assumptions such as constant inner diameter or constant diameter enables analytical and design team to cope with a complex engineering problem. The large investment made in such design approaches has resulted in their persistence in present day gas turbines. Over time, the body of Knowledge of the processes actually occurring in axial compressors grew and economic factors related manufacturing costs became more important. Also, manufacturing processes and materials advances gave more freedom to compressor designers, and thus a different approach to design was taken. The results of this different and more sophisticated approach are axial compressors with variable axial velocity distributions and variable inner and outer diameters. Such compressors are generally capable of providing superior performance characteristics with fewer stages than earlier compressor. However, design of
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gas turbine compressor design is fixed for a particular machine by the manufacturer. It is not an option to be selected; rather it is part of an overall piece of equipment. The compressor must be designed and operated so as to avoid ‘compressor surge’. The surge characteristic of compressor is a complex function of many compressor design features. It can also be affected by compressor deterioration and blade fouling in operating units. In order to avoid compressor surge, each stage of the compressor must maintain its overall flow situation. When a breakdown of this overall flow situation occurs in any one stage, the result is usually disruptive to the flow situation in subsequent stages and surge occurs. Under such conditions, intermittent flow and conditions such as ‘puffing or rotating stall(in which only a part of one ore more stages is in surge while the remainder is in steady flow ) can occur . In such conditions are found to occur at rotational speeds high enough to pose the danger of mechanical damage to the combustion turbine unit, the situation can be alleviated by :1) providing inter stage relief valves at one or more points along the compressor or 2) equipping or more stages of is based on years of research and development and the compressor stationary blade rings with variable geometry provisions. The first method maintains a viable axial velocity distribution by relieving axes flow from the later stages of the compressor . The second method controls the flow produced by the variable geometry stages to be within the capacity of the later stages of the compressor. The method of surge control is inherent to the design of each particular gas turbine and is left to the manufacturer. Combustion Chamber (Combustor) In large industrial type gas turbine, two types of combustion chambers are used. With an annular type combustors, combustion occurs in numerous combustion chambers located radially around the compressor discharge casing. Some manufacturers use a single or dual ‘silo’ type combustor, which is a large
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chamber, mounted on top of the turbine. Each combustion chamber includes fuel nozzles, and ignition system and flame detectors. Each combustor is enclosed in a casing and liner for and insulation. Compressor discharge bleed air is used to cool the combustor. There are advantages and disadvantages to silo-type combustors when compared to multiple in-line type combustors. Silo-type combustors provide better combustion when using liquid fuel. Nox emissions are not easily controlled with a silo-type combustor. However, external silo-type combustors have greater pressure drop than then in-line type combustor arrangement. Turbine Section The turbine expands the hot gases from the compressor and combustor and drives the compressor and generator. The turbine is constructed similar to the compressor. Turbines are multistage design and consist of individual discs or wheel mounted on a shaft. Each disc holds the turbine blades (buckets), which extend radially from the shaft. This complete assembly is called the turbine rotor and is closed in the casing. The casing holds stationary nozzles, which direct the gas, flow against the turbine blades causing the rotor to rotate. Generator Assignment The generator can be located either at the hot and (exhaust) or the cold end(inlet) of the gas turbine. With a hot end generator, the exhaust system becomes more complicated and direct axial exhaust into the HRSG is not possible with a hot end arrangement. Using a cold end generator arrangement may complicate the air inlet system. It may also complicate arrangement of the starting drive and/or accessory gear. Either arrangement is acceptable and the location is the manufacturers choice based on his experience.
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8. Waste Heat Recovery Boiler INTRODUCTION What is waste heat? It is heat that is discharged in to the atmosphere without recovering the available energy that might be economically converted in to useful purposes. Then what is wastes heat recovery system? it is a system of heat exchangers which converts this heat in to a more usable form of energy. The utilization of the wastes heat recovery system in combine cycle power plants with gas turbines is a relatively recent development brought by the development in the field of gas turbines resulting in gas turbines becoming larger in size and output, the large volume of heat available form gas turbine exhaust and rising fuel costs. One of the main features of the gas turbine is the high mass flow of air used over the stoichiometric quantity of air. Typically volume of [excess air factor] for gas turbine combustion lies between 3 to 4. this means there is more than sufficient oxygen to secondary combustion, if required. This high mass flow in turn makes the gas turbine exhaust an ideal candidate for heat recovery steam generation. Description The basic wastes heat recovery boiler (WHRB) consists of an evaporator which is the basic steam generator, and an economizer which heats the incoming feed water to the evaporator. Super heaters are also furnished, and condensate preheaters (low pressure feed water heaters/ are sometimes used depending on need and circumstances.)
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A complete wastes heat recovery boiler would also include an inlet gas by- damper, with by stack and silencer, exhaust stack with monitoring provisions and all interconnecting ducting between the gas turbine and exhaust stack. Other WHRB accessories include the drum level sight glasses and level controls, pressure gauges, continuous blow down control and sampling stations. The WHRB evaporator consists of a steam drum, interconnecting unheated down comer tubes, interconnecting heated riser tubes, enclosures casing and accessories. The evaporator receives the incoming heated feed water from the economizer and generates saturated steam which is taken form the top of the steam drum, the evaporator will operator at constant saturation temperature corresponding to the drum operating pressure. The steam drum is locked at the top of the evaporator to enable natural circulation to take place water steam level will be maintained in the steam drum during operation. The economizer is a feed water heater operating at the steam drum pressure utilizing the heat left in the exhaust gas after the gas leaves the evaporator. The economizer will heat the incoming feed water to the evaporator Super heaters are basically steam heater receiving saturated steam from the steam drum and producing superheated steam. Super heater are located ahead of the evaporator in the WHRB exhaust gas flow stream in order to develop required superheated steam temperatures. Low pressure feed water heaters also called condensate perheaters’ are sometimes used on combined cycle units to heat the desecrator feed water thus the cycle required less desecration steam making it more efficient. WHRB exhaust gas by – are commonly provided to isolate the WHRB form its heated source. If furnished this isolation will permit running the gas turbine at times when’s steam is not required or when WHRB is not available. Normally the
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by dampers will have a straight open/close function though some designs can also provide degree of modulation. A by stack complete with silencer is required and provided when by- dampers are used Duct burners can be used to raise the temperature of the exhaust gas to produce more steam. However, duct burners are generally not used on combined cycle plants due to the low rankine cycle efficiency as compared to higher combined cycle efficiency. Design considerations a)
Number of steam cycle pressures: The general design philosophy of the WHRB is to exhaust heat form the gas to the fluid at the highest temperature difference available. This can best be accomplished by making the gas and the fluid [steam or water] temperature gradients as nearly parallel to each other as possible. With a single pressure level boiler, the choice is very limited since the super heater, evaporator and economizer have to be placed in descending fluid temperature along the gas path, in a multi pressure level unit this general order is maintained, but various section may be interchanged so that a nearly parallel relation between the temperature gradients can be achieved. In a single pressure boiler there is a limited amount of heat that can be extracted, because the exhaust gas temperature cannot be reduced below the steam saturation temperature. However, with a multipressure system the recoverable heat range can be extended by extracting heat at various levels, as the saturation temperatures are lower at each successive pressure.
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However each successive pressure level adds to the plant cost and complexity, which may make them uneconomical for most combine cycle applications. Due to this reason dual pressure WHRB’s are commonly used particularly in combined cycle application where moderate and low pressure steam is required. Dual pressure WHRB’s are basically two WHRB’s utilizing the same gas stream. The high pressure section of the WHRB would consist of an HP evaporator complete with HP economizer and HP super heater interconnected to produce HP superheated steam. The LP section would consist of LP evaporator with LP economizer interconnected to produce lp saturated steam. Both WHRB’s would be built in to the same gas steam and would normally take feed form a common condensate heating system. The hot gas path would be HP super heater, HP evaporator HP economizer, LP evaporator, LP economizer and condensate preheater, splitting the HP economizer in to section and the cooler section downstream of the LP evaporator can provide far more efficient utilization of the heat source. By Stack And Damper In an open cycle GT plant requirement of damper does not arise. In a combined cycle plant provision of by stack and by require to be examined: The major advantage a by damper and stack can offer are i)
To phase out the installation schedule of the GT open cycle and combined cycle module, and
ii)
The capability of running the G when the GT open cycle and obtained cycle module, and generator is out of service for scheduled maintenance or repair.
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In the case of the steam turbine down, it is possible to run a GT without a by stack if the steam can be used elsewhere or dumped. For combined cycle applications however, the use for the steam else where is generally nonexistent and dumping the steam involves sizing the condenser to take the full load steam in a steady state condition, running steam and circulation water cycles in an unproductive heat-dump mode of operation. Alternatively, the WHRB is required to be suitable for ‘dry running’. A further advantage the by damper offers is the provision to control, to a limited extent, the heat input into the WHRB and the quantity of steam produced partially bying the particularly important and useful during cold startup operations. Another distinct advantage of the by damper is the ability to isolate the WHRB from the GT during the initial preoperative GT testing, when the repeated stop-start operating of the GT can otherwise cause condensation to occur throughout the WHRB. A further possible advantage, dependent on circumstances, is the ability to isolate the WHRB from the GT is burning sulfurous fuel. Disadvantages in using a by duct and damper include the initial equipment cost, maintenance costs on the by dampers and the constant heat loss through both direct exhaust gas leakage and heat loss through the dampers. A value can be derived for the GT operating time gain utilizing the by on consideration of the expected WHRB and steam turbine down times which can be compared to the value of the expected constant heat and gas leakage losses, maintenance costs, and equipment costs. In summary, however there is no general position on this subject and the same should be evaluated specifically for each site conditions.
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Type of dampers The major choice lies between utilizing a single diverter damper or two isolating type dampers one for the WHRB duct and one for the by –stuck. The two damper concept offers the greater selection of damper types that can be used but has the disadvantage that should both dampers close at the same time the ducting would become over pressurized. In turn this necessities the installation of relief s or pressure relief type dampers, in addition to the electrical interlock to prevent such situation. In the event of malfunctioning/ failure of the above provisions a mechanical linkage system ensures that both the dampers are not closed at the same time, as a foolproof system. By – dampers are prone to high leak rates. losses from the dampers include both sealing losses through leakage and heat losses through the blade(s).For combined cycle application involving large gas turbines. Damper types cover a range including,multi louver type, diverter type and flap type dampers. The advantages and disadvantages of each type and the suitability for a particular application very with the application. The multi louver type damper normally has external linkage actuating all the louvers simultaneously and is good for regulating flow. However it will not be as good on sealing as the single blade type damper. Further more, any pressure differential would tend to force one part of the louver seating open, and would not assist the sealing. The losses from this damper would therefore be greater and furthermore, differential expansion on the linkage brings to question the possibility of linkage binding. The diverter valve offers the advantage that the duct cannot be closed off and would be open to either the by stack or the WHRB. Sealing is generally effective and the damper can be appropriately insulated to minimize heat losses.
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Flap damper have the same kind do flap operation and sealing action as the diverter but are single application operating only as a single isolation type damper. They offer an advantage on extremely large ducts by utilizing a seating grid in the damper and breaking the damper into two or more individual “flaps” of a smaller easier handled size. Disadvantages would be the pressure drop across the seating grid. Pressure relief type flap dampers are also available and offer the advantage of incorporating an over pressure relief flaps and avoiding the additional costs of adding the relief s elsewhere in the ducting. Normally they are associated with multi louver dampers. Generally the diverter and the louver type dampers can be set to a partially open position to control heat for or start-up. However the dampers are generally not suitable for continuous modulation such as load following, due to the size. All the dampers, of course will utilize either motor drive or hydraulic power for operation. Seals on the dampers are particularly important. Metal seals utilize the pressure differential in the duct together with a built in spring action to help seat the thin flexible metal seals against the seats. Normally the seals are attached to the flaps and the seat area is built into the fixed frame. Multi blade dampers have a greater seal footage than other types, and only part of the seal is helped by the differential pressure. Other parts of the seal must seal against the differential pressure. Because of this multi blade dampers tend to have a higher leakage rate than the flap or guillotine type dampers. Sealing efficiencies of 99.95% should be specified for metal in seals without air sealing. Dual seals are often provided to obtain a greater sealing efficiency. Air seals use a fan to create a small positive air pressure to help seal the metal seals. Efficiencies of 100% should be specified for the seals if air sealing is utilized. Several factors must be considered when selecting by dampers the value of thermal losses, safety considerations cost and operation and maintenance costs.
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Normally the diverter is preferred for the GT by- application, however the actual losses for each particular application must first be considered. Heat losses through the damper is an extremely important consideration and insulation of the damper or louver to minimize the heat loss is an important consideration. Heat losses will vary departing on the insulation material used, type of damper and temperature of exhaust gas. Manufacturers should be asked to specify the minimum heat loss through the insulated dampers they manufacture. The particular design and type of damper selected will vary with the application. For example, if air sealing is used. The cost of supplying the constant air supply must be considered and compared to the additional sealing savings. By Stack The height of stack, if not restricted by local regulations, will be based on economic considerations of the stack construction costs compared to the value of the gain in stack draft. Stack construction cost will increase with stack height, due to weight to be ed, increased wind loads, increased
material costs, and increased
handling cost and maintenance costs. The gain in stack draft will be the theoretical stack draft gain less the stack gas pressure drop and its value will be obtained from the value of the increased electrical output from the GTC due to less back- pressure. A theoretical stack draft effect will be produced for each meter of stack height due to the difference in the weight of the external air at atmospheric temperature.
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Stack gas pressure drop will be the friction loss plus the stack exit loss. Stack diameter will require selection prior to the determination of the pressure drop. In selecting the stack diameter a velocity between 2,000 to 5,000 ft/min should be assumed and pressure drops determined accordingly. In certain circumstances discharging the gasses at a height sufficient to disperse and dilute the exhaust may be a consideration. The wind load on a stack is a major consideration, and a high stack may often require a large diameter or other special provisions at the base for stability. The by stack construction could be of fabricated carbon steel [outer shell] with internal insulation and stainless steel liner. Otherwise it could be made of stainless steel with external insulation and aluminum/GT cladding. Differential expansion for the complete system is taken care by provision of an Expansion t, normally of flexible [fabric] type. In some design, exhaust side silencer is located in the by stack upstream of the damper. Pinch Point and Approach Temperature These are two factors which affect the cost and effectiveness of WHRB. Pinch point is the differences between the gas temperature leaving the evaporator section of the system and the saturation temperature corresponding to the steam pressure in that section. Approach temperature is the difference between the saturation temperature of the fluid and the inlet temperature of the fluid . Figure-17 makes the definition more clear. These two factors and the desire on the part of the engineer to keep them as low as practical cause more concern to the system designer than any other factors in the design of WHRB.
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As a general rule, lowering the pinch point results in an increase in the total heat recovered in that section. However, lowering the pinch point will require more heat exchange surface and a subsequent increase in cost and gas side draft loss. Multiple computations are required to select the most economical optimum. Another possible penalty which must be evaluated in lowering . then pinch point excessively is unstable operation. The very low design pinch point may indicate more heat recovery but if all other factors are not realized, it may result in poorer performance when steam production is critical. Lowering the approach temperature can result in more steam production at that pressure level as with a lower pinch point, this can only be realized at a higher cost and same increase in draft loss. Higher approach temperature would result in design stability. The higher approach temperature would increase the surface in the evaporator section and assure a higher level of stability. Al low load the gas turbine exhaust temperature will help drop but mass flow of the gas may not drop signification depended on type of gas turbine and intent vane range. The quantity of steam generates in the evaporator will drop. The economizer, will see nearly the constant gas temperature. Conditions but will have to heat a reduced fed water flow. The reduction in mass flow of the gas is not great as the reduction in feed water flow. This could cause steaming and water hammer problem in the economizer.
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Path of Flow Fig. 17 In an economiser section a higher approach temperature will reduce the probability of steaming at lower loads or during. Start up a design feature that could prevent this problem of seaming at lower load is to located the feed regulating station on the downstream of the economizer rather then the traditional location upstream of the economizer . This will have the effect of maintaining an economizer
pressure higher then evaporator steam drum
pressure particularly during the economizer low flow situations when the value would be party closed with a corresponding high pressure drop.
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Types of Circulation Circulation of the water through the evaporator tubes can be forced utilizing circulation pumps or can be natural utilizing gravity and density difference between water and steam to obtain circulation. Both types of WHRB’ s are common with combined cycle power plants. Natural circulation boilers must have vertical or inclined to the vertical tubing and will normally have higher initial costs. Forced circulation WHRB’ s using a water/steam drum normally use horizontal tubing pump pressure to circulate the water/ steam through the evaporator tubes. The advantages with horizontal tube design is the lower cost of the tube bundles and potential for lower cost of section renewal if request in later years. However, due to horizontal tube design used with this type of boilers there will be lack of tolerance. Pressure drops through the horizontal evaporator tube banks must be balanced and often require restriction on some of the tubing in order to achieve this balance. This increases the pressure drop through the system and water tend to separate to separate easily leading to further problem and minimum velocity through the tubes must be maintained to avoid this separation. If separation does occur, the bottom of the tube will have a steam surface and heat transfer through the top of the tube will not be as effective as heat transfer through the bottom of the tube. The top of the tube will become hotter, setting up thermal stresses in the tube leading to sagging and in some cases out right tube failure due to overheating the tube. Thermal inertia of the forced circulation WHRB is less then the natural circulation WHRB which facilitates faster loading and unloading of the WHRB. The table below gives the summary of advantages and disadvantages associated with each type of design.
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Summary –Forced circulation Vs National circulation Function
forced
Circulation
Position Adv.
Natural
Circulation
Dis.
Position
Adv. Dis.
Circulation pump purch and inst. Costs REQ’D
X
NOT REQ’D
X
Circulation pump operating costs
REQ’D
X
NOT REQ’D
X
Balanced tube side press drop
REQ’D
X
NOT REQ’D
X
Potential for tube failure
HIGH
X
LOW
X
Potential for steam/ water separation HIGH
X
LOW
X
Initial cost
LOWER
X
HIGHER
X
Replacement cost
LOWER
X
HIGHER
X
Tolerance to changed conditions
LOWER
X
HIGHER
X
Sensitivity to adverse conditions
HIGHER
X
LOWER
X
Faster startup and shutdown
FASTER
X
SLOWER
X
(Less thermal inertia)
Unfired or fired Whrb As stated gas turbine at high excess air. Hence, the exhaust gases have very high oxygen content of the order of 16 to 20% which is sufficient to if any fuel firing desired in the WHRB or its duct. A WHRB designed to recover only the heat available in the exhaust gases is termed as unfired WHRB. Where are heat in the exhaust gases is supplemented
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by firing of additional fuel in the WHRB duct, the WHRB is termed as a fired WHRB. It must be recognized that the overall combined cycle plant efficiency will drop when duct firing due to the fact that Rankine cycle efficiency is lower than then combined cycle efficiency especially when the steam is used in a relatively low pressure induction type steam turbine. However the cost of installing a duct burner is extremely small and other equipment costs are nonexistent. Ducts burning can be advantageously used in combined cycle units where a need exists for peaking capacity. For evaluation of duct burner firing for peaking in a combined cycle value of additional KW produced by the steam turbine should be compared to the cost of generating the steam. Costs of additional steam turbine generator capacity should be added, if applicable. Choice on Condensates Pre-heater Decision about adding a condensate pre-heater is dependent on the being available in the back end of the WHRB,the requirement for heat to be added to the condensate return and economic justification. In most HRSG’s the back end heat is usually available and justification for the heater is based on need due to the temperature of the return condensate and economic justification. Economic justification for a condensate pre-heater should evaluate and compare all the costs associated with the installation of a condensate pre-heater against the value of the steam saved by the utilization of the condensate preheater. The steam saved by utilization of a condensate preheater will be the difference between the steam used by deaerator operating with the condensate preheater and without the condensate preheater. Without the condensate pre-heater additional deaerator steam would be used to heat the incoming condensate (feet water) to the saturation temperature required for dearerator operation.
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Condensate pre-heaters are subject to similar low load problems as experienced by the economizer. The problem is aggravated in the condensate pre-heater, however by a temperature rise in the exhaust gas at low load as the economizer will use less heat front the gas with the reduced
feed water flow. The
condensate pre-heater will therefore be subject to hotter gas together with reduced condensate flow which can cause severe steaming and steam hammer in the heater. In many cases the chosen solution to the condensate pre-heater low load problems is a planned by- of the condensate preheater during the infrequent low load operations permitting the condensate preheater torn day. Alternate solutions to this problem, dependent on the excepted frequency of low load operation would include running a full flow feed through the pre-heater at all times and dumping the surplus heated water to the condenser.
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9. Gas Turbine Generator Synchronous Generator The air cooled generator Type WY 18 L is of simple but robust construction. The generator and the gas turbine 13D are placed on a common and plain concrete foundation, with the same center line level for both the gas turbine and the generator rotor. Therefore, the space below the active part could be used to house the four cooler sections. The outline drawing shows the principle design and the main dimensions. Generator Design The generator is built in compliance with the latest standards of the IEC, ANSI and VDE, and is designed to operate continuously at rated load but also it withstand a wide range of abnormal operating conditions, including short circuits. Stator casing and Iron core The horizontally spilt stator casing of the active part consists of two welded sheet steel constructions, which are screwed together. Inside the casing sheet, steel ribs are arranged as guide plates for optimum cooling air distribution to the active sections. The four identical air recooling elements inserted into the casing below the active part can be withdrawn horizontally. The low-loss core segments are punched from silicon alloyed electric sheets. They are carefully deburred and are given several coats of a heat resistant varnish which can withstand temperatures much higher than the normal operating temperature. By this manner, the individual segments are dependably
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insulated from each other, so that even after many years; operations no short circuits or even hot sports can occur. After stacking the core segments outside the casing, vibrations and inseased temperatures are applied when the core is compressed between heavy pressplates. The press-plates are made of low-loss non-magnetic material and press by special fingers the whole cross section of the core. The resulting core remain rigid over its whole length, especially the highly stressed end parts, even if subjected to varying load conditions or daily multiple starts. Distance profits are welded to thicket segments, so that in constant distances radial slots are formed for flowing through of the cooling air. The would core will be inserted as a ready unit into the casting and will be welded with the caper part by use of adjusting pieces.
OUTLINE DRAWING OF ALTERNATOR Figure-18
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Stator-Winding Bars and Installation of Bars The mechanical energy generated by the turbine is converted to electrical energy by the generator and appears in the stator winding in the form of current and voltage. The winding is capable of withstanding all thermal and mechanical stress arise from normal operating or from external faults, with a generous factory of safety. The stator winding consists of individual bars installed in rectangular slots distributed uniformly along the inside circumference of the stator core. Each winding bars consists of solid conductors, insulated from each other. The conductors are first wound with resin-impregnated glass fiber, then assembled together and pressed in heated modulus to form compact bars. During bar assembly, the conductor’s positions in the bar are transposed, or twisted, so that each conductor occupies each possible position once in the straight part of the bar. Thus, the same amount of current appears in each couch and no unnecessary losses occur. The bars so formed are called Roentgen bars, after their inventor. The main bar insulation is built up following the Micadur (R) system, which was developed by BBC and which has been successfully used for many years. In this system, the bars are continuously wound, in an automatic tape winding machine, with the required number of layers of mica-coated glass-fabric tape.
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STARTOR SLOT CROSSECTION Fig.-19 The straight part of each bar is pressed to size, the bars then are dried in an oven, then they are impregnated in vacuum, by a special epoxy resin, and hardened in an oven by means of circulating air. The finished insulation complies with the Insulation Class F specifications of the IEC, ANSI and VDE standards.
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ENDWINDING FIG-20 Terminals The winding connections and the terminals are connected by means of flexible copper strips. The flexible strips absorb possible relative motions between casing and winding which may be caused by different heating of casing and winding. The terminals are designed to withstand forces in case of a short circuit at he terminals. The insulation is designed fir the 1.5-fold test voltage of the generator, and consists of impregnated mica-glass strips. Both ends of the terminals have two parallel connecting plates, inside the generator for connecting of the flexible strips, and outside for the connection to the bus bars to the star point, respectively.
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Rotor Body The rotor balances the torque of the gas turbine. It leads the magnetic flux, and carries the field winding and the necessary parts for conducting the field current. The rotor body is a single forging of heat-treated alloy steel having very good mechanical and magnetic properties. After heat treatment, the rotor body is tested using the most up-to-date methods to ensure that the required properties have been obtained. As an example, the rotor is subject to ultrasonic testing over its complete length and cross-section. Therefore, a central bore for testing purposes is not required. Longitudinal slots are milled in the two winding zones of the rotor surface to carry the field winding. The different slot shapes in the pole zone and winding cause different moments of inertia relative to the direct and the quadrature axis. In order to achieve uniform bending of the rotating rotor relative to these two axes, transverse slots are milled in the pole zones. This acts to reduce the amplitude of the double-frequency flexural vibration in a smooth-running rotor. The driven shaft end of the rotor is provided with an integrally-forged coupling flange for connecting the turbine shaft. At both ends of the rotor are seats for the fans and also journals for the bearings, the ends of the rotor body are machined to take the retaining rings. The nondriven shaft ends has a central bore to take the rotor filed current lead, while radial holes are drilled for the lead studs which connect the rotor leads with the field winding.
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Rotor Winding The rotor winding is composed of hollow conductors which carry the excitation current required to produce the armature field. The conductors are made of drawn silver containing oxygen-free copper having high-grade properties. In the active region, the hollow conductors are insulated from each other and from the rotor iron by Nomex, a polyamide paper, fulfilling class F requirements.
ROTOR WINDING FIGURE-21 The rotor winding is directly cooled by air howing from both ends of the rotor through all the conductors. At all the air inlets and outlets, the creepage (leakage)
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paths between the bare copper and the rotor mass are rather long. This ensures goods dielectric strength of the winding even after long operation time. The filed current is conducted from the slip rings to the rotor windings by means of two half-round copper lead bars fitted in the shaft bore. At one end of the lead bars, connection is made to the slip rings by two leads studs and flexible-plate connectors. At the other end, connection is made to the rotor winding by similar lead-studs and flexible-plate connectors located under the retaining ring. At each end, one stud is provided for each polarity. The stud is made of flexible-plate connections compensate all mechanical and thermal stresses. Retaining Ring Retaining rings which carry the centrifugal forces of the rotor end windings are fitted at each end of the active part of the rotor. Each retaining ring has a cylindrical section and disc-shaped cap end. The retaining rings are shrunk onto the rotor ends and maintained in their axial position by means of a bayonet attachments. A special key prevent any movements of the rings in the tangential direction under short circuit conditions. A special feature is the insensibility of the used steels against stress corrosion. Cooling System The axial fans at both ends of the motor provide for two cooling circuits. The air which cools the rotor enters the end-coil space between the end bells and the shaft. In the vicinity of the slots it flows through slits into the hollow conductors, where the air flow is being divided. One flow path is through the hollow conductors in the slots, entering the air-gap at the middle of the rotor through
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radial ages in the windings and slot wedges. The other one leads through the hollow conductors in the end-coils, finally leaving the space under the endbells through slots n the rotor body and entering the air-gap.
AIR FLOW SCHEME OF THE ALTERNATOR FIG-22 Some of the air which cools the stator is led straight into the air-gap. Together with the cooling air from the rotor end-coils, it leaves this region through radial ages in the frame. From there the air returns via the coolers to the fans. The rest of the air cools the stator end-coils and flows through axial channels into the second frame chamber. From there it flows through the laminations towards the air-gap where it splits up again. One half flows back through further slots into the first frame chamber, the other half mixes with the air coming from the rotor at the middle of the machine. From there the air es through the laminations to the third frame chamber, the coolers and aging to the fans. Each of the cooling elements consists of a finned tube bundle and two water chambers. The materials for the coolers will be selected depending on the quality of the water. The coolers can be cleaned easily with brushes.
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Bearings The generator is equipped with two journal bearings. The bearing at the driven end is part of the immediate block of the gas turbine. The bearing on the nondriven end is insulated to prevent shaft currents. The rotor is earthed via earthing brushes. The bearings are connected to the lub and jack in oil systems of the turbine. Slipring Unit The equipment for conducting the excitation current (direct current) into the rotating rotor in order to produce the magnetic field in the rotor winding is located in the slipring unit at the non-driven end of the generator. It consists mainly of two slip rings shrunk onto the shaft (but insulated from it), a set of carbon brushes in with the slip rings and carried in replaceable holders, and the brushholder s. The actual transfer of current takes place between the brush surfaces and the slipring surfaces. The constant pressure required between the surfaces is provided by rolled-strip springs on the brushes. The heat generated in the slip rings and brush gear is removed by a dual exhaust fan which draws cooling air through each side of the unit. During operation, the brush gear can be inspected through safety-glass windows provided in the casing. Temperature Monitoring Temperature sensors are provided in the stator winding and the cooling air circuit. The connecting wires to the sensors are brought out through bushings to the instruments terminal box.
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10. Excitation System General Description The synchronous generator is shunt excited. The power required for the excitation is taken from the generator terminals and fed to the field winding through the excitation transformer and the thyristor-controlled rectifier units. The thyristor-controlled by the voltage regulator through the grid control unit and pulse amplifier stages, so that the generator voltage is held almost constant from no load to rated load. A fully-controlled 3ph AC bridge circuit is used for the rectifier circuitry. This permits reversing of the polarity of the excitation voltage during rectifier operations ensuring rapid control during build-up or discharge of the field. A three phase AC auxiliary gird is required for field flashing for the generator excitation which is supplied via the transformer and the field flashing circuit breaker until the generator is excited to such an extent that the excitation equipment can operate automatically. When a fault occurs, the generator can be deenergized quickly with the field circuit breaker and the voltage dependent resistor. The short circuit device protects the excitation equipment and the thyristors from excess voltage by short circuit the field windings through the limiting resistor. Description of Operation Automatic Voltage regulator (A.V.R) Regulator rack
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The actual value and set point are compared in the Unitrol Section and the difference between the two is amplified in the control amplifier. The output voltage of the amplifier is limited to a value set by an internal potentiometer. The generator is controlled as follows below the cut-off frequency F. the control voltage is proportional to the frequency, and above this the voltage is held constant to a set value (see fig. 23)
Figure-23 The cut-off frequency can be adjusted by an internal potentiometer. The set point adjustment is made using a reference value transmitter Unitrol, which is installed in the regulatory rack. Adjusting elements allow the level and range of the voltage regulator to be selected (see fig. 24)
Figure 24
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Adjustable compound resistances are connected in the circuit to ensure the required reactive load distribution during parallel operation with other generators. The resistors allow the regulator drop to be changed. AP. I.D. filter and a differential amplifier are used for stabilizing, which permits optimum matching of the controlled generator. A power pack supplies a stabilized voltage to the set point potentiometers. Grid control unit automatic channel and manual channel the output signal of the regulator rack or manual control potentiometer is fed to the input of the grid control unit Unitrol together with a 3ph A.C. voltage from the filter Unitrol which is synchronous with and has the same phase as the supply for the power rectifier. From this, 6 phase voltages are formed, which are compared with the regulator or hand control signal (see Fig. 25). The differences is formed into square wave form voltages by an operational amplifier with controlled limits. Six trigger pulses are thus formed at 60 (electrical) intervals by the change in polarity.
Figure-25 The trigger pulses can be shifted depending on the size of the AC and DC voltage. For safety reasons, adjustable limits are provided for full rectifier or DCAC converter operation. The weak pulses of the grid control units 1001c-P are
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amplified in the pulse stages of Unitrol. Control relays in the unit allow the pulses to be suppressed, i.e. blocked, or to be reversed in DC-AC converter operation. Pulse end stages The output pulse of the grid control unit are fed to the parallel-connected pulse end stages of Unitrol. These end stages are designed to control up to a maximum of 8 thyristors in parallel per branch by means of the appropriate pulse gradient and power level, i.e. a 3ph AC bridge with up to 48 thyristors can be controlled. The pulse of every end stage can be suppressed by means of a blocking relay. The corresponding thyristors therefore remain isolated. Current rectifiers Three current rectifiers unit are fitted in section of the lead rectifier cabinet. The thyristors are directly fitted to the heat sinks. The current strips are connected to the DC AC busbars, the auxiliary and control cables are lead to the appropriate terminal blocks. Each unit is ventilated by its own fan. The fans are supplied through the fuses, which are directly connected to the secondary voltage of the excitation transformer, the transformer, and the protection circuit breaker. The air flow from the fans is monitored by a wind vane relay. Regulator limit rack Limit criteria All limits act equally in that they influence the voltage control and alter the excitation accordingly. The criteria are :
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•
Too large a stator current
•
Too large rotor current
•
Too larger a load angle for stability
In the over-excited range, the rotor or stator current limiter reduces the excitation and in the under-excited range the load angle limiter increases the excitation. The rotor or stator current limiter acts through a time delay and integrator to maintain both good regulation in the event of load changes, and also the dynamic behavior of the generator when intermittent grid faults occur. This allows the protective system to operate selectively. Load angle limiter Unitrol The load angle, the angle between the generator longitudinal axis the grid voltage vector, can be determined by forming a vector diagram of the generator and the external reactance. The generator voltage and current are fed to the input of the load angle control limiter. The quardrature reactance xq of the generator is the determining parameter, and can be adjusted between 0.5 and 3.0 p.u. While the external; reactance xe can be adjusted between 0 and 3.0 p.u. by means of potentiometer: The two voltages so formed enclose an angle between them which corresponds to the load angle. This angle is transformed in a transducer to a DC voltage signal. This angle is transformed in a transducer to a DC voltage signal. This signal is compared to an adjustable set point. When the measure value exceeds the set point, the limiter comes into operation and acts upon the voltage regulator. The excitation is then increased until the load angle corresponds to the adjusted set point. Despite increasing generator voltage, the voltage regulator remains ineffective.
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Rotor current limiter Unitrol This limiter protects the rotor winding from thermal overload. The excitation current is measure on the AC side at the supply to the controlled rectifier units, and converted into a current proportional DC voltage within the limiter. This is then compared with an adjustable set point, amplified and fed to the input of the voltage regulator. The limiter acts with a time delay, so that the dynamic control is not affected in cases of fault. Stator current limiter Unitrol The stator current is converted by a transducer into an equivalent DC voltage which is polarized positively or negatively according to the over-or under-excited operating condition. This forms the actual value for the regular in which the comparison with the adjustable set point is carried out the difference signal first amplified. The output of the amplifier for limiting the stator current influences the regulation of the voltage in such a way as to reduce he excitation. Temperature-dependent current limiter The temperature-dependent current limiter is used to protect the generator winding against thermal overload, should the ambient air temperature exceed 40 deg. C. The current limiter, by an appropriate reduction in excitation current, prevents the windings from overheating. On gas turbine systems, on which the limiter is used, the active power of the turbine decreases with increasing air intake temperature, and the limits given by the generator are always in the specified range. Thus any effect on the active power nominal value is not necessary. The current limiter includes supervision, which incase of irregularities such as nominal/actual deviations, gives a command signal to shut down the turboset.
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Basically the system consists of two units : (a)
Nominal value formation and nominal/actual value supervision of the stator current.
(b)
Nominal value formation and nominal/actual value supervision of rotor current.
Excitation the system consists of two units: Excitation with field flashing The supply for power rectifier is from the 3ph excitation transformer which receives its power from the generator. Before excitation takes place at rated speed, there is only the residual voltage of the generator available. This is insufficient for starting the excitation equipment. The thyristors of the controller remain blocked. An auxiliary voltage of 414 V, 50 Hz is available for field flashing, which supplies sufficient excitation current through the matching transformer and diode bridge for 35% of the generator voltage. The on command is given by the start-up automatic, or from the control room, via the decontic control. The command is only given when the criterion “ready for connection” is fulfilled. The field breaker is switched on. At 70% generator voltage the voltage relay switches the field flashing breaker off. Switching off must take place within 20s, otherwise the time supervision switches the field breaker off. The excitation process will be interrupted. De-excitation De-excitation can follow through the following criteria :
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(a)
By the command Excitation off via the or from the automation or from the control room.
(b)
By a fault within the excitation equipment.
(c)
By the generator protection.
The field breaker is immediately opened and the field discharges through the voltage dependent resistor. The excitation circuit must not be broken as long as current is flowing, since this can cause high voltages which would damage the thyristors or the rotor winding. When the filed breaker is switched off, a switches the resistors in, parallel with the rotor winding. A second breaks the supply from the rectifier. The resistor is sized so that the over voltage of the filed breaker. The resistor is designed as nonlinear and voltage-dependent, whose resistance decreases with increasing voltages. In this way the deenergizing time constant becomes shorter. The blocking voltage of the thyristors lines above this cut-off overvoltage. The overvoltage short-circuit device, parallel to the rotor winding, offers additional protection as undesired break in the rotor circuit occur during operation. When the short circuit device acts, the limiting resistor is switched in parallel to the rotor winding. It acts between the allowable switching voltage of the field breaker and the cut-off overvoltage, so that it is only activated when a fault occurs. Power supply Automatic Voltage Regulatory (AVR) Automatic voltage regulator and manual voltage control have two independent power supply units each, one AC-DC converter and one DC-AC converter. The AC-DC converters Unitrol are connected to the secondary side of the excitation transformer via transformer. The DC-AC converters Unitrol are fed from the 220
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V DC distribution AC-DC converter Unitrol connected to the 220 DC distribution serves for power supply for the protection. Control Voltage The 220 V DC supply voltage in the control cabinet is provided by a 220 V DC distribution Systems. The control voltage is monitored by, which gives a signal the voltage is low. Every function has its own control circuit, so that when failure occurs, only a part of the system is out of operation. When a failure occurs in the circuit it must be decided whether the excitation equipment can remain in operation or not, according to the importance of the faulty circuit. The faulty control circuit must be brought back into operation again as soon as possible. The protection switches are monitored by the built-in auxiliary s. When a protection switch cut out, the alarm “Protection switches Voltage” is illuminated in the control cabinet and in the electrical control room the excitation warning signal is given. Normally only a small amount of power is taken from the battery for the controls. Switching process require approx. 10 KW for a short period. Method of operation of the complete equipment When the generator is stationary the circuit diagram shows that field breaker is open. The generator must be brought up to rated speed before it can be put into operation. Above 95% rated speed, the excitation process can be started by switching on the field breaker. The generator is excited up to 35% rated voltage by the filed flashing. Above this value the electronics function reliably, and the regulator takes over the control of the generator voltage. At 70% generator voltage the field flashing is switches off by the field flashing breaker. The set point is compared to the actual value. During the excitation process the generator
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voltage is low. The control amplifier Unitrol 2010 gives a higher signal to the grid control unit Unitrol. At first a celling voltage of approximate. 40% of max. excitation voltage at rated generator voltage is present. The generator voltage and thereby the exciter voltage continually rise. As soon as the output voltage of the voltage transformer reaches the adjusted set point, the control pulses to the thyristors are pushed back until set point and actual generator voltage are equal. When manual control has been selected, then the firing angle of the thyristors can be adjusted with the hand potentiometer, which corresponds to a certain exciter voltage. The actual value of the generator voltage has no influence on the firing angle-in contrast to the automatic control-since with manual control, there is no closed control circuit. When the potentiometer is changed as a result. The changeover to manual channel can be made at any time. However the pulse difference between the output of the grid control unit automatic channel and the grid control unit manual channel must be zero. The pulse difference can be read off on the comparative voltmeter via the measurement Unitrol 0040, comparator Unitrol and measuring converter. The comparison is made in the control room via the contractor by adjusting the potentiometer. When zero voltages is shown on the comparative voltmeter, a smooth changeover can be made to manual control. Protection and Monitoring equipment The excitation equipment is monitored and protected by various protective relays, motor protection relays, and various monitoring equipment. The function description=MKC01…..MKC05 gives information about the monitoring equipment provided, and the result when a fault occurs.
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Alarms The signals for the excitation equipment are shown on the circuit diagrams. They are illuminated in bottom for acknowledgment, lamp test, functional test and lamp reset. Protection of the excitation equipment and the excitation transformer Electronic inverse time overcurrent relay type IKC 062 delay trip (change-over to manual), instantaneous trip. Electronic short circuit relay type Unitrol. Temperature protection with PTC-Resistors fitted to the excitation transformer.
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11. Gas Plant Operation Gas Turbine, WHRB, Steam Turbine Starting Modes Basic conditions for plant operation are as follows:•
Start up or shut down of G/T, WHRB and S/T of each module is performed separately from the other module (except for S/T gland steam back-up and heating steam back up systems).
•
Start up/shut down mode is selected freely form among those mentioned.
The start up/shut down procedure for WHRB and S/T here mainly describes operating procedure for G/T by- damper, WHRB inlet damper and remote operated valve necessary for start up and shut down from G/T exhaust gas ission to WHR till rated load operation of S/T. For detail operating procedure for G/T WHRB, S/T auxiliaries and remote operated valve following procedures are followed. Start up Mode The start up mode of G/T, S/T and WHRB shall be selected from among the following as a rule through various other start up modes are conceivable according to power demand and operating principle. Outline of each start up •
Normal start up mode This mode is two unit (2G/T’s + 2WHRB’s +1 S/T) combined cycle start up mode which, after starting up one each of G/T and WHRB, starts up the
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other G/T and WHRB and brings the output of G/T and S/T the target output of the module. In this mode, S/T is loaded with one each of G/T and WHRB in operation, and after the intial load is achieved, the pressure control of HP/LP by- valves of both WHRB’S is changed to common pressure control and then S/T is loaded up. The timing of starting up the other G/T and WHRB is left to the discretion of the operator, and S/T is kept stand by at the initial load until the pressure control of both WHRB’S is changed to the common pressure control. •
Rapid start up mode This is a mode of starting up both G/T’s and WHRB’S at the same time, changing the pressure control of HP/LP by- valves of both WHRB’S to the common pressure control, then loading up and brining G/T the output of and S/T to the target output of the module is achieved. This mode is used only for start up after right stop or hot start up (with vacuum). In other start up modes; in which the time from G/T start up to S/T loading is long, simultaneous startup is not made to reduce heat lose at start up.
•
G/T/WHRB additionally start up mode This is a mode of starting up the other G/T and WHRB one until combined cycle of G/T, WHRB and S/T is in operation. In this mode, the HP Steam pressure and temperature of the other G/T and WHRB are raised up those of G/T and WHRB in operation, and then the pressure control of HP/LP
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by- valves is changed to the common pressure control, and S/T is loaded up. •
Single G/T/WHRB start up mode This is a combined cycle start up mode of starting up S/T with only one each of G/T and WHRB in operation. The other G/T and WHRB remain stopped.
•
Individual pressure control Individual pressure control means pressure control made automatically by using HP/LP by- valves of both WHRB’s and detecting pressure before HP/LPCV so that HP/LP steam pressure of both WHRB’S will be the same and constant respectively.
•
Loading of S/T S/T loading rate after HP/LP by- are fully closed and HP/LPCV are fully opened is determined by the load change rate of G/T, namely S/T load change rate increases as G/T load change rate increase. (Except for G/T/WHRB additionally start up mode). S/T unloading rate too is determined by G/T load change rate under HP/LP by- valves are fully closed and HP/LPCV are fully opened.
STARTUP PROCEDURE •
Normal start up mode Cold start up
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The condition in which HP turbine rotor temperature is below 120 degC is called cold condition. In cold start up, the HP turbine rotor needs warming, which is performed to raise the rotor temperature above the transition temperature of the rotor material 120deg.C and prevent brittle fracture in high speed operation to achieve this heat sock is done by holding rotor speed at 2000 rpm during start up of the steam turbine. The time of heat soak varies with rotor temperature before steam ission. Heat soak time of 2 hours is given on the assumption that the rotor is at the ambient temperature. Heat soak is performed with one G/T in operation to reduce start up loss and the other G/T shall be stared up just before that soak is completed. At cold start up, WHRB outlet steam temperature must be below 430 C because of temperature limitation at steam ission to S/T. Therefore, WHRB inlet exhaust gas temperature must also be limited accordingly. WHRB and S/T shall be started up after G/T load is held at a load (about 30%) at which boiler outlet steam temperature can be kept below 4300 C. •
Start up conditions Turbine vacuum
Without vacuum
Gland steam supply conditions
2.5t/h x 2000 C x 3kg/cm2 above
Steam ission conditions
60t/h 3000 C-4300 C x 39kg/cm2
WHRB inlet damper initial opening
100%
Steam evaporation at G/T 30% load
25t/h
SH outlet below valve capacity
35t/h x 29kg/cm2
Time required for pressure
About 40 minutes
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Rise (0-29 kg/cm2)
(at G/T 30% load)
WHRB temperature rise rate
4002 C/h
S/T speed rate
300 rpm/min
S/T load up rate
•
(Before HP by- valve full close)
1%/min.
(After HP by- valve full close)
2.5%/min.
Start up procedure Preparation for start up -
Start up the following common equipment which is necessary for start up of G/T, S/T and WHRB. a)
Raw water pump system (clarifier, pretreatment plant and water treatment plant)
b)
DM water transfer pump system.
c)
Compressed air system.
d)
Compressed air system.
-
Fill up condenser and deaerator with water.
-
Start up circulating water system CWP Condenser-cooling tower CWP
-
Start up chemical injection system.
-
Start up seal air fan.
-
Make sure that all preparations for start up have been completed.
WHRB start up
-
Start up HP BFP and LP BFP
-
All manual operation
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-
Start up boiler circulating pump
-
Make sure that by- damper is full open and WHRB inlet damper is full shut.
-
Start up G/T after all preparation for G/T start up have been complete.
-
Start turning S/T
-
Open HP/LP drum air vent valves and SH outlet air vent valves.
-
When WHRB inlet damper is open, automatically open HP steam pipe drain valve and LP steam pipe below valve.
-
After synchronizing G/T, fully open WHRB inlet damper. By- pre-heater and fully open HP/LP economizer re-circulation value.
-
Start up G/T up to 30% load and hold that load.
-
When HP steam pressure reaches 2 kg/cm2, automatically close HP/LP drum air vent valves and SH outlet air vent valve.
-
When HP steam pressure reaches 3kg/cm2 x 2000 C. Open the Hp steam stop valve and HP turbine inlet pipe drain valve to start warming of HP turbine inlet pipe.
-
After warming is over, start up gland condenser exhaust fan, and open gland steam stop valve to start gland steam supply
-
Start up vacuum pump after gland steam has been established.
-
Until HP/LP by- valves are put into operation, open SH outlet blow valve, HP steam pipe drain valve and LP steam pipe blow valve, and steam is blown into the WHRB blow down tank and S/V flush pipe.
-
At that time, opening of SH outlet blow valve and LP steam blow valve is automatically adjusted according to HP/LP steam pressure at start up of WHRB.
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-
Opening HP steam pipe drain valve is automatically adjusted according to HP steam temperature at start up of WHRB.
-
After vacuum has been established (550 mm Hg), open turbine drain valves and open HP/LP by- valve inlet stop valves to put HP/LP turbine by system is individual pressure control.
-
Open HP by- valve to start individual pressure control so that HP steam pressure will be constant of 29 kg/cm2.
-
Open LP by- valve to start individual pressure constant so that LP steam pressure will be constant of 5kg/cm2.
-
When HP/LP by- valves are put in operation, fully close by damper.
-
After HP/LP by- valves have been put in operation and SH outlet blow valve HP steam pipe drain valve and LP steam pipe blow valve.
-
When LP by- valve is put in operation open LP steam stop valve and LP turbine inlet pipe valve and LP steam pipe blow valve.
-
When LP by- valve is put in operation open LP steam stop valve and LP turbine inlet pipe drain valve to start warming of LP turbine inlet pipe.
-
After warming is over, open deaerator heating steam supply valve to start heating of deaerator.
-
At the same time, open condenser pre-heater re-circulating valve by the specified degree (at gas firing only)
-
When boiler evaporation is established and drum level control is switched to 3-factor control, it rate to pre-heater and fully close HP/LP economizer re-circulation valves.
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•
S/T start up
-
Make sure that all preparations for S/T start up have been completed
-
Make sure of turbine steam ission conditions 29kg/cm2 x 3000 C (below 4300 C).
-
After steam ission conditions have been completed, HP/LP turbine inlet pipe drain valves are fully closed.
-
Start steam ission to turbine (DEH on)
-
Start accelerating. Accelerate at 300 rpm/min. up to target of 2000 rpm. Make rub checks as necessary during acceleration.
-
Make heat soak at 2000 rpm for 2 hours. (Heat stocks time depends on mismatch temperature difference of turbine)
-
Startup the other G/T just before heat sock is completed and hold it at 30% load, increase steam temperature and pressure of the other WHRB and when HP/LP steam pressure reached 27/cm2 and 4.5/cm2 respectively, put HP/LP by- valves in operation. (To start up the other G/T and WHRB, follow the startup procedure described earlier).
-
After heating soak, accelerate up to 3000 rpm/min.
-
Check that preparations of generator have been completed.
-
Loading.
-
Maintain 5% initial load for 10 minutes.
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•
Load up
-
Put HPCV and LPCV (Governor valve) in upstream pressure control and sequentially close HP/LP by- valves so that the S/T load up rate will be 1% min. at this time, HP/LPCV automatically following up in pressure control. As a result, steam goes to S/T.
-
When S/T load reaches 20% full close all turbine drain valves.
-
When HP by- valve is fully closed, shift HP by- valve to backup mode.
-
When LP by- valve is fully closed, shift LP by- valve to back up mode.
-
Fully close HP main steam stop valves for other WHRB adjusting temperature and pressure differences.
-
Fully close LP main steam stop valve for other WHRB, adjusting temperature and pressure differences, fully close LP turbine by valve according to the predetermined programme and then switch to back-up control
-
When HPLP by- valves are fully closed, adjust G/T output to achieve target plant output.
-
At this time, adjust G/T load up rate so that the S/T load up rate will be 2.5% min. (GT load up rate will be approx. 2.9% min.)
•
Warm Startup The condition in which HP turbine rotor temperature is between 1200 C and 1400 C is called warm condition.
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Because of thermal stress, heat soak from 0 to 40 minutes is required, depending on the steam turbine rotor temperature. Startup procedure is basically the same as is cold start up. When making a weekend shut down of the plant, if a normal shutdown is made, the steam turbine rotor can be held at a relatively high temperature, and so heat sock time can be shortened. If, rotor metal temperature is above 300 C, and heat sock is not required. Because of temperature limitation at steam ission to S/T, WHRB outlet steam temperature must be below 4300C. Therefore hold G/T load at a load (about 30%) at which WHRB outlet stream temperature can be kept below 4300 C, and then start up WHRB and S/T. •
Start up conditions Turbine vaccum
Without vacuum
Gland steam supply conditions
2.5t/h x 2000Cx.3kg/cm2 above 75t/hx3000C-4300 Cx29kg/cm2 (10% initial load)
WHRB inlet damper initial opening
50%
Steam evaporation at G/T 30% load.
25t/h/
SH outlet blow valve capacity
35t/h x 20 kg/cm
Time required for pressure rise at
About 25 minutes (at
G/T 30% load.
G/T 30% load)
WHRB temperature rise rate
4000 C/hr
S/T speed rate
300 rpm/min.
S/T load up rate (Before HP by-
2.5% min.
Valve full close)
3.0% min. (Corresponding to the C/T load up rate of 30%/min.)
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•
Star up Procedure
-
Preparation for Start up •
Start up the following common equipment which is necessary for start up of GT, S/T and WHRB.
•
a)
Service water pump system or clarified water pump system.
b)
Compressed air sytem.
Start up circulating water system-CWP-Condenser-Cooling Tower CWP)
•
•
Startup cooling water and condensate systems.
•
Start up chemical injection systems.
•
Start up seal air fan.
•
Make sure that all preparations for start up have been completed.
WHRB start up −
Start up HP BFP and LP BFP.
−
Fill up HP/LP DRUM with water up to normal water level.
−
Start up boiler circulating pump.
−
Keep WHRB inlet damper fully shut
−
G/T start up have been completed.
−
Start turning S/T
−
After G/T is loaded, open WHRB inlet damper upto 30% opening.
Note : *Manual operation
−
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Start up G/T up to 30% load and hold that load.
113
−
Start drain blow Opening HP steam pipe drain valve and LP steam pipe blow valve.
−
When HP steam pressure reached 3kg/cm2 x 2200C. Open the HP turbine inlet pipe drain valve to start warming of HP turbine inlet pipe.
−
After warming is over start gland steam supply.
−
Start up vacuum pump after gland steam has been established.
−
Open SH outlet blow valve, HP steam pipe drain valve and LP steam pipe blow
−
Valve and steam is blown into the WHRB blow tank down tank and S/T flush pipe.
−
At that time, opening of SH outlet blow valve and LP steam blow valve is automatically adjusted according to HP/LP steam pressure at start up of WHRB.
−
HP steam pipe drain valve is automatically adjusted according to HP/LP steam temp. at start up of WHRB.
−
After vacuum has been established (550 mm Hg.), open turbine drain valves and open HP/LP by- valves inlet stop valves to put HP/LP turbine by- system in individual measure control mode.
−
Open HP by- valve to start individual pressure control so that HP steam pressure will be constant of 29kg/cm2g.
-
When HP/LP by- valves are put in operation, full open WHRB inlet damper and full close by- damper.
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-
After HP/LP by- valves have been put in operation, shut SH outlet blow valve, HP steam pipe drain valve and LP steam pipe blow valve.
-
After HP/LP by- valves have been put in operation, shut SH outlet blow valve, LP steam pipe blow valve.
-
When LP by- valve is put in operation, open LP steam stop valve and LP turbine inlet pipe drain valve to start warming of LP turbine inlet pipe.
-
After warming is over, open deaerator heating steam supply valve to start heating of deaerator. (Deaerator set pressure at 0.3 kg/cm2g)
−
At the same time, open condenser pre-heater re-circulating valve by the specified degree (at gas firming only).
•
S/T Start up
−
Make sure that all preparations for S/T start up have been completed.
−
Make sure of turbine steam ission conditions 29 kg/cm2g x 3000 C (below 43000C).
−
After steam ission conditions have been completed HP/LP turbine inlet pipe drain valves are fully closed.
−
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Start steam ission to turbine (DEH on).
115
−
Start accelerating. Accelerate at 300 rpm/min. upto target speed of 3000 rpm. Make rub checks as necessary during acceleration.
−
Start up the other G/T just before steam ission is completed, and hold it at 30% load, increase steam temperature and pressure of the other WHRB and when HP/LP by- valves in operation.
−
Loading.
−
Maintain 10% initial load for 10 minutes.
−
After initial load operation is over, open HP/LP steam stop valve of the other WHRB, and change pressure of the other WHRB and when HP/LP by- valves in operation.
−
Loading.
−
Maintain 10% initial load for 10 minutes.
−
After initial load operation is over, open HP/LP steam stop valve of the other WHRB, and change pressure control of HP/LP by valves to common pressure control.
•
Load up
−
Put HPCV (governor valve) in upstream pressure control sequentially close HP/LP by valves so that the S/T load up rate will be 2.5% min. at this time, HP/LPCV automatically following up in pressure control. As a result, steam goes to S/T.
−
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When S/T load reaches 20% full close all turbine drain valves.
116
−
When LP by valves is fully closed, shift LP by- valve to back up mode.
−
When HP by valves is fully closed, shift HP by- valve to back up mode.
−
After HPCV is fully opened, restore deaerator set pressure to normal pressure of 3kg/cm2, and put condensate pre-heater recirculation control back to automatic mode. (at gas firing only)
•
Rapid start up mode (Hot start up) The condition in which The HP turbine rotor temperature is above 4000C is called hot condition. In a hot start up, the turbine rotor temperatures is high enough, so the operating procedure required is to load and speed up the turbine in a shortest possible time using high temperatures team to minimize the mismatch temperature. Therefore hold G/T load at 40 MW at which this condition can be satisfied, and then start up WHRB and S/T. It is assumed that vacuum of condenser is maintained inlet condition.
•
Startup conditions Turbine vacuum
With vacuum
Gland steam supply conditions
Gland steam being supplied from WHRB
by
remaining
steam
pressure. Steam ission conditions
110t/h x 4000C above x 20 kg/cm2 (15% initial load)
WHRB inlet damper initial opening © PMI,NTPC
100% 117
Steam evaporation at G/T 50% load
25t/h
SH outlet blow valve capacity
35t/h x 29 kg/cm2
Time required for pressure rise
About 10 minutes
(10-2kg/cm2)
(at G/T 50% load)
WHRB temperature rise rate
4000 C/hr.
S/T speed rise rate
300 rpm/min.
S/T load up rate
2.5%/min.
(Before HP by- valve full close) (After HP by- valve full close) •
Start up procedure −
Preparation for start up Startup the following common equipment which is necessary for start up of GT, S/G and WHRB. a)
Raw water pump system clarifier, pretreatment plant and water treatment plants.
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b)
Service water pump system or clarified water pump system.
c)
DM water transfer pump system.
d)
Comp ressed system
118
-
Start up chemical injection system.
-
Start up seal air fan.
-
Make sure that all preparations for start up have been completed.
−
WHRB startup
-
Start up HP BFP and LP BFP
-
Start up boiler circulating pump
-
Make sure the by- damper is full open and WHRB inlet damper is full shut.
-
Start up G/T After all preparations for G/T start up have been completed.
-
Make sure of S/T turning.
-
Open HP main steam stop valve and establish vacuum, supplying vacuum gland steam by remaining steam pressure of WHRB.
-
After G/T is loaded, open WHRB inlet damper upto 100% opening.
Note : * Manual Operation -
Start up G/T up to 40% load and hold that load.
-
At boiler start up fully open LP main steam stop valve and HP/LP by- stop valves in operation automatically by individual pressure control.
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-
At that time, opening of SH outlet blow valve and LP steam blow valve is automatically adjusted according to HP steam pressure at start up of WHRB.
-
Hp steam pipe drain valve main automatically adjusted according to HP steam temperature at start up of WHRB.
-
Open HP by- valve to start individual pressure control so that HP steam pressure will be constant of 2kg/cm2.
-
Open LP by- valve to start individual pressure control so that HP steam pressure will be constant of 5kg/cm2.
-
When HP/LP by- valves are put in operation, fully close by damper.
•
Open the turbine drain valve.
S/T start up -
Make sure that all preparations for S/T start up have been completed.
-
Make sure of turbine steam ission conditions 2 kg/cm2 x 4000C above.
-
After steam ission conditions have completed HP/LP turbine inlet pipe drain valves are fully closed.
-
Start steam ission in turbine.
-
Start accelerating. Accelerate at 300 rpm/min. upto target speed of 3000 rpm.
-
Check that preparations of generator have been completed.
-
Loading.
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•
-
Maintain 15% initial load.
-
After initial operation is over, full shut turbine drain valves.
Load up −
Put HPCV and LPCV (governor valve) in upstream pressure control and sequentially close HP/LP by- valves of both WHRB’S that the S/T load up rate will be SH/min. At this time HP/LP by- valves automatically following up in pressure control.
−
As a result steam goes to S/T.
−
When HP by valve is full shut, shift HP by- valve to backup mode.
−
When LP by valve is fully closed, shift LP by- valve to backup mode.
−
When HP/LP by- valves are fully closed, adjust G/T output to achieve target plant output.
−
At this time, G/T load up rate will be 3.6% min.
G/T, WHRB additionally start up mode While the plant is in partial load operation, the number of G/T’s in operation is changed in some cases to increase efficiency of plant operation. When starting up G/T and WHRB in such case, the start up procedure must be such that minimize the temperature difference between high pressure steam of the plant in operation and high pressure steam in G/T and WHRB to be started up.
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•
Start up conditions -
-
Conditions of G/T and WHRB in operation 1.
G/T load …………………………100%
2.
WHRB steam condition ………………..29kg/cm2x 4950C
3.
S/T load ……………….. about 40t
Condition of WHRB to be started up …………… Cold condition.
Startup procedure •
WHRB startup
-
Startup HP BFP and LP BFP.
-
Startup boiler circulating pump.
-
Make sure that by- damper is full open and WHRB inlet damper is full shut.
-
Startup G/T after all preparations for G/T startup has been completed.
-
Open HP/LP drum air vent valves and SH outlet air vent valve.
-
When WHRB inlet damper is open automatically open HP steam pipe drain valve LP steam pipe blow valve.
-
After G/ is loaded, open WHRB inlet damper up to full opening.
-
G/T is 40% load hold on.
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-
When HP steam pressure reaches 2kg/cm2g, automatically close HP/LP drum air vent valves, and SH outlet air vent valve.
-
Until HP/LP by- valves are put n operation, open SH outlet blow valve, HP steam pipe drain valve and LP steam pipe blow valve, and steam is blown not the WHRB blow down tank and ST flush pipe
-
At that time, opening of SH outlet blow valve and LP steam blow valve is automatically adjusted according to HP/LP steam pressure at startup of WHRB.
-
After HP steam temperature reaches above 4000C, open HP/LP by valve inlet stop valves to put HP/LP turbine by- system in individual pressure control mode.
-
Open HP by- valve to start individual pressure control so that HP steam pressure will be constant at 27kg/cm2.
-
Open LP by- valve to start individual pressure control so that LP steam pressure will be constant of 4.5kg/cm2.
-
When LP/HP by- valves are put in operation, fully close by damper.
-
After HP/LP by- valves have been put in operation, shut SH outlet flow valve,
-
HP steam pipe drain valve and LP steam pipe blow valve.
-
When HP by- valve is put in operation, equalize pressure and temperatures of both WHRB’s, open the HP steam stop valve.
-
When LP by- valve is put in operation, equalize pressure of both WHRB’s and open LP steam stop valve.
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•
Load up
−
Put LPCV (Governor valve) in upstream pressure control and sequentially close LP by- valve so that S/T load up rate will be 2.5% min.
−
When by- valve is fully closed, shift LP by- vale to backup made.
−
Put HPCV (governor valve) in upstream pressure control and sequentially close HP by- valve so that the S/T load up rate will be 2.5% min.
−
When HP by- valve is fully closed, shift HP by- valve to backup mode.
−
When HP/LP by- valves are fully closed, adjust G/T output to achieve target plant output.
WHRB SHUTDOWN PROCEDURE •
Normal shut down mode Normal shutdown is mode used as planned shut down of both WHRB’s. In this steam turbine has to the taken out first since we have to shutdown both WHRB’s. The following steps are adopted in WHRB shut down.
-
Start reducing GTs loads at the of 3.8% min.
-
Maintain GT load such that the HP steam tem. Is 4800C.
-
Put HP/LP turbine by- of both WHRB’s in individual mode.
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-
Bring down S/T load to 15% by opening the HP/LP by valve of both WHRB’s
-
Take LP steam of both WHRB’s in out of service.
-
Open steam turbine breaker and strip S/T manually.
-
Open the by- damper of both GTs.
-
When ST speed is less than 400 RPM. Close WHRB inlet damper of both WHRB’s.
-
Close the HP and LP Turbine by- stop valves of both WHRB’s after closing of the by- control valves.
-
Break condenser vacuum when condenser vacuum is less than100 mm Hg.
-
When the HP/LP drum pressure become less than2kg/cm2 open HP/LP drum air vents.
-
Stop one each CWP, LP & HP BFP.
-
Confirm that HP & LP economizer recalculation valve are opened and condenser preheater is by-ed.
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UNIT –I
UNIT-II
GT LOAD DOWN START
GT LOAD DOWN START
H P STEAM TEMP.
HP STEMA TEMP.
4800C
4800C
HOLD GT LOAD
HP HOLD GT LOAD
HP/LP TB BY
HP/LP TB BY
INDV. MODE
INDV. MODE
LP STEAM OUT OF SERVICE
HP STEAM OUT OF SERVICE
DEA HEATING STEAM VALVE CLOSED ST MAIN BRKAR OPEN AND ST TRIP
DEA HEATING STEAM VALVE CLOSED
GT BY DAMPER OPEN ST SPEED ^400RPM
GT BY DAMPER OPEN
WHRB INLET DAMPER CLOSE
WHRB INLET DAMPER CLOSE
HP/LP TURBINE BY CLOSE
HP/LP TURBINE CLOSE
BY
VALVE
CONDENSER VACUUM BREAK COND. VACCUUM <-100 mm of hg Gland steam Group off HP STOP V/V CLOSE
HP STOP V/V CLOSE
HP DRUM PR.<2kg/cm2
HP DRUM PR <2kg/cm2 open
open HP drum air vents
HP drum air vent
LP DRUM PR <2 kg/cm2 open
LP DRUM PR Less then <2 kg/cm2
LP drum air vent
LP drum air vents STOP ONE CWP ONE LP7 HP BFP
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WHRB out off shut down In this shut down only WHRB is taken out of service. ST will be in service with second
WHRB. Corresponding
GT
may be
stopped as
per
the
situations. The following steps are adopted in the shutdown. Reduce the GT load 30% of the corresponding WHRB which is to be taken out of service. -
Change HP and LP by- to individual mode.
-
Change LP turbine by- to main and open gradually (ST load down rate should be 2.5% min.
-
When LP boiler pressure, PB is less than LP turbine inlet pressure, close LP steam stop valve.
-
Change LP turbine by to individual mode.
-
Change HP turbine by to manual. Open gradually, ST load down rate 2.5%/min.
-
PB less than PT is when the shutdown boiler HP stream pressure is less than the turbine inlet HP steam pressure close HP stop valves.
-
Change HP turbine by to individual mode.
-
Open GT by damper.
-
After stabilizing HP/LP drum level close WHRB inlet damper.
-
GT shut down as per the situations.
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CONFIRM HP CV
CONFIRM LP CV
SET 29 KG/CM2
SET IN 5 KG/CM2
SHUT DOWN SIDE WHRB GT LOAD=30% CHANGE HP TB
CHANGE LP TB
BY TO IND. MODE
BY INDV. MODE CHANGE LP BY LOAD DOWN (ST) TO MANUAL & GRADUAL RATE 2.5 min. OPEN PB< PT PB: BOILER PRESSURE PT TB: INLET PRESSURE
LP STEAM STOP VALVE CHANGE HP TB BY INDIVIDUAL>MODE CHANGE HP TB BY TO MANUAL
ST load down rate
AND GRADUALLY OPEN
2.5%/min.
PB
PB BOILER PR PT TB INLET PR>
HP STEAM STOP VALVE CLOSE CHANGE HP TB BY IND MODE GT BY DAMPER OPEN STABILISE HP/LP DRUM LEVEL WHRB INLET DAMPER CLOSE GT SHUTDOWN IF REQUIRED
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MALFUNCTION AND REMEDIES ABNORMALITY
CAUSES
REMEDY
1. High Drum Level LP or HP drum
a. Malfunctioning of level controller
1. Change over the level Controller into manual Mode and drop the water Level.
b. Sudden increase in load
2. Re adjustable of the level Controller. 3. Closed the corresponding HP/LP Stop valve if both WHRBs are in service to avoid tripping of ST while avoid carry over of water to steam turbine.
2. Drop in drum level (LP drum or HP drum)
a. Malfunctioning of drum level controller
1. Change over the level Controller into manual Mode and normalise the drum level
b. Trouble of feed water pump
2. Start the standby feed water pump
c. In sufficient quantity of feed water
3. Re adjustable of level controller
d. Boiler tube leakage e. Sudden decrease of load 3. Too high temp. of flue gas
a. Too much of exhaust gas flow
1. Investigate the gas turbine
b. Too high temp. of feed water
2. Reduce the temp. of feed water
c. Fouling of heating surface
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SHUT DOWN PRODECURE -
Except in an emergency shut-down, load must be reduced gradually.
-
At approximately 15% of rated load, open all turbine drains.
-
When all load been removed, shut-down the unit by tripping either manually or remotely. This closes all the turbine HP steam and LP steam valves and disconnects the generator from the line.
-
Make sure that the auxiliary oil pumps starts when the bearing oil pressure drops below 0.85+0.05 kg/cm2.
-
Refer to the generator instruction book for information on the water supplied to the hydrogen coolers.
-
In order that the turning gear will be remotely engaged, be sure that the rotor speed is below the turning speed before the control switch is turned to the “START” position. This will also supply oil to the turning gear.
-
Shut-down the auxiliary oil pump and make sure that the turning gear oil pump starts when the bearing oil pressure drops below 0.75+0.05 kg/cm2.
-
Open the vacuum breaker when the turbine speed reaches about 400 rpm. Shut down the air removal equipment.
-
When the vacuum has reached zero shut-off sealing steam from the HP steam header to the gland steam control value and shut down the gland steam condenser exhauster.
-
Shut - down the condensate pumps.
-
Shut - down the circulating water pumps.
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-
Regulate water to the oil cooler to maintain the temp. of the oil leaving the coolers between 350 C and 380 C, if possible.
Loss of Electrical Load to the System In the event of a complete or partial loss of electrical load, the following procedure should be noted: -
Since the governor is in such a position that the no load speed is higher than 3,000 rpm, the speed will rise will suddenly at a rate proportional to the electrical load lost.
-
This sudden speed rise will cause the acceleration response feature of the governor momentarily to assume control of the turbine, and close the both HP and LP control valves rapidly.
-
With the both LP and HP control valves closed, the speed of the unit will decrease at a rate depending on the residual load left on the gtenerator.
-
If desired, the unit may now be resynchronized. If it is decided not to put the unit back on the line, the normal shut – down procedure outlined above should be followed.
-
If the cause of the load dump cannot be determined, resulting in a delay in resynchronizing, the unit should be tripped automatically after a specified time, the shut – down procedure described below under “Loss of Pressure or Temperature” should then be followed.
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Loss of Pressure or Temperature If the initial pressure regulating system is cut – out or inoperative the following procedure will apply: -
The operator will begin to reduce load in an attempt to maintain pressure above 80% of normal or temperature drop to not more than 830C.
-
If the pressure falls below 80% of normal or temperature drops 830C, reduce the load and trip the unit.
-
Place unit on turning gear, note power input to turning gear motor and listen for rubs. If everything is normal, follow standard starting procedure to bring unit up to speed and synchronize.
-
For the case where the pressure can be held above 80% of normal and temperature to less than 830C drop, bring the unit up in normal manner after temperature and pressure have been restored to normal.
-
If the unit trips from a known cause resulting from testing, etc. or if the vacuum is lost, let the unit come to rest and place on turning gear.
-
Check the rotor for truth. If everything is normal follow “Normal Starting Procedure”.
-
Where vacuum and speed can be held, reset overspeed trips and proceed with synchronizing the unit.
-
For the case where vacuum is lost and the low vacuum trip mechanism trips the load and unit, let the unit come to rest and place on turning gear.
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-
Clear vacuum trouble and follow “Normal Starting Procedure”.
Protective Devices Function
-
This unit is equipped with protective devices, which trip the unit in the event of low vacuum. Low bearing oil pressure and axial shift.
-
In addition, the unit can be tripped, either with the trip solenoid or with hand trip lever.
-
The situation can also arise where by the unit is inadvertently tripped in the process of testing the protective devices if the overspeed trip test lever is released prematurely.
-
In the event the unit is tripped as noted in above and the trouble has been recognized, cleared and the vacuum maintained, relatch the unit and proceed to synchronize.
-
If in the even of a trip and the vacuum is lost let the unit come to rest, following the normal shut-down sequence. After the trouble is corrected, restart the unit in accordance with the normal starting sequence.
Shut Down of Steam Turbine CHECK ST OIL GROUP GROUP GROUP ON ST TURNING GROUP GROUP GROUP ON ST DRAIN GROUP GROUP GROUP ON CONDENSATE PRE HEATER IN AUTO HP 2nd
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Economizer and LP economiser Recirculation valve on auto. 1st GT
2nd GT
GT Load Down from UCD or U
DT Load Down form UCD or U
If GT Load = 50%
If GT Load = 50%
Take HP TB PC coned.
Take HP TB PC coned
“INDIVID”
“INDIVID”
Take HT TB PC coned.
Take HP TB PC coned
“INDIVID”
“INDIVID” Check GT & ST should not be in APR
Decrease HPNTB P/C set
Decrease HP TB P/C SV to 10
2
Valve to 10kg/cm /(g)
kg/cm2 (g) gradually at the rate of
gradually at the rate of
let1.5 kb/cm2 min.
1.5 kg/cm2min Decrease LP RB P/C SV to 2kg/cm2
Decrease LP RB P/C SV to 2kg/cm2
If LPTB by open=50% Class LPSSV, IV
Close LPSSV IV check LP check
Check LP steam inlet D/V; IV open
LP steam inlet D/V; IV open
Continue GT Load decrease in unit mode or U up to 20% to 30% ST load 15%
open generator, breaker from UCB
HAND TRIP ST from U Check
(i) HPCV close, HPSV close (ii) HPCV close, HPSV close (iii) Turbine Drains open
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OPEN GT BYP ASS DAMPER
OPEN GY BYP ASS DAMPER HP and LP drum level will go down. Increase SV to + 50mm as after 10 min closing the inlet damper the drum Level will min. and go further down.
CLOSE WHRB INLET DAMPER
GO FOR GT ETOP CLOSE WHRB INLET DAMPER As gland steam will bead be available for 15 to 20 min
Close HP TB by, I/V
Close HP TB by, I/V
Close HP TB by, I/V
Close HP TB by, I/V If ST SPEED 400 RPM
Condenser Vacuum Group OFF
see (i) condenser vacuum pump stop (ii) Vacuum breaker open If condenser vacuum 100 of Hg
Gland steam Group Off
See (i) Gland steam condenser Exhaust fan stop (ii) Gland steam supply close
-
Close deaerator Heating steam supply
-
See turning motor run and turning gear engaged. (Check at local also)
-
Check condensate PH by B/P value open and inlet close.
-
HP and LP economizer re-circulation value open.
-
START TOP and STOP AOP
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Close HP SSV
135
-
Keep only one see each of the following auxiliaries, and stop if more than one running. i)
Circulating water pump CWP
For one module
ii)
HPO Boiler feed pump HPBSAP
One set
iii)
LP Boiler feed pump LP BFP
One set
iv)
Condensate extraction pump CEP
One set
Some Important Alarm/Trip Set Value •
Bearing Vibration Hi
HI 125 (P-P) Alarm
•
Bearing oil pressure low,
0.75+0.05 kg/cm2
Alarm low,
0.52 kg/cm2 Trip
Turbine speed High,
3240 RPM alarm HI HI,
•
3330 RPM Trip •
Condenser vacuum low,
635 mm Hg Alarm Low, 550 mm Hg Trip
•
Axial shift Trip
•
GT No load trip: if generator breaker of Both GT ( of the Module of which
5.6 kg/cm2
ST is opened then ST of that module will trip on “GT NO LOAD TRIP” •
HP main steam pressure high
> 70 kg/cm2
HP Main steam Temperature High
> 5250 C
HP main steam Temperature low
< 1500 C
HP exhaust Temperature High
< 1750 C
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•
•
•
•
•
•
•
LP main steam pressure high
> 7 kg/cm2
LP Main steam Temperature High
> 1750 C
LP main steam Temperature low
< 1500 C
LP exhaust Temperature High
> 800 C
Condenser Hot well level abnormal Low NWL
150mm
High NWL
+150 mm
ST Parameters ST Eccentricity High
0.0750mm
Axial Shift Low
1.100mm 2.10+2.0kg/cm2
High
+0.700mm
HP Differential expansion
LP Differential expansion
Low Low
-2.30mm
Low
-1.70mm
High
+4.70mm
High High
+5.70mm
Low Low
-2.30mm
Low
+2.90mm
High
+5.10mm
High High
+5.90mm
HP TB by spray water pressure low
9kg/cm2
-
HP by valve outlet temperature High
1900C
-
HP steam to condenser temperature High
2000 C
Thrust bearing temperature high
990C
-
1070 C
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Bearing temperature High
137
•
•
-
Lubricating Oil temperature High
500C
-
HP Oil pressure low
16kg/cm2
-
Main oil tank level low FL
+2165mm
Deaerator level Low
NWL 300mm
-
Low Low
NWL 60mm
-
High
WL+300mm
-
Deaerator pressure High
3.2kg/cm2
-
Condensate flow low
100t/hr
Oil pump auto start : -
Auxiliary oil pump auto start
0.85kg/cm2
-
Bearing oil pressure low alarm
0.75kg/cm2
-
Turing oil pump auto start
0.75kg/cm2
-
DC Emergency oil pump auto start
0.52kg/cm2
-
Bearing oil pressure Low Low Trip
0.52kg/cm2
ST Run Back Run Back on ST will operate automatically if ST load > 50% and its load will come down to 45% in the following cases.
i)
Out of the 2 running CEP one trips and 3rd one does not starts on auto.
ii)
Out of the 2 running HP BFP one trips and 3rd one does not starts on Auto.
iii)
Out of the 2 running LP BFP one trips and 3d one does not starts on Auto.
iv)
STG transformer winding temperature High
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Soaking Time in Warm Start (in minutes (M.T.) S=
Where
5 + 8T 50
=
9M} {P {3 x 2 + 14} 200
T
= steam inlet temperature in 0C
M
= 1st stage metal temperature in 0C
P
= Intel steam pressure in kg/cm2
For conservative time estimation last term can be neglected which is negative (and negligible) S
=
5 + 8T
=
9 M Minuets
50 Exhaust Hood Spray This spray will start if exhaust hood temp. 700C This will close when turbine load 15% and exhaust hood temp 700C Exhaust hood temp. high alarm at 800C Curtain spray Certain spray will start when CEP is running and any of HP & LP. TB By valve i/v and control valve open. It will close when HP & LP by control valve close System Alarm/Trips The following alarm./trips are provided to safe guard the operation of steam turbine. © PMI,NTPC
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Main oil tank level high FL
+ 2565 mm
Main oil tank level low FL
+ 2165 mm
Thrust bearing wear alarm
2.10+0.2kg/cm2
Thrust bearing wear trip
5.3-5.6kg/cm
Bearing oil pressure low alarm
0.75+0.05 (bearing oil) kg/cm2
Bearing oil pressure low trip
0.52 kg/cm2
Turning oil pump auto start
0.75+0.05 (bearing oil) kg/cm2
Emergency oil pump auto start
0.65+kg/cm2
HP oil pressure low alarm
18.0 kg/cm2
Aux. oil pump auto start
0.85+0.05 (bearing oil) kg/cm2
Oil cooler outlet oil temp. high
500C at 900 rpm to rated 400C turning to 900 rpm
Axial shift alarm at
2.1+0.2kg/cm2 (trip control oil)
Turbine trip indication
3.0+0.2kg/cm2 (auto control oil)
Turbine reset indication
6.0+0.2 (auto stop oil)
Turbine trip test indication on
3.0+0.2kg/cm2 (at trip test)
Turing gear interlock
0.3kg/cm2 (bearing oil)
Journal bearing metal temp alarm
107o C
journal bearing metal trip
113oC
thrust bearing metal alarm
99oC
thrust bearing metal trip
107oC
bearing oil drain tem alarm
77oC
Main oil transfer pump takes suction from bottom of main oil tank and transfers the oil to the dirty oil storage tank. From dirty oil storage tank the dirty oil is fed to plant oil purifier and after purification oil is stored in clean oil storage tank. From clean oil storage tank the oil is transferred back to main oil tank by clean oil transfer pump. Main oil transfer pump, plant oil purifier, dirty oil storage tank clean oil storage tank and clean oil transfer pump are common for both modules.
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Turbine Gear Operation During Shut-Down In order to facilitate starting, the turning gear should be in operation and should preferably have been in operation through-out the shut-down period Even after the turbine is comparatively cool, this offers minimum time lost in starting and an assurance of safety in the even of any leakage of steam in to the turbine during the shut-down period. When it can be arranged without delaying work schedules, the turning gear and the oil circulation system should be kept in operation for not less then 48hours after shut-down. In the event that continuous rolling during shut-down is not practical, the turning gear should be started again at least an hour before rolling the turbine with steam. It should remain in operation sufficient length of time be fore itting steam to the turbine to allow the rotor to become straight with in the limits noted in the section on “Limit son operation. Recognizing that tha preceding schedule may not be practical in case repairs or adjustments are to be made to the turbine, the following schedules are recommended as the absolute minimum periods considered satisfactory under various circumstances. •
The turbine is to be dismantled. The turning gear should be kept in operation until the dismantling programme requires that it should be stopped.
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If an emergency necessities it, the turning gear may be shut down immediately. However, it must be recognized that this may subject the rotor to a severe bow. It must not be used for continuous rolling unless the bearing lubrication system is operating. Bearing oil circulation must be maintained after shut down to protect the bearing against overheating. The minimum circulation period should be 24 hours after shut-down. During this period oil temperature leaving the coolers should be held below 35oC. If possible •
Shut down of extensive repair or adjustment Both the turning and bearing oil circulation systems are to be kept in operation for a minimum of 12 hours; oil temperature from the cooler should be maintained below 35oC. if possible. Both the turning gear and the oil circulation may then be shutdown. If operation of the unit is to be resumed before the turbine is thoroughly cooled, the turning gear should be started 3-4 hours before rolling the unit with steam.
•
Shut-down for minor repair or adjustment Depending upon the nature of the work to be done, the following shedule may be adopted. Keep the turning gear and the oil circulation systems in operation for a minimum of 3 hours. Both systems may then be shut off for a period of
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not more than 15 minutes; if practical, however, oil circulations should be maintained. •
Immediately thereafter roll the turbine on turning gear for at least 2 hours. Both turning gear and oil circulation may then be stopped for not over 30 minutes; however. 15 minutes after stopping, the rotor is to be turned 180o with the gear. This 180o turn may be made without oil circulation, if avoidable.
•
Following the 30 minute shut down period as above, the turbine should again be rolled on turning gear with oil circulation for a minimum of 2 hours.
•
The system may then be shut down indefinitely, provided that at 30 minute intervals for the next 6 hours the rotor is turned 180o Deg. Turns may again be made without oil circulation if necessary.
•
In any case, if the turning gear has been disengaged, it should be put in operation a sufficient time before itting steam to the unit to allow the rotor to become straight within a limit of approximately 0.025mm on the truth indicators or within a limit of approximately 0.05mm on the eccentricity recorder indications.
System Alarms The following alarms and shutdown point are provided for gas turbine protection. -
Hand trip
-
Alarm/Shutdown
-
Fire
-
Alarm/Shutdown
-
Lube oil Pressure Low
-
Alarm
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-
Lube oil Temperature High
-
Alarm
-
Lube oil tank level Abnormal
-
Alarm/shutdown
-
Inlet Air Filter Abnormal
-
Alarm
-
Vibration high
-
Alarm
-
Vibration high
-
Alarm/shutdown
-
Exhaust gas temperature
-
Alarm/Shutdown
-
Gas Pressure Supply Pressure Low
-
Alarm/Shutdown
-
Liquid fuel Main Pump Trip
-
Alarm/Shut down
-
Gas Leakage
-
Alarm
-
Liquid Fuel Drain Tank Level Hi
-
Alarm
-
Over speed
-
Alarm/Shut down
-
Exhaust Gas Pressure Hi
-
Alarm
-
Exhaust Gas Pressure Hi
-
Alarm/Shut down
-
Flame Out
-
Alarm/Shut down
-
Flame Detector Abnormal
-
Alarm
-
Blade Path Temperature Spread Hi
-
Alarm
-
Disc Cavity Temperature Hi
-
Alarm
-
Turning Gear Trouble
-
Alarm
-
Bleed Value Abnormal
-
Alarm/Shut down
-
Acceleration Rates Insufficient
-
Alarm/Shut down
-
Auxiliary Motor Abnormal
-
Alarm
-
SW Position Wrong
-
Alarm
-
Inst. Air Pressure Low
-
Alarm
-
Electrical Fault
-
Alarm/Shut down
Emergency Operation The Gas Turbine System can be operated in an emergency during the following conditions: -
Operation of the turbine with a control or lube oil system leak.
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-
Immediately tighten nuts, bolts or fasteners, if leak cannot be stopped the gas turbine must be shut down and repairs made.
-
Operation with a turbine with a fuel leak.
-
If a fuel leak occurs prompt action must to taken to contain and repair the leak. If the leak cannot be controlled the gas turbine must be shut down.
-
Operation of a gas turbine with abnormal vibration.
-
Many factors affect vibration, the cause of any abnormal vibration must be found and corrected. Unsteady blade path temperature, high bearing temperatures, turbine rotor rubbing against the turbine casing, speed and load will all directly or indirectly affect the amount of vibration.
-
Alarm and trip devices will shut down the unit if vibration exceed 0.200 mm (Peak to Peak).
-
Operation of a turbine during compressor surging.
-
If the gas turbine speed exceed its specified low frequency operating range, compressor surging can occur. If surging occurs the unit must be shut down immediately and inspected for damage.
-
Operation of a gas turbine in a under frequency condition.
-
If the unit is to run in under frequency mode particular attention should be paid to turbine blade resonance and compressor surging. If the turbine compressor enters a surge condition it must be tripped immediately.
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Turbine blade resonance is produced by blade conditions above or below the blades normal operating frequency range. Turbine blade resonance is a main concern of low frequency operation. When operating below Hz the unit must be monitored for abnormal noise and vibration. If unit frequency falls below 47.5 Hz for more than 15 seconds or falls below 47.0 Hz it will automatically trip. System Trouble Shooting Dependable, economic operation over long periods of time is only possible if the equipment is maintained and operated according to the manufacturer’s instructions. If problems develop, they should be traced and corrected as soon as possible. The following is a list of potential problems, possible causes and suggested checks to be performed by the plant equipment operator. Potential Problem -
Gas turbine starting motor will not start when “GTSTART” push button is depressed.
Possible Causes
-
Unit auxiliaries not running, starting motor circuit not operating Properly.
Operator Action
-
check all switch positions at unit auxiliary motor control centers, check starting motors breaker for standing switch gear alarms at breaker.
Potential Problem -
Gas turbine trips at ignition and light off sequence.
Possible Causes
“Fuel On” – “FLM2,”FLM3” out of control sequence,
-
CSO and main fuel oil pump discharge pressures are
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out of adjustment (liquid fuel firing only), blade path temperatures during light off abnormal. Operator Action
-
Check “Fuel ON” –“FLM2 and FLM3” lamp sequence and compare to previous operation, check for any extreme difference between “FLM2” and “FLM3” lamp sequence, check CSO and main pump discharge pressure,
check
blade
path
temperature
spread. Abnormal Operation The response characteristics of turbine-generator units and their auxiliaries to power system emergencies and the procedures to follow during such periods are of particular interest to the operating utility. In general, question may arise concerning the following: -
Low-frequency operation
-
Maintaining low auxiliary load on the unit
-
Steam turbine motoring
Low Frequency Operation Limits for operating frequency and time at frequency are given for the protection of the turbine and generator during under-frequency over-frequency operation. For the turbine prevention of resonant operation of the tuned blades is the major consideration. Operating limits have been developed form engineering analysis coupled with laboratory testing and extensive field telemetry testing.
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Time limits are based on fatigue life consideration and have been apportioned to the more prevalent under-frequency operating mode. For specific blading styles, more conservative off-frequency operation limits have been imposed than previously published. Generator operating limitations are also presented which must be observed during off-frequency situations in order to prevent undesirable thermal excursions of generator components. The primary consideration in operation of a steam turbine under load at other than the synchronous frequency is the protection of the long tuned blading in the low pressure (L.P) turbine element. In the design and design verification process, great care is taken to properly tune the tapered and twisted blades that constitute the last several stages in the L.P. turbine element. Operation of these stages under load, at a speed that causes a coincidence of blading natural frequency in a lower mode and multiple of the actual turbine running speed frequency, will lead to blading fatigue damage and ultimately blading failures. Auxiliary Load Operation Most turbine generator units are designed to withstand complete isolation from the system and remain in service at no load or at an auxiliary load level. However, it should be recognized that some of the larger plants now in operation, and under construction, have plant control interlocks which automatically trip the complete plant in the event of load separation.
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The transient stress in the turbine rotor is a factor to be considered when suddenly dropping from full power out to auxiliary load and the subsequent rapid application of load after the connection to the system is reestablished. The immediate affect is an instantaneous drop in first stage temperature of about 120 degree c. This drop in stage temperature after initiation of the
produces a peakstress in 10to 15 minutes
transient which decays slowly to zero
in about
one
hour, if auxiliary load is maintained for an hour or more, the rotor is forcecooled to new equilibrium state. The subsequent reloading should then be performed at a moderate rate in order to avoid
a large thermal stress in the opposite direction. To minimize
the accumulation of rotor fatigue damage, it is recommended that auxiliary load operation following
an electrical out occur only when the
system
conditions make it absolutely necessary. Steam turbine motoring Motoring occurs when the steam supply to the turbine is shout while the generator is still On the line. Since there is
no steam energy available
to
supply the turbine-generator
losses, the generator will act as a synchronous motor and drive the turbine. Although this condition is generally
defined as generator motoring, the
protection is not for the generator, but to prevent overheating low pressure turbine or high pressure turbine blade. Monitoring can caused by closing steam valves without taking the unit off the line. This can be done by.
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-
Closing all governor valves during shut- down operation.
-
Tripping the turbine while not tripping the generator breaker.
Our general policy in connection with motoring of a turbine generator unit is as follows: -
Closing all governor valves during shut-down operation.
-
Tripping the turbine while not tripping the generator breaker.
-
Operation with high density steam entrapped within a high pressure casing may result in a rapid temperature rise of the steam and internal turbine parts.
-
A turbine- generator unit should not be motored for any extended period without ventilating steam being itted to all turbine elements.
-
It is recommended that such operation be limited to not less than one minute.
-
It is recommended that the opening of the generator circuit breakers for the purpose of avoiding or limiting a motoring condition be controlled by the closing of the main valves.
Interlock for turbine motoring is installed to protect turbines from the damaging effects caused by motoring and lack of ventilating system.
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Emergency Operations •
Black out (Grid Failure) Main Control Broad is connected to the grid with two 200 KV lines terminating 220KV and two 400 KV lines terminating at 400 KV. At Main Control Broad the interconnection between 220 KV and 400 KV Buses is through two ICTs each of 325 MVA capacity. Black out condition may occur when all the four lines trip or there is failure of supply at both 220 KVS/S and 400 KV S/S. In case both 220 KV lines trip and 220 KV side breakers of ICTs also get opened then a circuit called “House Load Circuit” gets triggered. This will cause tripping of both STs and desynchronization of all the GTs except one reselected GT which will supply House Load of Gas Plant. In all other conditions units at Gas Plant will trip on high frequency or low frequency (Grid sinking). In case of total power supply failure Gas Plant provision has been made of “Emergency Gas Turbine” (EGT) to run essential auxiliaries and to start one Gas Turbine. One “Emergency D.G.Set” has also been provided to keep the units on turning gear in case of non-availability of EGT.
Following operation shall be carried out in the condition of black out (i.c.,when all the efforts to save one of the GTs for House Load have failed ):-
Ensure that DC Lub. Oil pumps and DC Seal Oil Pumps have started in auto, if not start manually.
-
Ensure the starting of EGT in Auto/ Manual.
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-
Charge Station 6.6 KV SW/GR (one bus only) from EGT and charge auxiliary transformers one by one. Put on sufficient lights
-
Start cooling Water Circulating Pumps (110 KW) for GTs and then one IAC.
-
Ensure starting of AC Lub. Oil pumps of GTs and TG Oil Pumps of STs. Then stop DCLub. Oil Pumps. Start AC Seal Oil Pumps of all the units and then stop DC Seal Oil Pumps.
-
Ensure starting of turning gear of all the units.
-
In case EGT is not available then “Emergency D.G. Set shall be started. Other restorations are to be done only after power Supply is made available from the grid. IOCC and 220 KV S/S shall be ed for restoration of grid supply, so that start up power can be made available to the Gas Plant.
-
Start Raw Water Pumps and Clarified Water Pumps.
-
Start6 on PAC.
-
Line up one of the Gas Turbines for start up.
-
Start the selected Gas Turbine and keeping both 220 KV lines and ICTs out of service charge one 220 KV Bus (In case Grid supply has not been restored till that time).
-
Take one Station Transformer into service (which can feed to the other dead 6.6 KV Bus)and transfer station auxiliaries load to this transformer.
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Following shall be attempted only if Grid Supply has not been normalized. -
After getting a load of –7 MW on gas turbine, by starting station auxiliaries, one of the 220 KV lines shall be charged from Gas Plant end with proper communication to 220 KV S/S and they shall be advised to load the line at the rate of –5 MW/minute.
-
Start one more gas turbine and keep it ready for synchronization. This units can also be brought on bar if the previously running gas turbine has crossed a load of 20 MW. Then further load can be increased at the rate of 10 MW/minute.
-
After the grid condition are normalised EGS can be stopped. Get the other 220 KV line charged from the grid end and synchronize at Plant end. Similarly synchronize the 400 KV lines, Bring the combined cycle into service.
Tripping of one of the 220 KV lines(Station Load full) Normally when both the 220/400 KV ICTs are available, tripping of the 220KV lines shall not pose much problem. However it shall be ensured that the other 220 KV lines and ICTs are not getting overloaded. To reduce the station load one of the gas turbines can be desynchronized temporarily. If the overloading is minor, then station load can be reduced slowly. In case only one ICT is in service then changes of overloading 220 KV lines/ICT are high and one or two gas turbines can be desynchronized temporarily, depending on the amount of overload, to control the station the station load. After ascertaining the healthiness of the line units can be normalized.
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Tripping of both 220 KV lines (Station Load Full) Normally when both ICTs are available tripping of both 220 KV lines will overload the ICTs slightly. The amount of overloading will depend on the reactive power flow through ICTs. Depending on requirement, one of the gas turbines can be desynchronized temporarily or the load on all the units can be reduced gradually to a safe level. However , if only one ICTs is available at the time of tripping of both the 220 KV lines, immediately two GTs shall be desynchronized, load on remaining GTs shall be reduced and load on STs shall be reduced by operating HP/LP by valves. If fast action is taken ICT will also trip on over load causing “House Load Circuit” triggering. Another course of action can be, opening of 220 KV Bus coupler breaker. This action may however trip the units connected to the bus, where 220 KV lines were connected earlier. After ascertaining the healthiness of the line units can be normalized. Tripping of one of the ICT s {Station Load full} Same action as in above point no. 2 will have to be taken depending on the loading of remaining ICT and 220 K V LINES. Tripping of both ICT s {Station load full} Same action as above will have to be taken to avoid overloading of the 220 K V lines. High Frequency Operation At a Frequency of 51.2 Hz there is provision for GT s to run back to 60 MW. In case Frequency remains above 51.5 Hz for more than 2 minutes both GTs of a module will get automatically desynchronized
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causing the ST to trip on “GT
154
NO load ST Trip”. Before this happens efforts can be made to bring down the frequency by desynchronizing of the GTs manually and reducing be made load on remaining GT. Simultaneously IOCC shall be ed
to control the
frequency. During this process HP/LP stop values of the out of service WHRB shall be kept closed to a void tripping of ST on “WHRB HP/LP Drum level High “. If the frequency comes down the GT shall be synchronized and WHRB shall be taken into service immediately to avoid drop in outlet temperatures of WHRB. If all the efforts fail the remaining GT will also get desynchronized and ST will trip on “ GT No Load ST Trip”. During this time efforts shall be made to keep the WHRB s in steady operating condition by controlling the drums level and operating the HP/LP by valves suitably. When frequency comes, GT s can be synchronized and ST rolling can started to bring ST into service .
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12. Protection And Interlock General Philosophy Regarding Protection Electrical Equipment All electrical equipment must be protected against internal and external faults, as well as abnormal conditions which may endanger the equipment or the system. Appropriate relays and devices have to be provided, to detect the faults and potentially dangerous conditions and to isolate the concerned equipment cost the earliest in order to minimize the damage. The basic requirements of sensitivity and speed have to be met by provisions of carefully selected relays and relaying schemes, since failure of a protection operate cannot be ruled out, and it need lead to damage of costly equipment, it is necessary to provide back up in the form of redundancy of relays, local or upstream back up relays etc. however, continuity of power generation is also of utmost importance and it dictates that the risk of tripping (due to mal-operation of the protection) when a tripping is not really necessary be minimized. This risk is related directly to the number of relays and other elements in the protection scheme must be made as simple as possible. Redundancy and back ups must also be minimised and tendency of over protection checked. The protective relays must be connected to automatically isolate the endangered equipments if the damage is imminent or short circuit has already occurred. However, in cases where the equipment or system is not immediately endangered and can continue in service for some more time, the protective devices should be connected to initiate only an alarm. This would enable the plant operator to take corrective steps and prevent a tripping or to prepare the outage of an equipment (by changing over the stand by provisions or bringing down the unit load).
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It is also important that the control room operator is not burdened or confused with too many alarms. The alarm facial on the unit control board (UCB) are therefore to be used for annunciating only those abnormal conditions for which the UCB operator has to take some distinct preventive or restorative action. It is generally adequate and/or with hand reset flags on protective and auxiliary relays s or switchgear. Protection of Generator & General Bus Against Bus Duct Against internal Fault The generator bring very costly and critical items in a power plant, should be protected by duplicate protections. Phase to Phase Faults are to be covered by differential relays 87G and 87GT. In case of CGB scheme 87 GT covers up to CGB generator is not protected by87GT. In such a case voltage controlled over current protection is provided as back up. The stators earth faults would be covered by 64 G1 & 64G2. the earth faults in the stators winding close to natural are generally remove, however the chances of such faults can not be ruled out. It is therefore proposed to have at least one stators earth fault relay to detect faults anywhere in stators winding including neutral terminal. The relay 64G1 therefore, should be capable of detecting earth faults anywhere in the stators winding but 64 G2 may cover only 95% of the stators winding. There are various type of relays available which operate on different principles to detect earth faults close to neutral (64G1). One of the type has there elements built in. One element detects rise of the fundamental frequency voltage across the neutral grounding resistor and protects the generator winding about 95% (relay element similar to 64G2). Second element detects collapse of third harmonic voltage present on neutral and thereby detects the earth fault close to neutral. Third element which supervises
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the voltages, prevents the tripping command to exist during generator shutdown period. ASEA relay type RAGEA operates on as principle. Relays type PVMM, manufactured by English Electric Co., works on a more or less similar principle, except that the second element compares the third harmonic voltage element protects the entire generator stators winding except for a blind zone in the middle of the winding. Another principle to inject a low frequency signal (ariybd .01 pu) which biases the generator neutral point to earth. Through a minimal amount of current flows continuously due to generator and lead capacitance, any drastic increase in the current indicates the existence of a ground fault. BBC relay type GIX 103 ir GIX 104 & SIEMENS relay type 7UE22 are examples of relays working on this principle. Any one the above described type of relays can be considered for 100% stators earth fault protection. The relays are time delayed by about one second to prevent mal operation during transformer KHV side ground faults currents is limited to about 10A. The earth fault relay 62 G4 shall be voltage operated inverse time relay connected across the open delta winding of an auxiliary voltage transformer 9 (VT) on generator terminal. The generator bus ducts has an isolated phase construction throughout and hence cannot have a direct phase to phase fault. Their coverage by one 87GT (and partly by 87G also in case of GCB scheme) for short circuits is therefore considered sufficient. Earth faults in the generator bus ducts are however covered by two protections 64G and 64 G2. The configuration transverse differentials protection (50G) is proposed to be provided for generator internal fault protection.
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In case generators have only three terminals brought out in line as well as neutral sides (even if the stators winding is double star) it would not be possible to apply the conventional transverse differential protection for internal faults. The inter turn fault protection is not considered very essential since such faults are very unlikely in single turn coil construction adopted for the large generators. For earth fault protection of the generator rotor circuit a two stage relays (64F) working on the principle if insulation resistance
measurement of rotor is
provided. The first stage of the relay shall be connected for alarm in UCB. Further fall in insulation resistance of the rotor circuit initiate operation of the relay second stage which is recommended to trip the unit. However it is recommended that the machine be shut down for detection of fault location and rectification as early as possible on occurrence of first stage alarm because a second earth fault at another location could be quite harmful. Protection of Generator Transformer & EHV connection Provisions of duplicate instantaneous (main) protection is mandatory for all 220 KV/400KV elements. Accordingly the overhead EHV connections between generator transformer and switchyard would be covered by two differential protection 87GT and 87Tl each of would detect all type of short circuits. 87GT and 87LT would also cover HV side earth faults, HV/LV winding would be covered by generator earth fault relays 64G1 and 64G2. In addition to the above differential protections, the generator transformer would have the usual Buchholz relay (63T), winding temperature indicator and oil temperature indicator, each of these would have alarm and trip s. Winding temperature indicators would be connected to as repeater dial on the unit control board, and would also be used for transformer cooler control.
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Transformer would also have oil level indicator with alarm , and devices to detect abnormal, ties in coolers control. Protection of Generator and Generator Transformer Against External Faults and Abnormal Operating Conditions As the large-generator have a very limited capacity to withstand and negative sequence currents (caused by unbalanced loading, opening of phase, and uncleared system fault etc.) a precise negative sequence protection 46 G must be provided. It should have an 12 element to give a time delay alarm in case of a persistent negative sequence current exceeding the generator continuous 12 capability and 12t unit to initiate generator tripping before generator 12 2t capability limit has been reached. While the generator has a certain overload capacity, continued operation at a load higher than its rated current can lead to overheating and consequent insulation failure. The protection against stators over heating would be provided by the generator temperature monitoring system. No protection or separate temperature monitoring for bus ducts is necessary since they would have a naturally cooled rating slightly higher than the generator rating. A three-phase single zone impedance relay (21G) is to be provided for the back up protection of generator against external three phase and phase to phase faults in the EHV system, which may be hanging on due to failure of their own system primary protection, The zone of 21G should be extend beyond the EHV switchyard. It should be connected to trip the generator after a time delay of 1.0 to 1.5 seconds so that the generator is tripped only if the EHV line protection has not cleared the fault even in second zone.
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The back up protection against resisting earth faults in the EHV system is provided by an over current relay 51 NT connected to the current transformer (CTs) in generator transformer HV neutral. Local breaker backup (or breaker failure) protection 50Z is to be provided for the generator breaker EHV as well as GCB). It would facilitate fast back up isolation of faults not cleared by the instantaneous primary protection. Loss of excitation protection 40G is provided as per universally accepted practice. In case steam turbines ratings high (<=200MW), a Pole slipping protection is also provided to cover the protection against instability in EHV system. The generator can develop high over voltages in the even of mal operation of automatics voltage regulator, or a load thrown off while generator excitation is under manual control. An over voltage relay 59G set at 110% is to be provided to detect its condition and give an alarm in case it persists for 2-3 seconds. The severe over voltage would be covered by over fluxing protection which is explained in next clause. Over fluxing of the generator transformer and unit transformer can occur due to over voltage on generator terminals, or due to excitation application while turbo generator is at lower speed. Its persistence can cause gradual over heating and damage to the generator and transformer. An over fluxing protection 99T is to be provided to protect the generator and transformer against the condition. The generators to be tripped if voltage/frequency remains above 1.2 p.u. for about 2-3 seconds. Time delays necessary in order to allow AVR to correct the situation. In case of GCB scheme separate over fluxing relays need to be provided for generator and generator transformer.
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Motoring of the turbine generator can occur due to storage of steam/fuel supply. While it does not affect the generator, it can cause gradual over heating of turbine blade. The low forward power relay 37GA along with a time delay is used for detecting this conditions. While reserve power relays have conventionally been used for this protection, a low forward power relay (LFPR) is proposed to be used since both are equally effective for this purpose. In fact LFPR is better, since turbine blade over heating would occur even if the unit is in generating mode at a load at which LFPR operates about 0.5% of rated output. Any other electrical protection which have to be provided for turbine safety is the under frequency protection. The turbine manufacturers normally stipulate stringent limitations regarding under frequency operation of turbine, and it would be safer to trip the unit whenever the frequency tends to drop to a value dangerous for turbine operation. A multi stage relay is proposed to be used for under frequency protection (81G) the first stage of the relay gives an alarm in unit control board indicating that the system is operating in an under frequency regime. The operator should inform the Regional Load Dispatch Centre regarding this so that they can take timely action by way of load shedding to improve the frequency. The second stage will stage will again give an alarm permit operation for a set time after which the machine will be tripped. The third stage when reached will instantaneously trip the machine. These levels of operation are normally defined by the turbine manufacturers. Relay are also available which have df/dt functions. However it would also be beneficial to work out am overall operation strategy, during under frequency conditions, in association with the Region Load Dispatch Centre and other utilities in the region.
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Monitoring of generator VT fuses would also be provided to give an alarm in UCB, in the even of fuse failure. VT fuse failure may maloperate back up impendence stators earth fault (64G2), low forward relays and under frequency relays therefore these protections would be blocked on VY fuse failure. Besides the above electrical protections, the generator and its auxiliary systems (hydrogen cooling, bearing lubrications, seal oil, gas etc.) would be protected in accordance with manufacturers recommendation. Fire protection system shall be provided for generator transformer and its shall be give for class A tripping. Generator have certain MW/MVAR capabilities which are indicated in capability curve. The operation beyond its capability is harmful to the generator. In order to give a clear picture to the operator about the actual operation of the generator with references to its capability, it is proposed that DDC MIS should have a facility to show the actual operating point to generator plotted on the generator capability curve. Protection of Unit Auxiliary Transformer The unit auxiliary would have separate differential protection (87UT) as their main electrical protection against phase-to-phase faults. 87UT shall also have set instantaneous element. Earth fault bt their HV winding be covered by 64G1 and 64G2 and the earth faults in each LV winding shall be covered by restricted earth fault protection 64 RUT. Back up to 64RUT and HT system earth faults shall be provided by earth fault 51 NUT connected to a CT in transformer neutral. Backup to 87 UT for transformer faults and to HT incoming breaker protections for HT system faults would be provided by overcurrent relays 51UT with a definite time delay to coordinate with down stream protection. Besides the above electrical protections, the transformers would have the usual protective devices like
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buchholz relay with alarm and trip s, winding and oil temperature indicators with alarm and devices to detect cooler abnormality. Winding temperature indicators would have a repeater dial on UCB and will also be used for cooler control. The H.T. bus ducts between the unit auxiliary transformer and the associated H.T. Switchgears would also be protected by the transformer protections 87UT, 51UT, 64NUT. Unit auxiliary transformer is so designed that they would not normally get overloaded. However, winding temperature indicator (WTI) and oil temperature indicator (OTI) are provided alongwith alarm to detect any overloading and annunciate. Fire protection system shall be provided for unit auxiliary transformer and its shall be given to class A tripping. Generator Tripping Logic Scheme without GCB In the absence of any circuit breaker on generator on terminals, the generator and associated transformer become one integral unit as far as their protection in concerned. In case of short circuit anywhere in the generator bus ducts, generator transformer, EHV connections upto the circuit breakers, unit auxiliary transformer and H.T. bus ducts it is necessary to immediate isolate and H.T. unit incoming breaker instantaneously. This is commonly known as class A tripping and the following protections must be connected to initiate the same: 87G, 87GT, 87LT, 87UT, 63UT, 95G, 51UT, 64G1, 64G2, 64RUT, and 51NUT. Tripping of the circuit breakers as mentioned above would suffice for electrical isolation of the fault, the tripping of the turbine is not necessary. However, the electrical fault which caused the tripping would require investigation, and tripping
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of the turbine would be required subsequently in any case. Therefore, there is no point in trying to keep the turbine in operation after a class trip and it can also be tripped alongwith the circuit breakers in case of class A trips. Besides the risk of dangerous turbine over speeding upon disconnection of generator from the grid is higher when turbine is not tripped. This risk is considerably reduced when turbine is tripped simultaneously with circuit breakers. Scheme with GCB In case generator or circuit breakers are provided on generator terminals, EHV and 6.6 KV auxiliary breakers should not be tripped for generator faults/trouble since 6.6 KV supply needs to be retained for running of steam turbine. It is therefore, required that following protections need only be connected to tripping of generator circuit breaker, field breaker and turbine (class A1): 87G, 95G, 64G1, 64G2. However, the protections which covers generator transformer, UAT’s and associated systems beyond GCB upto EHV breaker such as: 87G, 87UT, 63UT, 51UT, 64RUT, 51NUT need to be connected to trip EHV breaker, field brenker, UAT LV brenker and turbine alongwith GCB (Class A2). Protections 64G, 21G, and 51NUT require isolation of the generator from a persisiting external fault, for which tripping of the EHV breakers alone would normally be sufficient. However, the fact that these ultimate back, up time delayed protections have operated indicates a serious failure of EHV switchgear or relays, since they would operate only if all the normal back ups EHV system (i.e. second main protection, second zone of distance relays and local breaker) has already failed to open (after having received a tripping command from the bus bar differential protection or local breaker back up), in which case there would be no point in giving another tripping impulse to the same breaker.
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Considering all these aspects, it is proposed that the back up protections under consideration (46G, 21G & 51NT) be connected to initiate class A/Class A2 trip. In case of over-fluxing, ideally only the generator excitation need to be rapidly cut back. Since this is not feasible when AVR is not functioning properly (which is evident from the very fact that 99T has operated), tripping of the excitation becomes necessary. However, this would lead to loss of excitation condition unless the generator is completely tripped at the same time. Therefore, in effect, over-fluxing protection (99T also have to be connected to initiate tripping of all generator circuit breakers. Considering the fact that serious abnormality in the excitation system is indicated, and generator back in service, it is proposed that turbine also be tripped simultaneously. 99T would therefore initiate class A/class A2 trip. The generator would work as an induction machine and there is a loss of excitation. The turbine generator speed would rise, the MW output would come down due to governor action and the generator would start drawing a large MAVR from the grid. While the generator can sustain this condition for a short time at much reduced active load, it is nevertheless to keep such an abnormal condition in which severe torque oscillations are produced, current are induced in the rotor core and overheating takes place. Since the protection operation indicates major failure in excitation systems protection should also be connected to class A/class A2 trip. Pole slipping of provided on steam turbine proposed to be connected for class A tripping since it is a condition in which turbine generator may have suffered mechanical damage and the unit has to be brought back only after careful checking.
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Unit Tripping Logic As mentioned earlier, most of the electrical protection of generator transformer unit have to connected for class A mode of tripping, where all circuit breakers and turbine are tripped simultaneously, as soon as the fault has been detected by protective relay. The electrical fault in the generator transformer are comparatively rare, and most of the unit tripping take place due to other causes generally related to problems in boiler, turbine take place due to other causes generally related to problems in boiler, turbine and their auxiliaries. For such problems, it is not necessary to trip the generator breakers instantaneously. Infact, it is desirable to delay their tripping, as explained below: In a class A trip, the turbine-generator gets instantaneously disconnected the power system, stops delivering power and is then free to accelerate. The turbine emergency stop valves take a finite time to close and some steam enters the turbine even after initiation of a trip. This steam alongwith the steam which is already present in the turbine expands to energy in accelerating the turbine. There after the turbine would start slowing down if the steam Valves have closed fully and there is a case all the valves have not closed fully and there is a leakage through any of the valves, the turbine may not slow down but start accelerating further. The dangerous possibility is now widely recognized and a sequential trip (also known as class B trip) the turbine valves are allowed to close first. When the steam inside the turbine has exhausted, the power output of the turbine generator comes to zero. This condition is verified by sensitive LFP, and then only the generator circuit breakers are tripped. This kit is insured that the turbine does not over speed. Another advantage of class B trip is that grid suffers the loss of generation as a cramp down over a few seconds, and not as a step. Two LFPR are provided to provide backup to each for the purpose.
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However in case of Gas turbines, since a large compressor load is connected on it’s shaft such dangerous over speeds are not expected and as such no necessity of all the protection are connected to trip electrical breakers as well as turbine simultaneously. The trip s of WTI and OTI of generator transformer (49T), the excitation system troubles if not urgent, and all abnormalities in generator auxiliary system (hydrogen cooling, bearing lubrications, seal oil, gas etc.) which require unit tripping should follow class B mode. Excitation system troubles which require immediate tripping of excitation would initiate class A tripping. In OTI of the unit transformer would be connected for tripping only associated H.T. circuit breakers (but not de-enrgising the transformer. Two separate hand reset lockout relays 86G and 186G would be provided for class A/Class A1/Class A2 tripping. They would operate in parallel but on separately fused 220VDC supplies, the various protections would be distributed on these two DC supplies such that even if one of the supplies, faults. The VT supplies, DC supplies and healthiness of lockout relays would also be monitored. Protection of Station Transformer The station transformers would have differential protection (87) as main electrical protection against phase to phase faults, 87 shall have one instantaneous high set element. Since EHV system is solidly grounded, the HV winding earth fault shall also be covered by 897. However, a restricted earth fault protection. Back up to incoming breaker protection for H.T. system faults would be provided by definite time delay over current relay (51) on all three phases on HV side of transformer. For earth faults in LV winding, restricted earth fault relays (64RST) shall be provided, Backup to this and H.T. system, earth fault shall be provided by a
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definite time delay really (51N) connected to a CT in transformer neutral, which shall be connected to trip EHV breaker. Besides the above electrical protection, the transformer would have the usual protective devices like Buchholz relay with alarm and trip s, winding and oil temperature indicators with alarm and the , oil level, indicator with alarm and devices to detect cooler abnormality. Winding temperature, indicator would have a repeater dial in central control room and also be used for cooler control. Station transformer are so designed that they would not normally get overloaded. However, WTI & OTI are provided alongwith alarm in order to detect any over loading and annunciate. Fire protection system shall be provided for station transformers and its shall be connected to trip its EHV breaker. Local breaker back up (or breaker failure) protection 50Z is proposed to be provided for the EHV breaker in the HV side of the station transformer. Protection of H.T. Motors Very divergent practices are adopted by different organizations in this respect. On one extreme, CTs are provided on only two phases and just one single element relay is provided for short circuit protection. Overload protection is provided only for those motors which have a distinct possibility of getting overloaded and not for every motor. The other extreme, CTs are provided on all phases as well as on the cable, separate short circuit elements are provided for each phase, overload protection is provided on all the phases as a general rule (whether the motor has a possibility of getting overloaded or not), stalling
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protection is provided for all motors, and a sensitive earth fault protection is provided using the core balance CT on the cable. A moderate approach should however be adopted. Low resistance natural grounding is being adopted for the HT system. CTs are to be provided on all phases, Proposed provision of relays would be as described below, the guiding philosophy being provision of some back up but a voidance of superfluous elements. For example, only balanced three phase overloading is expected for which provision of overload relays on one phase would be sufficient. However, for backing up against relay failure, overload relays proposed to be installed on two phases. Protection of overload relay on the third phase is considered absolutely redundant and has therefore been avoided. Selection of the types relays would be based on their reliability record and ability to work satisfactory under the prevailing environmental conditions. For protection against phase fault, all motors up to 2000KV rating should have instantaneous high set over current relay (50) having low transient over reach to avoid mal operation due to DC component in motor starting current on all phases. The motor rated above 2000KW need to be protected by high speed differential relay (87), which would be connected to H.T. switchgear CTs on one side and the CTs on motors natural bushings on the other side in order to have better sensitive of the protection. Provisions of relays on all phases would give inherent instantaneous back up for relay failure. For protection against earth faults, all HT motors should have sensitive earth fault relay (560N) connected o core balance (zero sequence) current transformer to be put in H.T. switchgear. CBCT is recommended in order to have better sensitivity of the protection for the cases where earth fault current is linked (low resistance or high resistance grounding).
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For overload protection, all motors would have inverse time over current relays (51) on two phases, and their Characteristics would be adjusted selected to lie between the hot withstand curve and the starting curve of the motors. The stalling or locked rotor protection will be provided only for those motors which do not have sufficient gaps between their hot withstand and starting curves or which have a distinct possibility of stalling. It will comprise of an over current relay (50/LR) having plan adjustable definite time delay, which would operate in conjunction with speed switch with adjustable setting on motor shaft. One more over current relay (50/OL) will be provided for overload alarm, for all motors, it would alert the operator in advance, so that corrective action may be taken or preparations made before a motor is tripped out by the over load relays (51) Provision of 51 on two phases and 50/OL on the third phase would provide inherent back up against relay failure as well protection against faults near the motor neutral which may not be detected by the other protections. Single phasing protection is not necessary for motors controlled by circuit breakers having a common operating mechanism for the three poles. Negative sequence current protection is also not considered necessary for power plant auxiliary motors since the supply voltage would always be balance in the close proximity of generators. The large motors lasted above 1000KW would be having winding temperature monitoring, with automatic scanning by the embedded RTDs. These would be connected for alarm only winding temperature monitoring is not considered necessary for smaller motor, which would normally be self cooled with little possibility to overheating as long as they are not overloaded. However, adequate monitoring of the cooling system shall be provided where water coolers are involved.
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The proposed provision of CTs and connection of relays for the motors rated up to 2000KW and for motor above 2000KW are shown in fig. 3&4 of annex-C respectively in enclosed sketch. Composite relays are also available which have all the features as mentioned above. These relay are static and/or microprocessor based having built in on line testing facilities, and have thermal memory for overload protection of motor. Such relays, when employed, should meet the environmental conditions at their location. Transient overreach requirement for short circuit protection and the initial cost are some of the additional factors needing attention. Protection of Auxiliary Transformers (6.6 KV/0.433KV) The H.T. feeders to auxiliary transformers would have instantaneous high set overcurrent relays (50) on all phase for protection against phase to phase fault. The LT transformers rated above 5 MVA, if any, shall be provided with differential protection instead of short circuit protection (50). Definite time delayed low set overcurrent relays (51) would be provided on all phases for protection against LV side faults. The latter would also provide backup protection against faults in the LV system. Inverse time overcurrent relays are not proposed to be used since time varies with the fault current and low intensity faults are cleared after a considerable time delay. Use of overcurrent relays with a definite adjustable time delay for 51 would provide clearance of all type of faults in less than one second (Ref. Annexure-D). For protection against HV side earth faults, all LT transformers would have an instantaneous earth fault relay connected to core balance CTs for all the LT transformers. For protection against LV side earth faults, 6.6KV/0.433KV transformer would produce current of the same magnitude as the faults (since the neutral is solidly
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grounded) and would recover by overcurrent relays 51 mentioned in cl. No. 11.1.0. for the protection against earth faults inside the LV winding, a definite time delayed earth fault relay is proposed which should be connected to a CT in transformer LV neutral side. This earth faults relay shall be fully coordinated with maximum fuse resting of outgoing motor feeders. Besides the above electrical relays, the liquid filled LT transformers would have Buchhloz relays (63) with alarm and trip s, winding and/or oil temperature indicators with alarm and trip s. Dry type indoor transformers would have only winding temperature indicator, with necessary s for alarm and trip. The LT transformers normally so sized that they are not over loaded. However, winding temperature and oil temperature are monitored and alarms are given for temperature high in case of any overheating. Protection of LT Motors The 415 V motors rated above 125 KW would be controlled through circuit breakers. They would be protected against phase-to-phase faults either with instantaneous short circuit released in the circuit breakers, or with instantaneous high-set over current relays (50), provided on all phases. For a sensitive earth fault protection these motors would have instantaneous overcurrent relay (50) in the residual CT connection. For overload protection, the breaker controlled motors would have inverse time overcurrent relay (51) on two phases for tripping and a definite time overcurrent relay (50/OL) on the third phase for alarm. The arrangement would be similar to that described earlier for the H.T. motors. The smaller 415 V motors (rated upto 125 KW) would be controlled by ors. They would be protected against phase-to-phase and earth faults by
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HRC fuses. The fuse would be selected as not to be affected by repeated motor starting. The overload protection would be provided with three phase thermal (bilmetallic) overload relays. Single phasing protection should be provided for all or controlled motors which are protected by fuses. It would be ensure that this protection is not too sensitive to negative sequence current, and is adequately stable during motor starting, normal voltage unbalance etc. Undervoltage protection is proposed to be provided on all the H.T. and L.T. breaker controlled motors in order to avoid spurious tripping of reverse supply breaker (due to drawl) of starting current of all the motors connected on particular bus), when it is closed on a dead bus. The under voltage protection would be connected on bus VT and would trip all the motors connected on the bus VT and would trip all the motors connected on the bus after a time delay. Time delay is provided in order to allow the changeover to take place on up stream switchgear and voltage dip due to the fault anywhere in the system. Protection of Auxiliary Supply System For phase to phase faults on H.T. bus and back up protection for outgoing feeder on these buses, a definite time delay over current relay need to be provided in all phases on H.T. tie and incoming breakers. Earth faults on H.T. bus and backup to all outgoing feeders on unit/station bus would be covered by a definite time delay overcurrent relay need to be provided in all phases on H.T. tie and incoming breakers. For phase to phase faults on 415V incomes & buscoupler breakers. Tie to 415V MCC’s/boards, if controlled by breakers, shall also have short circuit release fitted in the breaker. The releases shall be definite time delayed magnetic/solid
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state type. Definite time delayed over current relays as real so acceptable in place of releases, Earth fault current in the 415V system would produce currents on same magnitude as for phase to phase faults, and therefore would be covered by phase faults relays. Earth fault protection therefore is not necessary to be provided in incomes, buscouplers and tie feeders to MCC/board. Time graded protect as detailed above, results in a very high operating times (about 1.0 sec.) for relays located at income. Through switchgear have one second through faults withstand capability, they can not be expected to withstand internal faults for such long time without serious damage. It is therefore recommended to adopt a reverse blocking principle to cut down the grading times. For this arrangement all the relays as described above located on ties, bus coupler, income, shall have instantaneous (with a short delay of about 100as) and delayed (coordinated with downstream) stages with the instantaneous stage of up stream blocked by all the down stream protection current element. With this arrangement a faults last any point in the system can be detected within 100m.sec. Protection of Emergency Diesel/Gas Turbine for Black Start Facility For phase to phase fault protection three phase high impedance differential protection should be provided for these generators. This protection shall be extended upto 6.6KV breaker so as to protect 6.6KV cables also, For6.6 KV bus phase to phase faults and back pup to out-going feeders protection, voltage controlled overcurrent relays should also be provided. Voltages control over current relays are recommended for back protecting because fault current form generator would reduce considerably owing to its very high steady state impedance. Since emergency diesel generator neutral is kept undergrounded, in a single earth fault only capacitive current would blow. Earth fault in this case shall be
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detected by the voltage relay provided in the neutral VT which shall be connected for an alarm in the central control room. Above arrangement would facilitate the diesel generator operation under emergency conditions even if a simple earth fault exists in the system. Though necessary interlocks shall be provided to avoid motoring of the diesel generators, possibility of such occurrence cannot be ruled lout. Single phase reverse power relays are therefore proposed to be provided for diesel generators. Protection, described above shall be connected to trip 6.6 KV breaker as well as diesel engine fuel solenoid valve since only tripping of 6.6KV cable fault. Interlock shall be provided so as too allow the engine to be restarted only after manual resetting of tripping relay located at diesel generator control . The reverse power relay shall be connected only to trip diesel generator 6.6KV breaker.
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13. Spare Parts Management Spare parts management is an important aspect for trouble free operation and minimum breakdowns or shut down time in a plant. For optimizing power generation, availability of required number of quality spare parts is of paramount importance.
While quality is normally ensured, availability is mostly poor
because of inadequate planning, Selection of spares in right quantity is equally important, otherwise the plant may have high inventory of spares, still may not have a needed spares. In case of NTPC; due to diversity of equipment and use of highly abrasive coal, a large variety of spares are to be stocked. Unavailability of even a small item could cause a total or partial unavailability of the units, resulting into huge loss of generation and consequential industrial loss. The process of spare parts management is governed by number of factors, e.g. maintenance planning equipment history and behaviors, spare parts planning, forecasting, procurement policy and procedures etc. Efforts have been made here to dwell upon some of the important aspect and present them as a recommendatory system of NTPC power stations which may be taken as general guidelines. The stress is on the maintenance planning, spare forecasting, planning, stocking and inventory control. Procurement of Initial Spares A provision equivalent to 4% of the equipment cost is made towards the purchae of initial spares in our feasibility reports and detailed project reports. These initial spares include consumable spare parts, unit replacements and insurance spare. The spare parts are recommended by Operation Services based on past O & M experience,
manufactures
recommendation
and
potential
indigenous
development of such items.
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The provision of 4% of equipment cost of initial spares have been found to be inadequate and as a result, the quantity of initial spares are restricted to the provisions made by CEA, Even though the prices quoted by vendors for mandatory and recommended spares are very competitive since they are subjected to bid evaluation were are not able to procure sufficient quantity of initial spares due to financial constraints. Mandatory and Recommended Spares During initial periods, the spares parts were procured based on the recommendations of the manufacturers who were quoting very exorbitant prices as their virtual monopoly over these
prices which were not subjected to any
evaluation. In NTPC, to obtain, more competitive prices of spare parts, it was considered desirable to prepare a list of spare parts which will normally be required and procured along with the main equipment and are independent of equipment order. These lists include some of he high value insurance items and unit replacement assemblies. Such spares which are included in technical specifications as part of supply are known as Mandatory Spares. The prices of these spares have to be competitive as they are subjected to evaluation. Once the order is placed on the successful bidder, he is asked to submit a detailed list of spares which will be required for day-to-day operation and maintenance of initial period of 3 years as well as any other special and high valued items not included in mandatory spares, the list of recommended spares including mostly fast moving consumable type of items like bearing, liners, gaskets, HRC fuses, contractors etc. This list is subjected too rigorous scrutiny and spare parts 3 year O & M are recommended
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Since the total cost of mandatory and recommended spares has to be within the provisions of 4% of the total equipment cost as per CEA guideline, this causes resource constraints which result in either reduction of quantity of spares of enhanced expenditure on spare parts. Basic for Procurement of Initial Spares The initial spare included essential insurance items, unit replacement and consumable of 3 years. The period of 3 years has been chose because of non availability of O & M personnel at site at the time of placement of order for main equipment and long lead time associated with imported spare parts. Also no consumption is worked out on the basis of past experience of similar equipment and the manufactures recommendations. Any period less than 3 years of procurement of initial spares could lead to undesirable situation of stock out. The system of thorough review of manufacture’s recommendation has helped NTPC to avoid the common pitfalls of lending up with the high inventories of those items which are normally not required and not having the items, unavailability of which could case total or partial outage of the equipment. While deciding upon the adequate quantities of consumables spares of 3 years consumption, the following kept in view:-
-
Anticipated or known failure rate
-
Succeptability of he items to loss/ damage
-
Self Life
-
Manufacture’s recommendation
-
Interchangeability with other similar spares
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-
Development potential
-
Operating condition and environment
-
Total installed quantity (Population)
-
Accessibility of skills, tools, time and other facilities of repair
-
Wear life of components
-
Sub assembly replacement and repair at later date
-
Consumption rate
-
Fast technology developments of items of imported origin specially C&I items The above considerations have helped in proofing down the quantities of various slow moving items. It is observed that space parts recommended by vendor needs to be reviewed judiciously, so as to optimize the recommended list of spares of procurement in respect of number and type of item and such review has helped in cutting down our inventory at least by 20-30% of the original recommendation. The actual values of spares for all the packages considered together as per past experience, works out to be in the range of 5.5 to 6% for 200MV stages and similar trend is emerging for 500 MW sets where relatively fewer orders have been placed so far. Since the initial spares that are being ordered includes Insurance Spares, unit assemblies and consumable spares for 3years, it has not been possible to limit the value of initial spares within the stipulated provision of 4% given by CEA. NTPC being committed to high plant load factor and considering the isolated location long lead time, multiplicity of make and type of equipment,
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sophisticated control & instrumentation and operational aids. The spares ordered initially exceed the provision to have a sufficient stock of spares, to avoid shortages and stock-out situations. Based on the above experience & judicious review of initial spare parts requirement, it has been proposed to CEA that the provision for initial spares procurement should be as follows for both 200 MW & 500 units. Mech.
Elect.
C& I
% to total
Packages
Package
Package
Equipment
Cost Indigenous Plants
6%
4%
8-10%
6%
Imported Plants
6-8%
4-5%
12-15%
8%
The matter is under consideration with CEA and it is hoped that as a special case of NTPC, the provision of initial spares may be suitably enhanced. Spare Parts Planning, Forecasting & Procurement In order to ensure availability of spares at the minimum inventory carrying cost, a systematic approach is necessary to be evolved on forecasting planning and procurement of spares. The main objectives of this approach are as follows:
-
To ensure availability of spare parts whenever needed so that plant availability is not affected.
-
To achieve optimum inventory turnover into i.e., consumption and purchase of spare parts be so matched that capital blocked in inventory is minimum.
-
To procure the spare parts at the lowest individual and lowest total cycle cost with consistency is quality.
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Maintenance Planning Spare parts planning is dependent on the maintenance planning function. The behavior pattern/ history of the equipment is available from the past operational feed back which helps in preparing maintenance and capital overhauling schedules. Requirement of spares for all type of maintenance. i.e. preventive, break down and capital repair & overhauling, can be planned accordingly in the following manner. Preventive Maintenance Scheduling of preventive maintenance of various equipment in the power station is done as per the norms worked out on the basis of experience of power station maintenance engineers or as per recommendations of the manufacturers. A detailed weekly preventive maintenance programme for all the equipment is finalized in advance and is strictly adhered to by the maintenance personnel. Certain fast wearing type of items like glands, gaskets, packing etc. are necessarily to be changed during these preventive maintenance and hence can be planned for availability. Capital Repair & Overhauls During capital repairs and major overhauls, many items are required to be changed depending upon the wear and tear, failure and past history of the equipment. Theses items are normally long lead items and hence a comprehensive planning for such spares should be done at least 2-3 years in advance to meet the overhauling requirements. 5year rolling plans for stations should be prepared, updated and reviewed every year for the purpose of
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ascertaining the need of spares, depending upon the plan condition and equipment behaviour. Break-down Maintenance It is very different to predict the requirement of spares for such jobs as the pattern of breakdown occurrence is uncertain. However based on regular condition monitoring predictions can be made Regarding residual life of the equipment. Accordingly, requirement of spares can be worked out for future period depending upon past history and failures. Spare planning & Inventory Control Maintenance Planning will indicate the schedule of maintenance and quantity of spares required to meet schedules. These quantities are to be cumulated for total quantity of spares required to meet the schedules. These quantities are to be cumulated for total quantity and analysed for deciding the order quantities keeping the current stocks, quantity due in against pending orders and lead period. The inventory for the consumable spares should be such that it meets the management objectives of no stock outs and minimum inventory holding. Since there is always some gap between the time when order is placed and the spare is actually reviewed, the role of spares planning is very vital. The planning cycle and inventory level fixation depends upon various factors. Location of the plant
The location of the plant effects the lead time & local source development. NTPC plants located at remote place have to carry high inventory of spares.
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Nature of spares
Proprietary items or imported spares which calls for cumbersome import procedure, have high lead periods and should be planned well in advance.
Lead Time
The time elapsed between indent raising and availability of spare part for use is an important factor which governs the stocking policy: This time comprises internal lead time of indent processing & placement
order,
apart
from
the
manufacturer’s delivery period. Equipment history and
The equipment behaviour and failure rate usage rate determines the usage rate. It provides the guidelines for inventory stocks. However, usage rate varies with the age of plant.
Organization and Budgeting
Spare
parts
planning
set
up
for
identification Budgeting and placement of
orders
for
spares
help
in
curtailinistrative and procedural delays.
Similarly
adequate
Budget
provisions and availability of funds for spares leads to optimum inventory stocks.
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Analysis and Classification For the purpose of planning, it is necessary to conduct an analysis of spares with respect to consumption; critically movement and their availability etc. ∗
F.S.N. Analysis
The spares are classified into Fast, Slow and non moving categories based on the movement rate.
∗
Fast Moving
Items which have moved at least once in 1-2 years
Slow Moving
Items which moved at least once in 2-4 years.
Non Moving
Items which have not even once in 4-5 years.
Type of Spares: Insurance Spares
The spares which normally not required but are vital for the equipment as in case of any miss hope or accidents non availability of these items will lead to long out ages. For example turbine rotors exciters etc.
Unit Replacement
In order to reduce equipment down time complete assembly is replaced. This assembly/sub assembly can be repaired later on and can be used again.
Consumable Spares These are fast wearing out items and have reduced life. These spares are replaced. ∗
ABC Analysis
Based on the annual consumption or usage values, spares are replaced a new when found worn out.
Non-Moving Items
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A.
Contributing to 60-80% of total spares consumption. Industrial annual consumption Of more than 50 thousand.
B.
Covering about 25% items contributing to 15-30% of total usage value. Industrial consumtion between
C.
5000-50000/-.
Annual usage value of each spares less than 5000/-.
Criticality Analysis
The spares categories according to their criticality of usage In the plan.
Vital
Spares which cause complete stoppage of plant and service risk in operation are vital like control value speed governors load limiters.
Essential
In absence of such spares, the plant can run but perhaps at substandard performance level like spares of ID fan, mills etc.
Desirable
These are the spares whose non-availability may not Adversely affect the plant performance but may increase down time later on.
SDE Analysis
Categorization depending upon sources and leadtime of procurement:
Scarce
Normally proprietary or imported spares with long lead time.
Difficult
Special efforts are to be put in the for procurement of these items. They are high tech. Items.
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Easy
These items can be procured from standard suppliers in a short lead time.
Inventory Control To meet the objective of spares planning, inventory levels should be so optimised that stock out as well out stocking are minimum. The inventory cost include carrying cost and ordering cost. Optimisation is to be achieved so as to minimise the total cost by controlling how much to order and when . To achieve this, it is necessary to classify the spares into above categories such as ABC, VED, and FSN and expected life. On the basis of these classification , Inventory levels safety stocks & recording level can be decided for each and every items. Based on the VBC and ABC analysis, a confidence level matrix have been adopted to determine the safety stock of
the spares.
Confidence matrix V E D
A 85% K=1 75% K=0.75 70% K=0.7
B 95% K=1.7 85% K=1.0 76% K=0.75
C 99% K=2.5 95% K=1.7 80% K=0.8
Periodic Review System The spare parts planning and forecasting is proposed to be done on fixed review period basis. It is envisaged that all the consumables spares will be reviewed only once in a years. A calendar for the review of spares have been prepared which provides review of spares segregated into seven packages such as SG, TG, Elect., C&I, Off-sites etc. Apart from these, high consumption critical spares
© PMI,NTPC
187
of Bowl mills, coal handling plant and ash handling plants will be reviewed on quarterly basis. The maximum inventory level has been fixed on the basis of consumption rate, lead time and safety stock. 1.
2.
Average consumption during lead period
Average consumption during review period A.
total population ------------------------- x Review per Expected life 1.5 years –2 years 1 year
Review period
=
1year
Safety stock =KM = face or depending upon ABC /VED analysis = consumption during lead period
maximum Level = lead period consumption stock
5.
x Lead period
= =
Safety stock where K M 4.
=
total population ______________ Expected life
Lead period i) BHEL spares ii) Other than BHER
B. 3.
=
+
review period Safety Stock
Reorder quantity Reorder quantity
© PMI,NTPC
=
Maximum level –(present stock –quantity due).
188
N.B in case the present stock is less than lead period consumption , recorder quantity should be taken equal to review period consumption only. The identification of consumable spares, their classificatication schwas VED, ABC, FSN and expected life havebeenentrusted to site Ormond the fixation of maximum in vent ore safety stock etc. have been entrusted to Operation Services. The periodic annual review of spare will be carried out by Site O&M Planning &Materials Deptt. The objective of the spare development is to make a systematic effect to develop the spare parts indigenously or in house for all categories of spares whether Indian or Imported so as to achieve the following objectives. −
Cost reduction
−
Alternative source
−
Import substitution
−
Availability
−
Reduction in Lead time
It has been proposed that items which are common to 2 or more than projects or which have a annual consumption value of more than 10 lakhs will be taken up for developments from Corporate Centre while the items having less consumption value or particular to individual projects will be taken up for development by project itself. The responsibility of preparation of drawings and technical specification will be O&M planning at Sites and COS at Corporate Centre while replacement of order will be the responsibility of site materials Deptt. And Materials department at projects and Coordinate Centre respectively. Once the source standardisation is done, projects can place order on these parties for their annual requirements.
© PMI,NTPC
189
Already the jobs of spare parts developments has been taken up by all the projects & coordinate Centre with encouraging results and the number of spare parts developed will increase with the availability of drawings and material specification of spares which is a very elaborate process. Vender development The spares being im ported by the plant can be boradly as -
Spares for which there is not enough capacity witmn the country though the required specifications are available
-
Spares for which there no specification is available but capacity is a vailable within the country (most of these are propriety nature spares imported as per suppliers part numbers).
A study conducted shows that the category(b) constitutes about 80% of the total import budget of spare and thus requires special attention towards import substitution drive. We had developed in our project many of the spares which would lead to foreign exchange savings to the tune of several lack per annum. The interesting feature of this development management is that ones specifications of an item is established, the item will go out of the import-list for all time to come leading to saving foreign exchange over a number of years. Dependence on continued import and indigenous proprietary procurement is not only expensive but also very time consuming and leads to long delivery per period and available inventory as well. The development spares required continuous efforts right from identification of spares for development to the trial runs, assessment of performance of the spares supplied by the vendors and
© PMI,NTPC
190
accordingly enlisting successful vendors for such development items and continue to explore new competent sources. Identification for Spares for Development Spare parts be identified for development purpose on the following criteria: (a)
Spare of imported origin.
(b)
Spare of indigenous proprietary origins. − Spare of vital/ essential in nature. Long/ irregular delivery period from original equipment manufacturer/ Oes and not of high tech-components. − Spare common among various NTPC projects. − Spares generally bought out items and only assembled by OEM/OES. − High consumption spares. − High value spares and not of very sophisticated technology involved.
Identification of Sources − Press ments, Exhibitions, State wise yellow pages, DGTD. Manufactures/ trade and NSIC directories and s/ other projects past experience. − Introductory letters received from vendors and ments published in leading newspaper journals. − Information obtained from DGTD regarding items licensed for same/ allied items.
© PMI,NTPC
191
− Magazines like Industrial /Electronic product finders, Computers Today’s, Industrial Equipment News, National Product News, Telemetric India, etc. Registration of Vendors − The vendors shall be required to furnish the details such as technological expertise, financial soundness, manufacturing/ testing facilities and past performance with regard to supply and quantity of same/similar items in the prescribed Performa. − Screening committee consisting of representatives from Finance, Material, O&M, QA not below the level of Manager/ Supdt. At Site/OS will finalize the list of verdors for undertaking the development of items, based on their capability and capacity. Suppliers work may be visited if necessary. − Vendors ed with DGTD/NSIC and known to NTPC site/OS need not be reassessed Enquiries Enquiries can be floated for spare development to all the ed vendors for similar items. In case more than one vendor is not available in the approved vendor list for a particular item, enquiry can also be floated even to single vendor with approval. Placement of Development Order − The negotiation/Tender committee (as per DOP) places the development orders. The original price or spares/ items, estimated costs, specific investment and infrastructure by the vendors will be considered
© PMI,NTPC
192
− The committee can propose to issue more than one PO against single item in case of multiple offers (all being L1). − Negotiation committee can recommend issuing of more purchase order for single items at different rates to different vendors.
© PMI,NTPC
193
14. Date Sheet for Gas Projects S. NO.
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
a) Net plant capacity, Base load (MW) at design condition (i.e. 270C ambient temp. 60xRM, 320C CW temp. at site elevation)
413.3
652.56
645. 36
817.48
648.6
b) Combined Cycle net heat rate at base load design condition of 15 kg/sec. per Gas (K cal/KWhr.)
1796
1796
1771
1748
2.
Fuel & its Source
Natural Gas tapped from HBI pipeline
Natural Gas tapped from HPI pipeline
Natural Gas from BHEL pipeline.
3.
Gas Requirement
4.
Water Source
5.
Condenser cooling system
2.25 MCMD (Annual Average) Etawah branch canal Closed circuit
6.
Mode of operation of plant
1.75 MCMD (Annual Average) Kota right main canal Hybrid (Closed circuit/once through) Base load and/or two shift or peaking duty depending on technical and/ economic considerations
Natural Gas /NGL Natural Gas is supplied From HBJ 2.25 MCMD (Annual Average) Hazira Branch canal Closed Circuit
Base load and/or two shift or peaking duty depending on technical and/ economic consideration
Base load and/or shift or peaking duty depending on technical and/ economic consideration
7.
Main Plant awarded on
MHI Japan on 03.09.90
8.
Orientation of GT Exhaust
GEC-Alsthom on 05.03.90 Axial
1.
DESCRIPTION
© PMI,NTPC
ABB, West on 20.8.87 Axial
Axial
1834 (This figures are with DM Water injection turbine) Natural Gas From Gandhar Gas Basin.
3MCMD (Annual Average) Net Canal Closed Circuit
1.5MCMD (Annual Average) Wareada river Closed Circuit
Base load and/or Two shift operation or two shfit or peaking duty depending on technical and/ economic consideration. Siemen AG, West Marubani, Corporation on 26.3.92 Axial Axial
194
S. NO.
DESCRIPTION
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
270C asb. Temp. 60% RH 253 M above seal level, 320C CW temp. ABB, 98.9 89.25
270C asb. Tmep. 60% 270C asb. Tmep. 60% RH, 140M above sea 60% RH, 5.5 M above Level, 320C CW temp. sea level, 320C CW temp. MHI, Japan Eropean Gas Turbine 121.51 116.9 111.76 105.62
270C asb. Tmep 60% RH 210 M above sea 320C CW temp. Siemens AG, . 142.42 131.31
2770
2623
2697
2692
2617
31
32.78
31.9
31.95
32.8
6.76
8.2
7.8
9.6
9.4
3000 12.4 370 505
3000 13 412 513
3000 12 388 540
3000 10.2 462 560
3000 14.3 500 525
GAS TURBINE AND AUXILIARIES 9.
Design Conditions
10.
Manufacturer
11. 12.
ISO base rating at Net O/C base output of GT at design condition (MW) Net O/C heat rate at base load and design conditions (K cal/Kg. Hr.) Net efficiency of GT (%) (O/C, at base load, design conditions) Natural gas consumption (O/C, base load design condition kg/sec, assuming cal value of gas as 8700 K cal/m3 and density of gas As 0.856 kg/m3 Turbine speed (rpm) Compression Air bass flow (O/C base Exhaust gas temp. (0C)
13. 14. 15.
16. 17. 18. 19.
© PMI,NTPC
270C asb. Tmep. 60%RH, 23.25M above seal level, 320C temp. ABB, 147.8 131.36
195
S. NO.
DESCRIPTION
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
20.
Ignition of speed of the shaft (RPM)
600
600
510
400
600
21.
Self sustaining speed of the shaft
1500
2100
1950
2100
2000
22.
start-up mechanism
frequency Converter
Starting motor
Frequency Converter
Frequency converter
Starting motor
DESIGN FEATURES 23. 24. 25. 26. 27. 28.
No. of compressor stages No. of turbine stages No. of air cooled stage i) Guide ii) Rotating blades GT Generator Rating MVA GT Generator Cooling by TYPE OF AIR FILTER Type of Air Rating MVA system.
18 5
19 4
17 3
16 4
21 5
2 3 125 Air
2 2 141.08 Hydrogen
2 2 171.875 Air
2 2 165.5 Air
3 3
Self cleaning Cartridge type
Self cleaning Cartridge type
Self cleaning Cartridge type
Self cleaning Cartridge type
Self cleaning Cartridge type
Diverter type
Diverter Louver
Diverter type
Diverter Louver
Diverter type
Forced circulation/ Dual Pressure
Forced circulation/ Dual Pressure
Forced circulation/ Dual Pressure
Forced circulation/ Dual Pressure
Air
BY DAMPER 29.
Type WHRB
30.
Type
© PMI,NTPC
Natural circulation/ Dual Pressure
196
S. NO. 31.
DESCRIPTION
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
Manufacturer
Wanger Biro/Austria
MHI. Japan
CMI. Belgium
BHEL, India (with Wagener Biro design)
a) HP Super heater Out let temp. (0C) Outlet pressure (bar)
485 63.97
495 60
520 70
530 65
504.5 76.9
b) LP Drum/Super heater Out temp. (0C) Outlet Pressure
210 5.91 bar
158.1 7 kg/cm2
193.2 7 bar
200 5.7 bar
220 6.1 bar
32.
STEAM TURBINE Make
ABB, Germnay
MHI, Japan
ALSTHOM,
BHEL, India
ABB,
33.
Type
Tandem compound Dual pressure Condensing type
Tandem compound Dual pressure Condensing type
Tandem compound Dual pressure Condensing type
Tandem compound Tandem compound Dual pressure Dual pressure Condensing type Condensing type
34.
Mode of Operation
Sliding Pressure
Sliding Pressure
Sliding Pressure
Sliding Pressure
35.
HP CYLINDER
Single flow 25 Stages
Single flow 25 stages (Impulse)
Single flow 13 Stages Impulse
Double flow 24 Stage (Reaction)
Single flow Single flow
36.
LP CYCLINDER
Double Flow 6 stage (Reaction)
Double Flow 6 stages (Reaction)
Double Flow 5 stage (Reaction)
Double Flow 7 stage (Reaction)
Double Flow
37.
Rated Speed
3000rpqm
3000rpm
3000rpm
3000rpm
3000rpm
38.
Based load out put at
153.28 MW
109.3MW
116.1 MW
154.51 MW
227 MW
© PMI,NTPC
KHI, Japan (with Henry vogt design)
Sliding Pressure
197
S. NO.
DESCRIPTION
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
39.
Make
ABB
MHI
DELASWAER
BHEL
ABB
40.
Condenser back pr. At load and design condition.
100 bar
72mm hg abs
92 a bar
0.1122 kg cm2
100a bar
41.
Cooling water flow
29700 M3/hr
23340M3/hr
23061 M3/hr
25000 M3/hr
37728 M3/hr
42.
Design Cooling water temp.
320C
320C
320C
320C
320C
one (1)
one (1)
one (1)
one (1)
one (1)
SULZER
BHEL
3x50%
3x50%
DEAERATOR 43.
No. of Deaerator/module FEED WATER PUMP A.
HP FEED WATER WATERPUMP
44.
MAKE
SULZER
45.
No. of HP feed water
3x50%
TORISHIMA JAPAN 3x50%
46.
Capacity
307.3m3/hr
210 T/hr
182.2 M3/hr
290 M3/hr
47.
TDM
94.3 bar
80 kg/cm2
110 bar
107 bar
TORIHIMA JAPAN 3x50%
SULZER
KSB
3x50%
3x50%
B.
LP FEED WATER PUMP
48.
MAKE
SULZER
49.
No. of LP feed water
3x50%
© PMI,NTPC
3x50%
198
GAS PLANT FAMILIARISATION
Power Management Institute Noida
CONTENTS S. No.
Description
Page No.
1.
Introduction to Gas Power Plant
1
2.
Gas Turbine Starting System
10
3.
Fuel System
12
4.
Fire Detection and Protection System
17
5.
Salient Features NTPC Gas Based Power Projects
22
6.
Modern large size Gas Turbine
25
7.
Gas Turbine Compressor
50
8.
Waste Heat Recovery Boiler
67
9.
Gas Turbine Generator
82
10.
Excitation System
92
11.
Gas Plant Operation
103
12.
Protection & Interlock
156
13.
Spare Part Management
177
14.
Data Sheet for Gas Projects
194
1. Introduction To Gas Power Plants Introduction The development of the sector in the country, since independence has been predominantly through the State Electricity Boards. In order to supplement the effects of the states in accelerating power development and to promote power development on a regional basis to enable the optimum utilisation of energy resources, the Government of India decided to take up a programme of establishment of large hydro and thermal power stations in the central sector on a regional basis. With this in view, the Government set up the National Thermal Power Corporation Ltd., in November 1975 with the objective of planning, construction, commissioning, operation and maintenance of Super Thermal and Gas Based Power projects in the country. The availability of gas in a large quantity in western offshore region has opened an opportunity to use the gas for power generation, which is an economical way and quicker method of augmenting power generating capacity by natural gas as fuel in combined cycle power plant in a power deficit country like ours. With this intention in mind the Government has asked NTPC to take up the construction of Kawas, Auraiya, Anta, Dadri and Gandhar Gas Power Project along the HBJ Gas pipe line. The power plant will consist of gas turbine generating units waste heat recovery boilers, steam turbo generator, ancillary electrical and mechanical equipments. The power generated at this power station will be fed over 220 KV AC transmission system associated with this project to distribute the power in the various Regions.
© PMI,NTPC
1
In the Power Sector, the use of gas turbine drive generators has just started and will go a long way in meeting the country’s requirements. Gas turbines range in size from less than 100 KW up to about 140.000 KW. The gas turbine has found increasing application due to the following potential advantages over competive equipment. •
Small size and weight per horsepower
•
Rapid loading capability
•
Self-contained packaged unit
•
Moderate first cost
•
No cooling water required
•
Easy maintenance
•
High reliability
•
Waste heat available for combined cycle application.
•
Low Gestation Period
•
Low Pollution Hazards
The function of a gas turbine in a combined cycle power plant is to drive a generator which produce electricity and to provide input heat for the steam cycle. Power for driving the compressor is also derived from gas turbine. Combined Cycle Combined Cycle power plant integrates two power conversion cycles namely. Brayton Cycle Gas Turbines) and Rankin Cycle (Conventional steam power plant) with the principal objective of increasing overall plant efficiency.
© PMI,NTPC
2
Brayton Cycle Gas Turbine plants-operate on Brayton Cycle in which air is compressed (process 1-2, in P-V diagram of Figure-1), this compressed air is heated in the combustor by burning fuel combustion produced is allowed to expand In the turbine (process 3-4) and the turbine is coupled with the generator. Without losses the theoretical cycle process is represented by 1’ 2’ 3’ 4’ In the actual process losses do occur. Deviation from the theoretical process, results from the fact that compression and expansion are not performed isentropically but polytropically which is conditioned by heat dissipation (expansion) and heat supply (Compression) caused by various flow and fraction by losses. In the combined cycle mode, the Brayton Cycle is chosen as the topping cycle due to the high temperature of the exhaust of the gas turbine (point 4 in the P.V diagram). In modern gas turbines the temperature of the exhaust gas is in the range of 500 to 550oC. Reference to the T.S. diagram may indicate the amount of heat that is produced, converted into mechanical energy and extracted from this process. For the evaluation of the cyclic process, two parameters are of greatest importance; 1)
Thermal efficiency
2)
Process working capacity
Thermal efficiency is obtained from chemical binding energy of the fuel and mechanical energy available at the shaft of the gas turbine. Thermal efficiency (th) as follows:
© PMI,NTPC
3
th = Energy at GT shaft Chemical Energy of fuel =
Q Input. Q output
1 — Q Output
Qinput
Q Input
Working capacity is also obtained from the difference between the amounts of heat supplied and removed. This is achieved by increasing P2 that is increasing gas inlet temperature T3.
BRAYTON CYCLE FIG-1 Rankine Cycle The conversion of heat energy to mechanical energy with the aid of steam is carried out through this cycle. In its simplest form the cycle works as follows (fig.2). The initial state of the working fluid is water (point-3) which, at a certain temperature is compressed by a pump (process 3-4) and fed to the boiler. In the
© PMI,NTPC
4
boiler the compressed water is heated at constant pressure (process 4-5-6-1). Modern steam power plants have steam temperature in the range of 500oC to 550oC at the inlet of the turbine.
COMBINE CYCLE FIG-2 Combining two Cycles to Improve Efficiency We have seen in the above two cycles that gas turbine exhaust is at a temperature of 500 –550oC and in Rankine Cycle heat is required to generate steam at the temperature of 500-550oC. so, why not use the gas-turbine exhaust to generate steam in the Rankine cycle and save the fuel required to heat the water ? Combined Cycle does just the same. © PMI,NTPC
5
The efficiency of Gas Turbine cycle alone is 30% and the efficiency of Rankine Cycle is 35%. The overall efficiency of combined cycle comes to 48%. Types of Combined Cycles It is basically of two types, namely Unfired Combined cycle and Fully Fired combined cycle. Unfired combined Cycle The basic system is shown in figure- 3. in this system the exhaust gas is used only for raising steam to be fed to the steam turbine for power generation. The conventional fossil fuel fired boiler of the steam power plant is replaced with a ‘Heat Recovery Steam Generator’ (HRSG). Exhaust gas from the gas turbine is led to the HRSG where heat of exhaust gas is utilised to produce steam at desired parameters as required by the steam turbine.
UNFIRED COMBINED CYCLE FIG-3
© PMI,NTPC
6
However, non-reheat steam turbine is the preferred choice for adopting this type of system as usually the live steam temperature for HRSG will be solely controlled by the gas turbine exhaust temperature which is usually around 500oC. In recent development, with the introduction of Dual Pressure Cycles more heat is recovered in the HRSG and steam with higher pressure and temperature can be generated. But higher capital investment and sometimes necessity of supplemental firing system makes the system complex and costly. Fully Fired Combined Cycle Fig – 4 shows the basic schematic of this cycle. In this system the heat of exhaust gas from gas turbine is used for two purposes as described below: Heat contained in exhaust gas is used to heat feed water to a desired temperature at the inlet to the boiler. This leads to the reduction or elimination of the extraction steam requirement from the steam turbine. In case, the steam turbine has a larger steam swallowing capacity to generate more power the amount of steam which is being extracted from steam turbine for regenerative feed heating could be made to expand in the turbine to increase its base load capacity and improve the overall efficiency. In case the steam turbine does not have the capacity to swallow extra steam available due to cutting down of extraction, the fuel being fired in the boiler can be cut down to generate less steam by an amount equivalent to steam required for extractions and thus improving the overall efficiency due to less consumption of fuel. Gas turbine exhaust contains about 14 to 16 % oxygen (by weight) and can be used as hot secondary air in the conventional fossil fired furnaces. So the heat required to heat the secondary air will be saved and can be used for other purposes. FD fan power consumption will also be reduced to a great extent.
© PMI,NTPC
7
BOILER REPOWERING SYSTEM EXHAUST HEAT EXCHANGER Fig – 4 Fuels Gas turbines are capable of burning a range of fuels including naptha, distillates, crude oils and natural gas. Selection of fuel (s) depends on
several factors
including fuel availability, fuel cost and cleanliness of fuel. Natural gas is an ideal fuel because it provides high thermal efficiency and reliability with a low operation and maintenance cost. Liquid fuels, particularly heavy oils, usually contain contaminants, which cause corrosion and fouling in the gas turbine. Contaminants, which cannot be removed from the fuel, may
© PMI,NTPC
8
leave deposits in the gas turbine, which reduce performance and add maintenance costs. Dual fuel systems are commonly used, enabling the gas turbine to burn back-up fuels when the primary fuel source is not available. Dual fuel systems can also be designed to fire both fuels simultaneously. Advantages of Combined Cycle Plants Apart from the higher overall efficiency, the combined cycle power plants have following advantages: Low installation Cost: power from a combined cycle power plant is approximately 70% of a conventional coal based power plant of same capacity. Low Gestation Period: Power from a combined cycle power plant can be obtained in two phases, i.e. two third power, obtainable from gas turbines, is available within 16-18 months and the balance in next 1 month. This is much lower than the gestation period for conventional thermal power plants, which is around 48-60 months. Better Reliability: Combined cycle power plants are considered to be highly reliable with the reliability factor of 85-90% as compared to 60-65% for conventional coal Fired Power Plants.
© PMI,NTPC
9
2. Gas Turbine Starting System The function of the starting system is to crank the gas turbine upto the required speed until : it becomes self sustaining. One method of starting large gas turbine is by using a motor driven hydraulic starting system. Alternatively, the GTG can be started by using a frequency converter to rotate the generator which drives the turbine for starting. A typical hydraulic starting system for each gas turbine consist of the following: •
Starting motor, electric AC induction motor
•
Hydraulic torque converter
•
Auxiliary Gear
•
Couplings
The electric starting motor drives the hydraulic torque generator through a coupling. The hydraulic torque converter consists of an impeller, which forces the fluid against hydraulic starting motor. The hydraulic torque converter is coupled to the accessory gear, which is connected to the gas turbine shaft. The torque converter receives hydraulic fluid from hydraulic and lube oil reservoir during operation. When gas turbine reaches self-sustaining speed the starting device is disconnected and shut down. To break the inertia of the starting motor and reduce the starting current a pony motor is provided. Gas turbines of GE and WH designs are provided with starting motor system for cranking purpose.
© PMI,NTPC
10
The static frequency converter serves the same function of starting, accelerating running at preset speed by starting the generator as a synchronous motor by feeding variable frequency current drawn from the connected grid. Gas turbines of ABB and SIEMENS make are provided with frequency converter s for the GT cranking and normally 2 * 100% static converters are provided for the power station. Black Start System To start a gas turbine in the event of AC-power failure an emergency black start system is provided. It also helps in safe coasting down of the gas turbine and its auxiliaries following a ‘trip’ in the event of grid collapse. The black start system consist of a separate diesel engine or a gas turbine driven synchronous generator connected to station switch gear bus. It can be operated manually from local or remote and also it automatically comes into operation following a black out condition. Capacity of the black start unit should be such that it can supply the total auxiliary power required to start a gas turbine from standstill condition. The NTPC’s Auraiya project gas turbine is provided for emergency black-start purpose and all other projects are provided with diesel generator set for the same duty.
© PMI,NTPC
11
3. Fuel System Fuel System of Gas Turbine The function of the fuel system is to deliver fuel to the combustion chamber(s) of a gas turbine at quantity and pressure as required by the control system Liquid fuel system The liquid fuel system consists of the liquid fuel storage and handling system. The liquid fuel storage and handling system provides means for unloading , storage and distribution of the fuel oil within the plant and typically composed of the following major components: •
Fuel oil unloading pumps
•
Fuel oil transfer pumps
•
Fuel oil storage tanks
•
Flow meter
•
Strainers
•
Pressure and Level control stations.
•
Distribution piping
The number and size of equipments is site dependant. The storage facility is dependant on such factors as the location of the site, proximity of the oil supply and the reliability of the oil supply. Fuel oil may be supplied to the plant by a pipeline, oil barriages, oil tankers, rail/road or high way trucks. Fuel oil unloading pumps may not be required if the oil arrives by a pipeline, an oil barge, or an oil tanker since they have their own pumps.
© PMI,NTPC
12
Naphtha Pressurising System By the forwarding pumps,Naptha is pumped upto the GTs and kept under recirculation. For firing the naphtha is the GT,separate pressurising system with high pressure pump, allied filters and measurements and recirculation system is used. This pressurising is required because in the naphtha burner this fuel is mechanically atomised. There is no other medium like air, etc are used for atomisation. Naphtha drain system Since the flash/fire point of this fuel is very low it is designed to drain the oil from the piping burner etc when the system is stopped. Separate drain tanks are kept at pumping stations, which will collect these naphtha and pump back to the main storage tanks. Mode of firing It is provided to fire natural gas or naphtha or in mixed mode of the fuel in the GT Combustion Chamber. FUEL GAS SYSTEM The purpose of the fuel gas system is to take gas fuel from the custody transfer point, process it to the quality and pressure conditions required by the gas turbine and to distribute it and monitor its use. Fuel gas system consists of the off base system and the on base system.
© PMI,NTPC
13
Off base fuel gas System The off-base fuel gas system is typically composed of the following major components: •
An emergency stop valve at the gas inlet to the plant knock out drum
•
Filters
•
Pressure-control station
•
Distribution piping
Optional Components are •
Gas metering station
•
Gas Heaters
The need for any of the optional components is dictated by the conditions specific to each individual application. The final supply pressure required at gas turbine is a function of the gas turbine compression ratio and the control valve and nozzle pressure drops. Large gas turbines typically require a gas supply pressure of about Kg/cm2 with pressure regulated with in +10 percent of set point. The emergency stop valve at the in let to the station is provided to completely shut off the gas supply in an emergency. The gas fuel then goes through a knockout drum. A knock out drum will remove the free liquid and some solids from the gas stream. However, it will not separate out much of the entrained liquids. The liquids separation from gas stream takes place by decreasing the gas velocity in its transit through the drum. Impurities collect at the drum bottom and are periodically automatically purged A level controller opens a drain valve which allows the waste to Adrian tank vented to the atmosphere.
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The filter separators must have minimum of two stages of separation. The initial stage being utilised for removal of large liquid droplets and the final stage for mist separation . Any solid particles carried with the gas stream are separated in the first stage . The first stage elements consist of hollow tubes of glass fibers through which gas flows radially from outside. The separation of carried solids and liquid particles is by interception, diffusion and/or inertial impaction of the droplets on the fibers. The liquid droplets coalesced on the surface drain on to the shell and from there to the drain pot. The second stage mist separator is preferably of the vans type in which liquid separation takes place by subjecting the gas path to multiple changes of direction. The gas analysis is carefully checked from the stand point these point of hydrate formation or icing at the pressure reducing station under the worst ambient temperatures prevalent at each site. If there is any tendency for the above, a suitably, rated gas fired indirect heater is provided ahead of the pressure reducing station. The gas heater ensures a gas temperature sufficiently high such that the after pressure reduction the gas temperature is at least 20” C higher than hydrate forming temperature or the gas dew point whichever is higher. The heater normally utilises natural gas for firing. Heat from the fire tube be utilised to heat a water bath in which the U tubes for carrying the gases to be heated are immersed. The control system ensures constant temperature of the outlet gas for gas flows ranging from zero to full gas flow. A full capacity by is provided so as to enable operation of the fuel gas system is designed to deliver gas fuel to the gas turbine combustion chamber(s) at the proper pressure and flow rates to meet all the starting, acceleration and loading requirements of gas turbine operation.
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Naphtha forwarding system The forwarding system is mainly remote controlled from the GT LCR and CCR. Although there is a possibility to control the main devices locally by a switching the selector switch on local position in the local . By means of three way valves a tank selection is possible. To ensure the standby position of the lines keep the value open. The pumps are protected by the automatic recirculating valves. The condition of the strainers as well as of the filters is monitored by the differential pressure indicators and switches. The safety valves protect the line against the line against over pressure.
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4. Fire Detection And Protection System The combined cycle power station represents a large capital investment in high technology plant and equipment, the loss of which would entail disruption to the power system and a larger reduction revenue. It is necessary to protect this investment against fire damage and to this effect an elaborate fire detection and protection system is being provided throughout the power plant. The areas to be protected by fire detection and protection system in a combined cycle gas project are 1.
Power house building consisting of gas turbine generator and its auxiliaries, steam turbine and its auxiliaries and waste heat recovery boiler equipment and its auxiliaries. •
Gas regulating station
•
Liquid fuel storage and unloading area
•
Cable galleries/spreader room
•
Transformer switchyard and other transformers located in the plant area
•
Various control rooms
•
istrative office and other auxiliary equipment buildings
•
Covered and uncovered stores
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The various fire detection and protection system are : Hydrant System The Hydrant System essentially consists of larger network of pipes both underground and over ground which feeds pressurized water to number of hydrant valves which are located throughout the power station at strategic locations. In addition, monitors are provided to protect gas reducing station and WHRB. Hydrant system is designed considering the plant as “ORDINARY HAZARD” High Pressure Water Spray System The high pressure water spray system applies water in the form of a conical spray consisting of droplets of water travelling at high velocity. The automatic operation of flow control/deluge valve can be through “wet” Detection or “Dry” detection-Initiation System. Separate 2x100% capacity pumps and header is provided for H.V.W. spray system. This system is provided to protect transformer and gas reducing station (for indoor installations) and lub oil storage tanks. Medium velocity Water spray system The medium velocity water system applies water in finely divided droplets travelling at medium velocity to control the fire and the same time renders adjacent areas safe by cooling action. This system is operated manually and protect tanks containing flammable/combustible liquid such as Naptha. The discharge density shall not be less than 10.2 litres per minutes per sq. metre.
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Sprinkler System The sprinkler system consists of a pipe work array filled with water having sprinkler needs at laid down intervals which operates at a predetermined temperature. Conventional cable galleries are not provided in the combined cycle power plant, however the spreader rooms where power cable are is protected by sprinkler system. Detection of fire in cable spreader room is through combination of smoke detectors i.e. “Ionisation” type and “optical” type detectors fully cross-zoned. Beside this, linear thermal sensors are also used. The discharge density for this system shall not be less than 12.2.Ipm/sq.M Foam Injection System To extinguish the fire inside tanks containing flammable/combustive liquids, foam is produced by mixing foam concentrate with water in the required proportion and aerating the resultant solution. Low expansion floroprotein in foam concentrate is used as foam concentrate. Foam injection is carried out manually only. Foam extinguishing system is provided for floating roof tanks storing Naptha, tank car unloading manifold and the dyke area surrounding the tanks. Halon System In NTPC’s earlier gas project, automatic inert gas extinguishing system using Halon 1301 was used to protect electrical and electronic equipments. However, recent scientific researches have indicated that man chemicals such as Halon and Chlorofluorocarbon deplete stratospheric ozone layer. In view of adverse Environmental impact, NTPC has stopped using Halon flooding as well as Halon extinguishers for all future projects.
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CO2 Extinguishing System After deflected of halon extinguishing system for all future NTPC projects, modular CO2 extinguishing system is envisaged in control s, fuel gas control block of Gas turbine (under sound proof enclosure), GT sound proof enclosure, bearing housing within exhaust diff and the cable space below electronic equipment room. This system is designed to achieve 34% concentration by volume in the areas to be protected and are equipped with optical type and ionisation type smoke detectors. Temperatures detectors are installed in the bearing housing within exhaust diff. Fire Alarm System An automatic fire detection system is provided for an early warning throughout the power plant area. This system detects the outbreak of fire at the inception stage and facilities to take remedial action at the initial stage to prevent/reduce damage of costly equipments. Since Combined cycle plant operation is simpler as compared to thermal power plant and the manpower required is less, the computerized early warning system is preferred than the conventional fire alarm system. Computerized addressable type early warring system which have following advantages: i)
The system has provision for automatic sensitivity compensation
ii)
Each detector is individually addressed
iii)
The system has facility for complete data logging.
iv)
The system is on modular basis and can be expanded at a later date.
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v)
The interface units are provided for connecting open type alarm initiating devices such as pressure switches, potentials free contracts etc.
vi)
Exact location of fire can be known from the fire alarm control .
Potable Extinguishers Portable pressurised water type and CO2 type extinguishers are provided at strategic locations inside the entire power station area. For 600 MW combines cycle power plant, approx. 75 nos. water type and 100nos. of CO2 type portable extinguishers are provided.
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5. Salient Features Of NTPC Gas Projects A.
ANTA GBCP STAGE I (413 MW) The plant is located near Anta Town of District Kota in Rajasthan. The nearest railway station is that of Anta which is approximately 1 km.from plant site. The branch offline of HBJ pipeline is approximately 15 kms.from the site, from where a tap offline is taken. The Kota right main canal which is the source of plant water requirements, runs along the plant boundary. In the main plant block, the gas-turbines and the steam-turbine are housed in separate buildings. There are 3 GTs in are one hall and one ST in another building, with GT hall towards the west. In the extension stage of the project, the mirror image of stage I is anticipated and after completion it shall have 2 GT, halls with 3 GTs on each side of the ST hall having 2 STs. The extension is planned towards the east of stage I. The GTs are placed transversally. The control building is adjacent to ST building. Each GT is having one local control centre located towards south of GT hall. The transformer yard is towards south of turbine hall and switchyard is further south.
B.
KAWAS GBCP.STAGE I ( 640 MW ) The plant is located near village Kawas in District Surat of Gujarat. The nearest railway station is that of Surat which is approximately 19 kms. From site. In the main plant block two modules each consisting of 2 GTs and ST have been provided. The turbine building houses two pairs of GTs at the east and west ends with 2 ST in the middle. Here all GTs and STs are
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placed longitudinally. The common control building is located adjacent to the ST hall towards north. Each WHRB is located towards south of the respective gas turbine. Each GTG is having one local control room which is located towards north, below the inlet air filter. The transformer yards is in north of turbine hall and switch yard further north. C.
AURAIYA GBCP-STAGE I (650MW) The plant is located in Etawah District of Uttar Pradesh. The plant site is approximately 2.5 kms. from the nearest railway station of phaphund. In the main plant block, two no of STGs are located in between 2 pairs of GTs. GTGs are placed transversally. The central control room with 220 KV switchyard control room is located close to STG-I. Local control of each GTS is located near the GT pedestal itself. The water pre-treatment plant and DM plant are located adjacent to both the power house and CW pump house. Induced draft cooling towers have been located considering the proper flow of cooling water, nearness to main power house & convenient routing of open return channel to CW pump house. The 200/400 KV switchyard has been located in front of the power station.
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D
DADRI GBCP-STAGE I (817MW) General Layout Plan In the main plant block two modules, each consisting of two GTGs placed on each side of 2 STGs. The central control room is located towards west of the ST hall. The transformer yard Is on the western side of the turbine hall, with switchyard further down west. Induced draft cooling towers have been located considering the proper flow of cooling water. Nearer to main power house & convenient routing of open return channel to CW pump house. The 220/400 KV switchyard has been located in front of the power station. The
220 KV switchyard control room is accommodated in the central
control room itself. Space has been kept for liquid oil installation and oil unloading facilities. The GAIL terminal for receiving gas is located within boundary of plant site.
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6. The Modern Large Size Gas Turbine, Type GTI3 The main feature of this gas turbine is the simple design, one compressor, one combustion chamber with one burner, a 5- stage turbine arranged on a common shaft with the 17- stage compressor, the shaft being mounted on two journal bearings, the axial thrust absorbed by one thrust bearing as well as an exhaust gas diff which permits an easy connection to a waste heat boiler. With the increasing operational experience the coupling output of this type could be considerably raised by gradually increasing the gas turbine inlet, temperature. This was made possible by cooling the blades of the 1st and 2nd stationary and rotating blade rows or total, with the typical gas turbine, by cooling the first three stages and by an intensified cooling of the rotor and the gas turbine blade carrier. The positive results expected by these measures such as higher power, higher efficiency and lower specific cost have been fully confirmed by the experience gained in operation. The single combustion chamber was mounted at the side of the gas turbine, and, via a U-shaped double piping, connected to the gas turbine. In the course of the further development of this type it was possible, in spite of the higher gas turbine inlet temperature, to reduce the size of the combustion chamber so that it is now arranged directly above the gas turbine. A dual fuel burner makes it possible, simultaneously or alternately, to use gaseous or liquid fuels at improved combustion conditions.
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The separation of flow surging, during partial speeds, may be prevented also with the aid of several adjustable stationary blade rows. Today,, economic reflections lead to the blow –off solution or to combined system with blade readjustment and blow off facilities in the case of industrial gas turbines. INTAKE AND EXHAUST SYSTEM A typical intake & exhaust system for an industrial gas turbine, comprise the following components: a) Intake system
b) Exhaust system
(i)
Inlet Air filter
(ii)
Inlet air silencer
(iii)
Inlet Duct
(i)
Exhaust duct
(ii)
Expansion t
(iii)
Dampers
(iv)
Exhaust silencer
(v)
By stack
Design, construction and operating principle of the above components are described in the following paragraphs. Inlet Air Filtration The need for filtration on gas turbine inlet arises because of four phenomena: foreign object damage erosion, fouling and corrosion. Foreign object damage is usually defined as the resultant blade damage caused by the ingestion of an object of suitable mass and hardness to create physical destruction.
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Erosion is typically defined as the actual removal of material by collision with a moving object. It is often observed on the compressor blades of a gas turbine due to the exposure to larger sized contaminant particles. Fouling is defined as the deposition of smaller particles on the compressor blades, there by changing the blade shape and degrading the performance of the compressor. It is usually not a permanent situation and can be corrected by cleaning of the compressor section. Corrosion is the loss of material in the hot sections due to chemical reaction. It is typically observed in off shore and near shore environments and is attributable to small salt practices and heavy metals. Design/ Construction and operating principles various type of filtration systems are described as follows: No Filtration In the early days of stationary turbine application, it was widely believed that inlet filtration was not needed, that the turbine could ingest almost anything as long as it was not a large object. Many early units were only provided with a simple screen on the inlet to prevent entry of animals, birds, and any other large object. Moving Media The first major problem undertaken was to deal with the larger particles which caused erosion of the compressor blades. By experience and past history, it is generally accepted that a 10 micro metre particle is a size of sufficient mass and inertia to cause actual removal of the metal in a compressor blade. Two of the earliest designs selected were the roll-type and the travelling screen filters.
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The roll-type filter utilizes a roll of dry viscous treated filter material inserted at the top, fed down through a ageway to take-up reel on the bottom. This material is advanced either on demand via the increasing pressure drop across it or automatically on a time basis. The travelling screen filter utilizes an endless belt of metal elements which es through a through of viscous oil, removing the accumulated contaminants and replenishing the viscous treatment. The visors treatment itself is the medium for tapping the airborne particle. The travelling screen type has many of the same operating parameters as the roll-type (efficiency, pressure drop) with one unique potential disadvantage, that of oil carry-over contamination. Both the roll-type with viscous impingement and the travelling screen type depend upon one primary mechanism to trap the airborne contaminants, an oily surface to which the particle will adhere. If the oil is not present or has been covered over with particles, the removal efficiency will drop off markedly. Both of these types have been used throughout the world with limited success. Today they are both basically obsolete. Inertial Separators Because of the shortcomings of the moving media types, in of large particle protection, the search turned to the inertial separator as a likely candidate for dealing with the larger particles. Inertial separators today are in basically two forms, the vane type and the spin-tube types. The vane type operates in two dimensions and the spin-tube type operates in three. Both use the principle of accelerating the particle and using its own inertia to remove it from the main air stream.
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The primary advantage with an inertial separator is its low maintenance cost and relatively trouble free operation, making it very suitable for remote locations. The need for a secondary power source for the scavenge blower is occasionally a disadvantage, however, the restriction across the inertial separator does not increase with operating time and remains constant under normal conditions. The inertial separator does have proven quite successful in of dealing with the larger particles and have gone a long way in reducing significantly the typical erosion problem with turbines. One major shortcoming has been in areas of extremely heavy dust conditions such as desert where the percentage of particles that do the inertial separator can still be harmful. High Efficiency Media Filter While the previously described inertial separators proved quite effective in dealing with the larger particles sizes, they lack the necessary separation efficiency when confronted with the smaller particles (under 10 micrometers). The advances in design and materials of gas turbines necessitated that fouling and corrosion effects be considered; hence the need arose for the high efficiency media filter. The majority of high efficiency media filters in use today are composed of randomly oriented small filters, either of cellulose or a synthetic material. Such filters depend upon the contaminant particle ing a fiber by one or a combination of three mechanisms: Inertia Impaction, Interception and Diffusion. Multi-Stage Filters While the high efficiency filters do an excellent job of protecting the turbine, they typically have a rather low dust holding capacity and are subject to frequent
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servicing. The solution to this has been to use some form of filtration ahead of the high efficiency filter to extend its useful life. In multistage filters a number of different concepts have been applied. All operate on the basic principle of having one or more stages of some type of device ahead of the high efficiency filter elements to help extend the useful life of the high efficiency elements. Self-Cleaning Filters The self-cleaning or Pulse-Cleaning Filter represents the latest technology in gas turbine filtration. The need for it arose when experience indicated shortcoming in multi-stage systems, namely their finite dust holding capacity. Regions such as the Middle East experienced the problems of excessive filter servicing, turbine damage, lack of proper maintenance, etc., all due to the fixed limit every conventional system has with regard to its dust capacity. The Pulse Cleanings system effectively eliminates the finite limit. The concept of the Pulse-Cleaning filter is relatively straight-forward, a high efficiency filter which, after loading with dust, is automatically back flushed with compressed air and effectively renewed. The Pulse-Cleaning system operation is described here with reference to a basic “module” or collection of high efficiency filters, cylindrical cartridges. The high efficiency filter cartridges are attached to the bottom of a horizontal plate and hang vertically downward: Above each filter cartridge, in the clean air plenum, is a venture flow nozzle. During normal operation, ambient air flows upward, radically inward through the cartridges and then upward through the venture and finally to the turbine. Up to this point the device is operating basically as just a high efficiency filter. Each cartridge is rated for 500-600 cfm at an initial
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pressure drop of 1.2-1.8 inches of water, hence the total number of cartridges required for any installation is controlled by this operating range. Skirts are suspended around the exposed sides of the cartridges to protect them from damage and to force the incoming air to travel upward. As the contaminants build up on the surface of the cylindrical cartridges, the pressure drop across the cartridges will continually increase. When the pressure drop reaches a preset limit (typically 2.2-2.8 inches of water above the “new and clean” initial pressure drop) the automatic Pulse-Cleaning sequence is activated. A pressure switch senses that this upper limit has been reached and activates the timer/sequencer. The timer/sequencer signals the solenoid value to open, releasing the pressure holding the pneumatic air valve. The release of the air valve (typically for only 50-100 milliseconds) allows a “blast” of compressed air (80-100 psig) to leave the air manifold and proceed out through the blow pipe. Above each venture is an outlet hole in the blowpipe. This “blast” of compressed air exist through the hole, enters the venturi, induces a secondary flow into the venturi and effectively “closes off” the flow through the cartridge for a fraction of a second. The compressed air pressure wave continues down into the cartridge and then radically outward, blowing the accumulated dust off the cartridge. In another fraction of second, the cartridge is back in service. A row of cartridges is pulsed at one time. Then 10-20 seconds later the timer/sequencer chooses another row of cartridges and the Pulse-Cleaning procedure is repeated. The timer/sequences continues to sequence the cleaning operation through out the elements until the overall pressure drop reaches a preset lower limit, at which time the cleaning process ceases until called upon again by a high-pressure drop reading. The number of elements cleaned at any instant and the time interval between are chosen so that no significant distortion of the airflow is presented to the turbine.
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Since the pulse-cleaning filter renews it self as needed, the old concept of how much dust a filter can hold no longer applies. The system will maintain the average pressure within its upper and lower limits virtually independently of the ambient dust concentration. Augmented Self clearing system American air filter company has introduced an augmented self cleaning air filtration system (ASC) In which self-cleaning principle is combined with inertial separation, and the separated dust is taken out from the operational area by an exhaust system. Dust laden air enters the ASC intake system and is cleaned by ing through the pack filter elements with in the housing. The clean filtered air exit through the ventures in to the clean air
plenum and on to the turbine in take.
However, by means of the integral secondary air cleaning circuit about 75% by weight of the dust is removed from the incoming air by inertial separation before it reached the pack filter elements. The inertial separated dust together with a small amount of scavenge air is conveyed into the secondary cleaning air circuit where the secondary air fan exhausts air fan exhausts it to the atmosphere at any desired point of discharge. The reverse air cleaning cycle is controlled by a solid state printed circuit timer which is housed in a control assembly. The timer sequentially energizes the solenoid valves that activate air valves to it sharp bursts of compressed air through vertical blowpipes located in the clean air plenum. Orifices located in the blowpipes direct the compressed air horizontally through the ventures into the filter chamber. The momentary pressurization and reversal of air flow dislodges the accumulated particulate from the filter elements, automatically reconditioning the filter for continued uninterrupted services. The dislodged dust is immediately
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sucked into the secondary cleaning air circuit together with the scavenge air. In this manner there is no re entrainment of the dislodged dust back into the filter elements. Entrance to the secondary air circuit is through a strategically located slot at the apex of the “V” formed by two ading pack
filter elements. This
secondary air slot leads into a vertical metal secondary air duct that conveys the particulate to a horizontal dust chute and exhaust fan. Pulse-Cleaning Filters The pulse-cleaning concepts represents the “heavy-weight” in the air filtration industry and is the closest idea so far to a universal filtration system. They are presently in use in the arctic, heavy deserts, off-shore, rural areas, industrial areas, major cities, etc. If there is one thought to keep in mind on turbine filtration, it is to add in favour of “better” filtration, not ‘lesser”,. Considering the cost of a modern gas turbine, the cost of ownership of a more expansive filtration system can be an excellent investment. Silencer A silencer is needed to attenuate the noise while allowing the medium to continue to flow in a duct pipe with least possible hindrance. Silencers work either in absorption principle or in resonance principle In a absorption silencer sound energy when comes in direct with acoustically absorbent material, the air molecules oscillate freely in the medium and thus sound energy is transformed into heat energy by molecular friction. The abortive material is therefore covered with acoustically transparent perforated
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sheeting. If the surface of absorption material is covered with an impermeable layer of dirt, the effectiveness of the silencer reduces. Thus the draw back of these type of silencers is their susceptibility to soiling. Resonator silencer attenuate noise by principle of resonance/interference. These silencers are not affected by loose non sticky dust unlike absorption type silencers. However, because of their effectiveness is limited to a narrow bank of frequency, much larger silencer volume is required attenuate same noise levels as compared to absorbent type silencers. This entails in increase pressure drop & increased cost of material. In gas turbine inlet and exhaust system adsorption silencers with splitter or baffle type arrangement are provided. To optimise pressure drop, and to attenuate noise levels over a range of frequency baffle/splitter are arranged in staggered form. Normally baffles are made of perforated stainless sheets filled with mineral wool felts and pads. Inlet & Exhaust Dust The GT engine, by its very nature, requires a considerable mass flow of air per KW delivered than other conventional engines. Thus, the duct sizes will be larger, if conventional power plant criteria for ducts, gas velocities are used. Of course smaller duct sizes require higher velocities & resulting greater pressure loss. Design consideration or pressure loss. Design considerations or pressure loss influences depends on Fluid dynamic and geometric consideration. The detailed producers for calculation of pressure loss in inlet and exhaust system is not presented here & standard literature on the subject may be referred in case it is required.
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Normally, when a GT unit is supplied on a turnkey basis, GT manufacture optimises the pressure loss in the inlet & exhaust system to maximize output & heat rate of the engine. However, in case GT inlet & exhaust system are ordered are ordered separately, the GT manufacturer specifies corresponding maximum losses in inlet & exhaust system respectively, at which output & heat rate of the machine are guaranteed. Accordingly allowable pressure drop in the dusting system is decided, taking into pressure losses in other system components. COMBUSTION CHAMBER Introduction In the combustion chamber the air delivered and compressed by the compressor is brought to the necessary process temperature by the combustion of liquid fuel (e.g. fuel oil) and/or gaseous fuel (e.g. natural gas). The simple combustion chamber is vertically arranged on the thermal block. Cooling air required for cooling of the hot gas turbine parts is byed. Design Features The combustio9n chamber is an all-welded sheet structure fig-5 and essentially consists of these outer
acket with combustion chamber cover, the bottom and
top combustion chamber liners. The complete finned segments insert and the burner with swirl insert fixed in the burner cover.
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GAS TURBINE UNIT Fig . 5 The combustion chamber outer jacket consists of a creep resistant, low-alloy ferrite sheet steel. A specific BBC structure (patented) is the ribbed body linear with the welded finned segments which has proved a successful design for more than 20 years. The finned segments insert encloses the hottest zone of the combustion chamber, the actual combustion zone, and consists of five rows of welded metal finned segment also called brick, the carrier, the lifting device and the intermediate jacket. Owing to the high thermal stresses of these parts, a special design and carefully selected material are required to ensure a long operating life. They have proven very successful for a long time. The finned segments (“bricks”) made of a highly heat resistant material are, on the fire side, connected by a cladding and protected from the hot flame.
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FINNED SEGMENTS figure. 6 Convective cooling of the finned segments is carried out on the rear side but cooling air flowing between the ribs, and open the fire side by a cooling air film which escapes on from the bottom side on the end of the bricks and is ed over the row located beneath. The finned segments can be easily replaced with the aid of a special lifting device. The upper combustion chamber inert consist of the outside unit and the doublewall interior unit is made of heat-resistant sheet metal welded together. The lower combustion chamber insert is a double-jacket structure made of crimped, non-scaling sheet metal, and is composed of an inner part and an outer part. It is connected to the finned segment insert. By means of the gimbal suspension the lower combustion chamber insert can expand freely . Outer and inner parts are centred against each other by a three-point mounting. For the observation of the combustion zone during operation, several sight glass units are mounted on the combustion chamber. Moreover, there is an automatic flame monitoring devices on the combustion chamber cover. The whole
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combustion chamber is fitted with a thermal- acoust insulation and encapsulated with galvanized sheet steel. For maintenance and checking purpose, the combustion chamber is equipped with a platform, which the access is ensured via a ladder or spiral staircase. Advantages of the Combustion Chamber 1.
Burner -
A burner for the gas turbine in Contrast to gas turbines with many combustion chamber, no measure for an accurate distribution of fuel and air are required to obtain a good temperature distribution.
-
Can be applied for any liquid or gaseous fuels.
-
Standard burner for oil and/or natural gas, consequently suited also for mixed operation.
-
Special burners are available in this field.
-
For liquid low-calorific value fuels such as ethanol or methanol standard burners can be used which only differ the oil burners by the size of their jet diameters.
-
The burner itself can be removed and checked which ensures easy maintenance.
-
The design ensure a good ensures a good atomising of the fuel and consequently, optimum evaporation and mixing with the air which is
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essential to good combustion. The combustion efficiency amounts to > 99.58, under full load > 99.98. -
Since there is only one fuel jet, it has relatively larger openings so that it is less prone to contamination.
-
One ignition for the whole system easier, i.e. higher ignore on reliability than with multiple combustion chamber systems.
Combustion Space The whole dimensioning of the combustion space results in the following advantages. •
No flame with the wall, which means that no combustion takes places near the wall and, consequently, the film cooling air cannot be misused as combustion air (this danger exists with small – size chamber chambers).
•
No oil spraying on the flame tube possible.
•
Combined cooling of convective cooling and film cooling by special cooling elements provided with ribs (ribbed bodies ), which form the flame tube.
•
Thus the flame tube does not consist of one ring, but is formed by individual elements. Therefore, no stability problems (dents) of the combustion wall as is the case with flame tubes.
•
Long dwell time ensure burning out of the fuel (the emission of CO and non-burnt hydrocarbons is practically zero).
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•
Flame radiation independent of the fuel since E of the flame (desire of emission) is nearly 1.0 (black body).
•
The combustion chamber is laid out for easy maintenance since it is possible, after removal of the further including swirl insert, to enter the combustion chamber, so that it can be checked in a very simple manner. Also the hot-gas housing and the first gas turbine stage are easily accessible. (See fig.7)
COMBUSTION CHAMBER Fig.7
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Burner Combustion a)
General description of the duel burner.
The dual burner with its auxiliary facilities is used for the supply of gaseous and liquid fuels to the combustion chamber. Simultaneously, mixing of the fuel with the “primary air “ is achieved with this burner, while the flame is stabilized by an appropriate air supply. b)
Constructional features (fig.8) -
Burner cover
-
Swirl inset
-
Fuel supply with fuel nozzle
-
Servomotor for liquid fuel
-
Gas burner jacket with inner jacket For the supply of the gaseous fuel
c)
-
Nozzle cap
-
Igniter
Combustion The air arriving from the compressor enters the combustion chamber from the bottom and flows upwards. Approximately 30% of the air between combustion chamber outer jacket and other part of the lower combustion chamber liner flow as secondary air via the mixing jets to the combustion chamber inner space while the remainder flows further upwards between inner and outer part. Further distribution takes places at the funnel shaped outlet of the lower combustion chamber outlet. Via an orifice on the lower end of the intermediate jacket another 30% of the air are branched off for cooling the finned segments. This air then is
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dispersed on the partition walls and flows between the cooling ribs downwards into the combustion space, while a layer of protective cooling air is formed on the flame side of the ribs which protects the ribbed bodies from adverse heating. The remaining 40% flow as “primary air” i.e. as oxidizing agent for the combustion, into the swirl insert in the air enters the combustion space in a turbulent state. This ensures a very good mixing with the fuel. Moreover, the swirl insert is used for a good stabilization of the flame. Another portion of the “primary air” flows through four orifices in the gas burner jacket into the annuls between inner jacket and inner wall of the gas burner jacket and enters the combustion space directly at the fuel nozzle with a turbulence caused by the inner swirl insert. By this type of air guide a very intensive mixing with the fuel is attained which ensures a complete and rapid combustion. The ignition of the fuels during starting is effected by the igniter with a propane flame.
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After ignition of the fuel, the hot combustion gases flow downwards. Before they enter the turbine, these gases are well mixed with the secondary air, coming from the mixing orifices and brought of a temperature issible for the turbine. Figure-9 shows a schematic diagram of the streamlines.
The air enters with swirl to the combustion space through the primary air inlet (swirl insert). Due to this flow scheme (cyclone principle), a back flow to the burner jet is produced. This reduces the flame front speed, which ensures a stable flame. The cyclone effect is terminated by the inflowing cooling air at the mixing orifices (D). Fig-10 shows a fuel jet liquid fuel is injected at a very high pressure and finely atomized.
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FIG-10 TURBINE In the gas turbine, the power for driving both the compressor and the generator is produced. The energy is supplied to the turbine in the form of the hot gases as they come from the combustion chamber. The kinetic energy of the gases is transferred to the rotor in the turbine by means of the rotating blades. In the following this process is explained with the example of a gas turbine stage. The combustion gases, coming from the combustion chamber or from the preceding stage, are accelerated in the stator blade to the speed C while, © PMI,NTPC
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simultaneously, the inlet pressure is reduced. By vectorial addition with the circumferential speed the relative speed w is obtained. Depending on the degree of reaction this speed is accelerated to W2 in the rotating blades row (fig.11). On
the other hand the absolute speed C2 is
obtained by the addition of W2 and U2: The pulse change of m.W1(mass by speed) into m.W2 generates the driving power acting on the blade.
TURBINE BLADES FIG.-11 PT = PV+PN = mR. ∆ ht . Pt
=
Gas turbine power in KW
Pv
=
Compressor driving power in KW
Pn
=
Generator effective power in KW
Ms
=
mass flow in kg/sec
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∆ht
=
Isentropic turbine gradient in kj / kg
=
effective efficiency,
it
=
internal turbine efficiency, approximately 0.80 to 0.87
it
=
mechanical efficiency, approx. 0.99
e
=
it
.
m
Example with partial data of the GT 13 D2 m
=
372kg/sec;
P1
=
11.6bar t1
=
990oC
P2
=
1 bar t2
=
490oC
∆ht
=
680 kj/kg for fuel oil
Hu
=
42000kj/kg
PT
= 372 kg/sec; 680 kj/kg; 0.83
=
212.5 MW
PN
= PT - PV
=
212.5-124.5=88.0MW.
DESIGN FEATURES Talking into its thermal load the turbine housing made of creep-resistant ferrite mater raise as welded structure or steel casting. On the exhaust gas side the inner cone of the turbine housing is connected to the outer wall by ribs. Inner cone and outer wall form in this area the annuals for the escaping exhaust gas. Because of the relatively high exhaust gas temperatures the walls and ribs of this duct in the gas turbine housing are cladding with a sheet metal lining. The
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sheet metal lining is made of a temperature resistant material and fastened in the turbine outer housing allowing for thermal motion. The bearing
housing
made of cast steel for the reception of the journal bearing on the exhaust gas side is suspended allowing for thermal motion in the inner cone of the outer housing bottom part. The stationary blade carrier made of alloyed ferrite cast steel is cooled with the aid of compressor air which enters through appropriate holes and air inlets. To keep the thermal flux and the temperature stresses in the stationary blade carrier(fig.12) low also during start-up and shutdown of the machine the side exposed to hot gas is provided with heat shields which are radically suspended.
BLADE CARRIER Fig-12 The turbine blading consists of five stationary and five rotating blades. Because of the continuously increasing turbine inlet temperature the number of cooled
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stages is increased(from a stage at the beginning with 13D2stages on 13D2to 3stages on the 13Etype). The cooled blades are manufactured of nickel-base alloys in a precision casting process. Different types of blade cooling methods (fig.13)
BLADE COOLING Fig-13 In convection cooling air flows through holes smooth or ribbed ducts (figure 14) and absorbs part of the quantity of heat transferred by the hot gas into the blades. Depending on the design, the heated cooling air is mixed to the hot gas on the blade trailing edge, on the blade tip and or an the blade.
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GUIDE VANE CONNECTING COOLING Fig-14 DEFLECTOR COOLING- which also called jet cooling is also type of convection cooling. However, the cooling air is ed through the sheet metal deflector plates. These deflector inserts are provided with holes so that the cooling air bounces as jets on the blade wall to be cooled.
TURBINE COOLING SYSTEM FIG-15
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7. Gas Turbine Compressor General The characteristic feature of this turbine generator set is the use of a single, composite welded shaft for the compressor and turbine gas rotor, carried in only two bearings. This enables the complete compressor/turbine assembly to be of very compact construction and ensures good shaft stability with a simple bearing arrangement of two journal bearings and one thrust bearing. The welded rotor concept has been in use for many years in the steam turbine field and this design has also proved superior for the gas turbine. In comparison to a rotor comprising several disc held together by a tie rod. The welded rotor has several important advantages: the welded discs have o holes in highly stressed areas, especially in the center, and dangerous stress concentrations are therefore avoided. The welding s the discs together into a single shaft without mechanical s in which misalignments caused by centrifugal force or thermal expansion cannot occur. This is particularly important with large shafts. The commonly- observed heavy vibrations caused by rapid acceleration and load application are completely avoided. The welded designs avoids all the problems which can otherwise occur due to the above- mentioned mechanical s. In addition, this design guarantees maintenance free service of the rotor. The compressor and turbine casing consist of several sections which are ed together to form a single unit. Design and function of Individual Components of the Compressor/Turbine Assembly
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Air Intake connection The air intake connection has a horizontal air inlet, and is situated axially in front compressor. It is a welded, solid steel plate construction split horizontally and held together with hexagon bolts and nuts. The attachment to the compressor casing is effected by two guide flanges welded on to the connection which locate in the appropriate slots turned in the compressor casing. Sealing tubes made of synthetic material are laid in these slots and form a seal against the environment. The air intake connection has four feet which are bolted with adjusting belts to steel plates embedded in the foundations. These bolts allows the height to be adjusted to that of the rest of the compressor/turbine assembly. Compressor casing The compressor casing, which is horizontally spilt as axis height is made of spheroidal graphite cast iron. This material possesses high tensile strength and good expansion qualities and for this reason is well suited to the stresses and high temperatures in the pressure and caused by the compression process. The upper and lower halves of the compressor casing are provided with robust flags and are held together by expansion studs with sockets head nuts. To bearing housing for the intake end journal bearing and thrust bearing is integrally cast with the
help of the compressor casing. The bearing cover
is bolted on to the bearing housing. The oil glass for the thrust bearing and the axial shaft position monitor are mounted on the. The flanges of the bearing cover and housing are sealed with an oil resistant ting The bearing cover and housing incorporate all the necessary grilling for lube oil purpose, for the sealing air of the intake end shaft seal and for temperature monitoring.
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The lower half of the compressor casing is also fitted with paws for the fixed s at the intake end and for the pin-ended s at the pressure end, as well as the flange for the king pin
which is situated at the intake end and
determines the fixed point form the complete compressor/turbine assembly. The compressor casing has three integrally- cast circular ducts, which are connected to the blade duct behind the fourth, seventh, and tenth rows of fixed blades. These ducts are closed to the outside by four blow-off values flanged directly to the upper half of the compressor casing. the blow-off values are controlled pneumatically and are open at lower speeds,i.e.up to approx 90%of the rated speed, so that a certain amount of the compressor output can be blown off. the air blown off flows into the machine room through the blow-off valve bonnet arranged around the valves and a silencers in that order. these blow-off valves enable a consideration reduction to be made in the external power input requirements during start-up, and they also avoid undesirable and dangerous flow conditions in the compressor which result in rotating stall and pumping effects. Turbine outer casing The outer casing is split into two halves the same as the compressor casing. the turbine and compressor casing are bolted together at a radial flange with expansion bolt. the casing parts are centered by means of eight round wedges in the circumference. The turbine casing is made of heat-resisting ferrite steel in order to withstand the thermal stresses, and is of welded construction. the upper and lower halves are bolted together at the horizontal flanges with expansion studs, expansion sleeves and socket head nuts. The horizontal flanges are of solid construction and are slotted at the drilled holes so as to avoid high thermal stresses. Horizontal and radial flanges of the turbine and compressor are metal-to- metal ts.
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The inner cone and outer wall of the turbine casing are ed together by ten ribs at the exhaust end, and the resulting ring duct between the two constitutes the flow age for the exhaust gas from the turbine. Due to the relatively exhaust gas temperature, the walls and ribs of the ring duct are lined with a special plating to prevent the temperature differences between various parts of the casing walls and therefore, the thermals stresses during transitional operating phase (e.g. starting up and shutting down); from exceeding the allowable limits. The planting is manufactured from temperature resistant materials and is attached to the turbine outer casing in such a way as to accommodate thermal expansion. A cast steel bearing housing is mounted in the inner cone of the lower half of the outer casing and provides the for the exhaust end journal bearing. The housing is free to thermally expand. The bearing housing is retained in the turbine outer casing by means of integrally cast guide at axis height and by a key collar bolts are provided to prevent lifting. The reaction forces of the journal bearing are transmitted through the lower half of the inner cone and the above-mentioned ribs to the turbine outer casing. From here are transmitted through the pin-ended s to the unit . The trip and mechanism for the two over speed trips, which are mounted at the rotor are installed on the bearing housing. The oil supply line mounted on a cover on the hearing housing enables the over speed trips to be checked while the turbine is operating by supplying oil pressure to the underside of the over speed bolts. Ventilation and oil wiper rings are inserted in the bearing cover and bearing housing and also in the intake end bearing housing and cover to prevent the escape of oil. Two air deflector’ plates are welded in to the lower half of the outer casing to stabilize the flow. In addition two paws are cast integrally with the lower half of the turbine outer casing, and the pin-ended s of the exhaust end are bolted to these paws.
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The vertically arranged combustion chamber is mounted on a horizontal flange welded on to the upper half of the outer casing. In addition, the outer casing has a circumferential coller for the blade carrier. Blade carrier The blade carrier for the turbine fixed blade is a ferrite steel alloy casting and is axially spilt the same as the compressor/turbine casing. It is bolted together with expansion bolts, expansion sleeves and hexagon socket head nuts. The blade carrier is fluxed axially by a collar in the turbine outer casing, which fits in to a salt around the blade carrier circumference. Paws are integrally cast with the parting flange of the lower half of the blade carrier at the same level as the outer circumferential salt. These paws have hardened pads fixed to them and together with the corresponding surfaces in the turbine outer casing they form the vertical for the blade carrier. An axial salt, is machined in the lower half of the blade carrier at the same level as the circumferential slot , and the guide pin fitted in the lower half of the turbine outer casing locates in this axial slot hold the blade carrier in its lateral position. The external pressure of the compressed combustion air on the blade carrier causes it to be pressed against the collar and forms a metal-to-metal seal to the exhaust end. This air also cools the blade carrier at the same time. A more intensive cooling of the blade carrier is achieved by allowing a small amount of air to flow through the appropriate holes and circular ducts. This air flow results fro the relatively small differential pressure between the compressor exhaust and turbine Intel and is sufficient to maintain the temperatures within the allowable limits. This air stream reenters the hot gas flow channel before the first row of fixed blades. Radically ed heat storage segments are fitted between the rows of fixed blades in the areas exposed to the gas flow, similar to the heat shield of the turbine end of the rotor. These segments contain the heat flow and thermal stresses within the blade carrier at a low level during start-up and shut-down.
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Owing the absence of centrifugal stresses these segments can be mounted in the blade carrier more simply than on the rotor. The cooling air deflectors fitted to the upper half of the blade carrier are to improve the cooling of the corresponding inner casing walls. Combined compressor- turbine rotor The combined compressor and turbine rotor comprises the individual discs which are welder together to form the compressor and turbine blading. The eleven discs constituting the body of the shaft are centralized with one another by circumferential recesses and connected together by a special welding process. The complete shaft comprises seven compressor discs, and a center drum connecting the two section made of a ferritic forged steel alloy. The end discs have forged journals, and the one at the compressor end is fitted with a coupling flange and thrust bearing caller. The complete shaft is stress-relieved after welding. The blade rotor is then over speeded to 20% above the rated speed and balanced. The compressor –turbine rotor is carried by two journal bearings which are force-lubricated and cooled. The bearings are each held by four fitted plates and are each prevented from rotating by a solid pin. The bearing are lined with white metal and the radial clearance is approx 1.5%. A lubricating oil film is formed by the pressurized oil, which enters the bearing from both ends and is forced in to two lube oil groves. The lower halves of the journal bearing are connected to jacking oil pumps which deliver high pressure oil to the bearing to lift the compressor- turbine rotor from the lower shells thereby reducing the initial breakaway torque.
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COMPRESSOR TURBINE ASSEMBLY Fig-16 The axial thrust of the rotor is taken up by the thrust bearing situated at the compressor end. This bearing is of the pad type and is also lubricated and cooled by pressurized oil. Since the thrust on the turbine balding is in opposition to that compressor balding the resulting differential axial thrust remains small. Moreover, this thrust always acts in the same direction, i.e. against the gas flow and
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towards the generator. The thrust bearing also fixes the axial position of the rotor in the casing. Both expand in the same direction since the fixed point is given by the king pin for the complete casing and is situated in the area of the thrust bearing. A shaft monitor is provide which measures the axial displacement. This gives an alarm in the event of abnormal displacement and then gives a trip signals for the unit when a maximum value is reached. The axial clearance required can be adjusted at the bearings. The surface of the rotor between the rows of turbine blades is protected against excessively high temperatures by heat storages segments. These subdivided the hot upper surface of the rotor into small rectangular zones. They can expand freely in all directions independently of the exhaust end of the shaft. In this way the major part of the heat which enters the rotor from outside is conducted away just below the surface of the rotor, so that the amount of heat reaches the actual body of the shaft remains small. This results in a relatively slow warm-up of the shaft body and therefore high thermal stresses cannot occur. A similar effect is experienced during shutting down when the flame is extinguished and a relatively cold air stream flows through the machine while it slows down. The lifespan of the rotor is extremely good with this method or construction. The cooling air required for the turbine part of the rotor, the feet of the turbine rotor blades and the first and second rows or rotor blades is taken directly from the compressor flow at the diff after the last row of fixed blades. The air flows through the channels between the compressor diff and the shaft cover to a ring nozzle, and then to the turbine part of the rotor. Here it es through drilling in the projecting shoulder of the rotor and flows through the cooling air ducting system between shaft and heat storage segments and also around the blade feet. After the fourth rotor blade row the cooling air exhausts into the gas steam from the rotor. It then flows over the feet of the last row of rotor blades and leaves with
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the exhaust gases. Part of this cooling air flow is used to cool the rotor blades in the first and second rows. The cooling air flow is monitored by measuring the pressure difference across the ring nozzle of the nozzle of the compressor diff. The sealing
air for the shaft seal cools the end of the rotor at the
exhaust end. Shaft seals are formed by thin sheet metal strips caulked into grooves in the rotor by means of caulking wire, operating at a small running clearance with either the shaft seal shell or the corresponding part of the casing, respectively relatively few sealing strips are required to give an adequate seal since the pressure difference at the shaft seal is small. The exhaust end shaft seals prevent the gas from escaping to atmosphere. Those at the intake end prevent the ingress of oil laden or otherwise contaminated air from the bearing regions into the flow duct/ both these shaft seals have a sealing air supply taken from the blow off duct after the fourth row of fixed blades in the compressor. At the exhaust end draft seal the sealing air supply comes through the shaft seal shell, thus cooling the rotor end as well as explained above. The shaft seal at the air intake end is divided into two groups of sealing strips. Here the sealing air enters the annular space between two groups via the appropriate ducts and holes in the compressor casing. The shaft seals situated between the compressor and turbine prevent the leakage of too much cooling air. Pressure tapping are located before and after the sealing strips of the shaft seal directly below the first row of turbine fixed blades. These are used to measure the pressure difference, which is necessary to monitor the correct functioning of the shaft sealing. Compressor balding The compressor balding is designed as reaction balding and consists of eighteen stages with one additional inlet guides row. The inlet guide row serves to correct the flow direction for the first row of rotor blades. The blades of the eighteen rotor and nineteen fixed rows are manufactured from high tensile ferric chrome steel.
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They are either precision forged or machined, and fit into slots which are turned in the shaft and compressor casing. The blades are separated from one another by spacers. The blades of the first seven rows are rooted directly in the shaft, those of the remaining rows are inserted in intermediate pieces. Turbine Blading The turbine blading is also of the reaction type, and consists of five rotor and five fixed rows of blades. Due to the high temperature of the incoming gas, the first and second rows of the rotor and fixed blading are cooled with air from the compressor. The cooling air is fed to the first and second rows of fixed blades through holes drilled in the blade carrier and to the first and second rows of rotor blades through drilling in the shaft and through the age underneath the heat storage segments. After entering the blade foot, the air es along several parallel ducts into the blade proper. Each air stream changes direction several times inside the blades in the first and second fixed blade rows and before finally blowing out through a number of slits in the trailing edge of the blade. Part of the cooling air in the first row of rotor blades and the second row of fixed blades and the entire cooling air flow in the second row of rotor blades is blown out through the blade tips. The first row of fixed blades is film- cooled as well, whereby the air leaves the blades through numerous small holes and forms a protective film over the blades surface. This method of cooling ensures that the leading and trailing edges of the blades in particular are thoroughly cooled, thereby avoiding cracks induced by thermal stresses. These cooled blades and the fixed and rotor blades of the other rows are precision cast in a nickel –based alloy. The fourth and fifth rows of rotor blades and the fifth row of fixed blades are precision drop-forged.
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The first row of fixed blades sits in an axial slot in the carrier segments, which are in turn fitted into a slot in the blade carrier. The specially-formed ends of these blades are ed in the shroud segments which are mounted in a radial slot in the compressor diff, which extends as far as this point. The remaining fixed blades have rhombic shaped feet which fit into the appropriate slots of the blades carrier. The first row of rotor blades are fitted into radial double. “T” slots in the rotor while the fifth row fits into angled fir tree slots. The so-called “super materials: and high alloy steels used in the turbine blading are all resistant to extremely high temperatures and to scale. Their anticipated service life is calculated on the respective prevailing temperature and resulting stresses. An additional protective measure has been taken to counteract corrosion in the form of a specially applied chrome layer on the first rows of blade. This increases the service life of the blades considerably. Compressor Diff The main function compressor diff is that of flow device, namely reducing the relatively high velocity of the combustion air from the compressor blading while keeping losses to a minimum, and ensuring a favorable flow into the turbine outer casing and combustion chamber connection. The compressor diff is branched into a radial and an axial section, so giving a favourable flow distribution which produces uniformly intensive cooling of the entire inner casing surface. The shaft cover and the inner shell of the compressor diff together form the annular duct for the cooling air for the turbine section of the rotor. The nozzle segments are fitted in circumferential slots in the diff at the outlet of this annular duct. These nozzle direct the cooling air flow with
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minimum losses to the rotor. At the inlet to the turbine blading, the end shroud segments of the first row of fixed blades and the inlet segments together form a heat shield for the wall of the compressor diff. Both segment types are inserted in circumferential slots in the compressor diff. The compressor diff and the shaft cover are heat resistant spheroid graphite iron castings. They are both splits horizontally, the same as the rest of the casings in the compressor/turbine assembly. The top and bottom section of the compressor diff are bolted together at the parting flange with expansion bolts. The complete compressor diff is flanged at the front end to the lower half only of the compressor casing, and is cantered by a collar on the front flange which fits into a corresponding recess turned in the compressor casing. Inner Casing The inner casing guides the hot gases coming from the combustion chamber to the turbine blading. It is a relatively thin walled welled construction made of heat resistant chrome nickel austenitic alloy. The outside of the inner casing is cooled by the combustion air coming from the compressor diff. It rests on s in the turbine outer casing is split axially, and is held together by expansion bolts made of high temperature resistant materials. All flanges bolts of the inner casing are secured by special locking devices so that they cannot become loose during operation. The inner casing is ed by two flange rings at its outlet which fit into corresponding recesses turned in the blade carrier and compressor diff. This prevents axial movement and at the same time guarantee good separation between the hot gas and compressed combustion air in the turbine casing. A drain connection is arranged in the lower half of the inner casing which is used
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for draining fuel which has not burnt due to ignition failure (only with oil or dual fuel operation). Special equipment and the associated guide bolts must be used when fitting or removing the inner casing, and the instructions must be followed implicitly. Exhaust Diff The exhaust diff is a single unit of welded steel plate construction to the turbine outer casing by two radial flanges forming metal to metal ts, and guides the relatively hot exhaust gases into the exhaust gas system. Expansion bolts with hexagon head nuts are used to bolt the diff to the turbine outer casing. The inner part of the exhaust diff, which is annular in shape, is essentially held by three oval- shaped, hollow plate ribs. These serve at the same time to permit access to the diff up the exhaust end bearing, for the installation of pipe- work, and also for ventilation purposes. The sheet metal cover of the thermal installation on the inside of the diff is welded over a third of its circumference to make it oil tight and so prevent smouldering which might occur as a result of oil leakage. A leakage drain is fitted to the lowest point of the diff unit. A traveling hoist can be fitted to one of the rails inside the exhaust diff and this assists in the assembly and disassembly of the exhaust end bearing during erection and overhauls so that bearing parts can be lifted in and out through the upper hollow rib. The exhaust diff is ed on the foundations through two adjustable spring s. These take up small relative movements between the exhaust gas system, exhaust gas diff, and machine foundations.
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The complete compressor/turbine assembly is ed on both sides at axis height by four pin-ended s and two fixed s: These rest on the unit s that are bolted to the anchor plates which are in turn cast into the machine foundations. The casing rests on two fixed s at the intake end between which is a solid king pin which fixes the position of the compressor/turbine assembly laterally and longitudinally. The king pin, which is situated below the machine in line with the central axis, takes up the forces which are transmitted by the exhaust gas system. Longitudinal movement of the compressor/turbine assembly is guided by the guide key below the turbine flange. In general, all s are principally of welded construction. Insulation The complete outer surface of the compressor/turbine assembly is thermally insulated by two layers of lagging mats. These are made of mineral fibre cloth with a sewn in mineral wool insult ant and are laid on the casings in a staggered pattern attached to one another by books and eyes. The method allows easy and non-destructive removal during overhauls. The exhaust gas diff is thermally insulated and soundproofed with the special insulating material consisting of several layers. This is covered with the galvanized steel sheet. Insulation of the air intake connection is similar to the exhaust gas diff. The enclosure over the compressor/turbine assembly, ie. the gas turbine unit, is free-standing on the machine foundation and can be
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walked on for access purposes. It reduces the noise level and protects the insulation from damage. It also improves the appearance of the unit. Compressor Section Compressors for gas turbines may be centrifugal or axial flow type, but all large gas turbine use axial flow compressors. Gas turbine compressors are multistage design and consists of individual discs or wheels mounted on a shaft. Each disc holds the compressor blades which extend rapidly. This complete assembly is called the compressor rotor and is enclosed in the compressing casing. The casing holds stationary blades which direct the air flow against the rotating blades. The stationary assembly is called the stators. As the blades force air through the compressor, the casing area decreases and air is compressed through each successive stage. Early axial compressor designs were almost all based upon constant axial velocity over the length of the compressor. This constraint simplified some flow analysis and together with other arbitrary assumptions such as constant inner diameter or constant diameter enables analytical and design team to cope with a complex engineering problem. The large investment made in such design approaches has resulted in their persistence in present day gas turbines. Over time, the body of Knowledge of the processes actually occurring in axial compressors grew and economic factors related manufacturing costs became more important. Also, manufacturing processes and materials advances gave more freedom to compressor designers, and thus a different approach to design was taken. The results of this different and more sophisticated approach are axial compressors with variable axial velocity distributions and variable inner and outer diameters. Such compressors are generally capable of providing superior performance characteristics with fewer stages than earlier compressor. However, design of
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gas turbine compressor design is fixed for a particular machine by the manufacturer. It is not an option to be selected; rather it is part of an overall piece of equipment. The compressor must be designed and operated so as to avoid ‘compressor surge’. The surge characteristic of compressor is a complex function of many compressor design features. It can also be affected by compressor deterioration and blade fouling in operating units. In order to avoid compressor surge, each stage of the compressor must maintain its overall flow situation. When a breakdown of this overall flow situation occurs in any one stage, the result is usually disruptive to the flow situation in subsequent stages and surge occurs. Under such conditions, intermittent flow and conditions such as ‘puffing or rotating stall(in which only a part of one ore more stages is in surge while the remainder is in steady flow ) can occur . In such conditions are found to occur at rotational speeds high enough to pose the danger of mechanical damage to the combustion turbine unit, the situation can be alleviated by :1) providing inter stage relief valves at one or more points along the compressor or 2) equipping or more stages of is based on years of research and development and the compressor stationary blade rings with variable geometry provisions. The first method maintains a viable axial velocity distribution by relieving axes flow from the later stages of the compressor . The second method controls the flow produced by the variable geometry stages to be within the capacity of the later stages of the compressor. The method of surge control is inherent to the design of each particular gas turbine and is left to the manufacturer. Combustion Chamber (Combustor) In large industrial type gas turbine, two types of combustion chambers are used. With an annular type combustors, combustion occurs in numerous combustion chambers located radially around the compressor discharge casing. Some manufacturers use a single or dual ‘silo’ type combustor, which is a large
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chamber, mounted on top of the turbine. Each combustion chamber includes fuel nozzles, and ignition system and flame detectors. Each combustor is enclosed in a casing and liner for and insulation. Compressor discharge bleed air is used to cool the combustor. There are advantages and disadvantages to silo-type combustors when compared to multiple in-line type combustors. Silo-type combustors provide better combustion when using liquid fuel. Nox emissions are not easily controlled with a silo-type combustor. However, external silo-type combustors have greater pressure drop than then in-line type combustor arrangement. Turbine Section The turbine expands the hot gases from the compressor and combustor and drives the compressor and generator. The turbine is constructed similar to the compressor. Turbines are multistage design and consist of individual discs or wheel mounted on a shaft. Each disc holds the turbine blades (buckets), which extend radially from the shaft. This complete assembly is called the turbine rotor and is closed in the casing. The casing holds stationary nozzles, which direct the gas, flow against the turbine blades causing the rotor to rotate. Generator Assignment The generator can be located either at the hot and (exhaust) or the cold end(inlet) of the gas turbine. With a hot end generator, the exhaust system becomes more complicated and direct axial exhaust into the HRSG is not possible with a hot end arrangement. Using a cold end generator arrangement may complicate the air inlet system. It may also complicate arrangement of the starting drive and/or accessory gear. Either arrangement is acceptable and the location is the manufacturers choice based on his experience.
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8. Waste Heat Recovery Boiler INTRODUCTION What is waste heat? It is heat that is discharged in to the atmosphere without recovering the available energy that might be economically converted in to useful purposes. Then what is wastes heat recovery system? it is a system of heat exchangers which converts this heat in to a more usable form of energy. The utilization of the wastes heat recovery system in combine cycle power plants with gas turbines is a relatively recent development brought by the development in the field of gas turbines resulting in gas turbines becoming larger in size and output, the large volume of heat available form gas turbine exhaust and rising fuel costs. One of the main features of the gas turbine is the high mass flow of air used over the stoichiometric quantity of air. Typically volume of [excess air factor] for gas turbine combustion lies between 3 to 4. this means there is more than sufficient oxygen to secondary combustion, if required. This high mass flow in turn makes the gas turbine exhaust an ideal candidate for heat recovery steam generation. Description The basic wastes heat recovery boiler (WHRB) consists of an evaporator which is the basic steam generator, and an economizer which heats the incoming feed water to the evaporator. Super heaters are also furnished, and condensate preheaters (low pressure feed water heaters/ are sometimes used depending on need and circumstances.)
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A complete wastes heat recovery boiler would also include an inlet gas by- damper, with by stack and silencer, exhaust stack with monitoring provisions and all interconnecting ducting between the gas turbine and exhaust stack. Other WHRB accessories include the drum level sight glasses and level controls, pressure gauges, continuous blow down control and sampling stations. The WHRB evaporator consists of a steam drum, interconnecting unheated down comer tubes, interconnecting heated riser tubes, enclosures casing and accessories. The evaporator receives the incoming heated feed water from the economizer and generates saturated steam which is taken form the top of the steam drum, the evaporator will operator at constant saturation temperature corresponding to the drum operating pressure. The steam drum is locked at the top of the evaporator to enable natural circulation to take place water steam level will be maintained in the steam drum during operation. The economizer is a feed water heater operating at the steam drum pressure utilizing the heat left in the exhaust gas after the gas leaves the evaporator. The economizer will heat the incoming feed water to the evaporator Super heaters are basically steam heater receiving saturated steam from the steam drum and producing superheated steam. Super heater are located ahead of the evaporator in the WHRB exhaust gas flow stream in order to develop required superheated steam temperatures. Low pressure feed water heaters also called condensate perheaters’ are sometimes used on combined cycle units to heat the desecrator feed water thus the cycle required less desecration steam making it more efficient. WHRB exhaust gas by – are commonly provided to isolate the WHRB form its heated source. If furnished this isolation will permit running the gas turbine at times when’s steam is not required or when WHRB is not available. Normally the
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by dampers will have a straight open/close function though some designs can also provide degree of modulation. A by stack complete with silencer is required and provided when by- dampers are used Duct burners can be used to raise the temperature of the exhaust gas to produce more steam. However, duct burners are generally not used on combined cycle plants due to the low rankine cycle efficiency as compared to higher combined cycle efficiency. Design considerations a)
Number of steam cycle pressures: The general design philosophy of the WHRB is to exhaust heat form the gas to the fluid at the highest temperature difference available. This can best be accomplished by making the gas and the fluid [steam or water] temperature gradients as nearly parallel to each other as possible. With a single pressure level boiler, the choice is very limited since the super heater, evaporator and economizer have to be placed in descending fluid temperature along the gas path, in a multi pressure level unit this general order is maintained, but various section may be interchanged so that a nearly parallel relation between the temperature gradients can be achieved. In a single pressure boiler there is a limited amount of heat that can be extracted, because the exhaust gas temperature cannot be reduced below the steam saturation temperature. However, with a multipressure system the recoverable heat range can be extended by extracting heat at various levels, as the saturation temperatures are lower at each successive pressure.
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However each successive pressure level adds to the plant cost and complexity, which may make them uneconomical for most combine cycle applications. Due to this reason dual pressure WHRB’s are commonly used particularly in combined cycle application where moderate and low pressure steam is required. Dual pressure WHRB’s are basically two WHRB’s utilizing the same gas stream. The high pressure section of the WHRB would consist of an HP evaporator complete with HP economizer and HP super heater interconnected to produce HP superheated steam. The LP section would consist of LP evaporator with LP economizer interconnected to produce lp saturated steam. Both WHRB’s would be built in to the same gas steam and would normally take feed form a common condensate heating system. The hot gas path would be HP super heater, HP evaporator HP economizer, LP evaporator, LP economizer and condensate preheater, splitting the HP economizer in to section and the cooler section downstream of the LP evaporator can provide far more efficient utilization of the heat source. By Stack And Damper In an open cycle GT plant requirement of damper does not arise. In a combined cycle plant provision of by stack and by require to be examined: The major advantage a by damper and stack can offer are i)
To phase out the installation schedule of the GT open cycle and combined cycle module, and
ii)
The capability of running the G when the GT open cycle and obtained cycle module, and generator is out of service for scheduled maintenance or repair.
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In the case of the steam turbine down, it is possible to run a GT without a by stack if the steam can be used elsewhere or dumped. For combined cycle applications however, the use for the steam else where is generally nonexistent and dumping the steam involves sizing the condenser to take the full load steam in a steady state condition, running steam and circulation water cycles in an unproductive heat-dump mode of operation. Alternatively, the WHRB is required to be suitable for ‘dry running’. A further advantage the by damper offers is the provision to control, to a limited extent, the heat input into the WHRB and the quantity of steam produced partially bying the particularly important and useful during cold startup operations. Another distinct advantage of the by damper is the ability to isolate the WHRB from the GT during the initial preoperative GT testing, when the repeated stop-start operating of the GT can otherwise cause condensation to occur throughout the WHRB. A further possible advantage, dependent on circumstances, is the ability to isolate the WHRB from the GT is burning sulfurous fuel. Disadvantages in using a by duct and damper include the initial equipment cost, maintenance costs on the by dampers and the constant heat loss through both direct exhaust gas leakage and heat loss through the dampers. A value can be derived for the GT operating time gain utilizing the by on consideration of the expected WHRB and steam turbine down times which can be compared to the value of the expected constant heat and gas leakage losses, maintenance costs, and equipment costs. In summary, however there is no general position on this subject and the same should be evaluated specifically for each site conditions.
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Type of dampers The major choice lies between utilizing a single diverter damper or two isolating type dampers one for the WHRB duct and one for the by –stuck. The two damper concept offers the greater selection of damper types that can be used but has the disadvantage that should both dampers close at the same time the ducting would become over pressurized. In turn this necessities the installation of relief s or pressure relief type dampers, in addition to the electrical interlock to prevent such situation. In the event of malfunctioning/ failure of the above provisions a mechanical linkage system ensures that both the dampers are not closed at the same time, as a foolproof system. By – dampers are prone to high leak rates. losses from the dampers include both sealing losses through leakage and heat losses through the blade(s).For combined cycle application involving large gas turbines. Damper types cover a range including,multi louver type, diverter type and flap type dampers. The advantages and disadvantages of each type and the suitability for a particular application very with the application. The multi louver type damper normally has external linkage actuating all the louvers simultaneously and is good for regulating flow. However it will not be as good on sealing as the single blade type damper. Further more, any pressure differential would tend to force one part of the louver seating open, and would not assist the sealing. The losses from this damper would therefore be greater and furthermore, differential expansion on the linkage brings to question the possibility of linkage binding. The diverter valve offers the advantage that the duct cannot be closed off and would be open to either the by stack or the WHRB. Sealing is generally effective and the damper can be appropriately insulated to minimize heat losses.
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Flap damper have the same kind do flap operation and sealing action as the diverter but are single application operating only as a single isolation type damper. They offer an advantage on extremely large ducts by utilizing a seating grid in the damper and breaking the damper into two or more individual “flaps” of a smaller easier handled size. Disadvantages would be the pressure drop across the seating grid. Pressure relief type flap dampers are also available and offer the advantage of incorporating an over pressure relief flaps and avoiding the additional costs of adding the relief s elsewhere in the ducting. Normally they are associated with multi louver dampers. Generally the diverter and the louver type dampers can be set to a partially open position to control heat for or start-up. However the dampers are generally not suitable for continuous modulation such as load following, due to the size. All the dampers, of course will utilize either motor drive or hydraulic power for operation. Seals on the dampers are particularly important. Metal seals utilize the pressure differential in the duct together with a built in spring action to help seat the thin flexible metal seals against the seats. Normally the seals are attached to the flaps and the seat area is built into the fixed frame. Multi blade dampers have a greater seal footage than other types, and only part of the seal is helped by the differential pressure. Other parts of the seal must seal against the differential pressure. Because of this multi blade dampers tend to have a higher leakage rate than the flap or guillotine type dampers. Sealing efficiencies of 99.95% should be specified for metal in seals without air sealing. Dual seals are often provided to obtain a greater sealing efficiency. Air seals use a fan to create a small positive air pressure to help seal the metal seals. Efficiencies of 100% should be specified for the seals if air sealing is utilized. Several factors must be considered when selecting by dampers the value of thermal losses, safety considerations cost and operation and maintenance costs.
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Normally the diverter is preferred for the GT by- application, however the actual losses for each particular application must first be considered. Heat losses through the damper is an extremely important consideration and insulation of the damper or louver to minimize the heat loss is an important consideration. Heat losses will vary departing on the insulation material used, type of damper and temperature of exhaust gas. Manufacturers should be asked to specify the minimum heat loss through the insulated dampers they manufacture. The particular design and type of damper selected will vary with the application. For example, if air sealing is used. The cost of supplying the constant air supply must be considered and compared to the additional sealing savings. By Stack The height of stack, if not restricted by local regulations, will be based on economic considerations of the stack construction costs compared to the value of the gain in stack draft. Stack construction cost will increase with stack height, due to weight to be ed, increased wind loads, increased
material costs, and increased
handling cost and maintenance costs. The gain in stack draft will be the theoretical stack draft gain less the stack gas pressure drop and its value will be obtained from the value of the increased electrical output from the GTC due to less back- pressure. A theoretical stack draft effect will be produced for each meter of stack height due to the difference in the weight of the external air at atmospheric temperature.
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Stack gas pressure drop will be the friction loss plus the stack exit loss. Stack diameter will require selection prior to the determination of the pressure drop. In selecting the stack diameter a velocity between 2,000 to 5,000 ft/min should be assumed and pressure drops determined accordingly. In certain circumstances discharging the gasses at a height sufficient to disperse and dilute the exhaust may be a consideration. The wind load on a stack is a major consideration, and a high stack may often require a large diameter or other special provisions at the base for stability. The by stack construction could be of fabricated carbon steel [outer shell] with internal insulation and stainless steel liner. Otherwise it could be made of stainless steel with external insulation and aluminum/GT cladding. Differential expansion for the complete system is taken care by provision of an Expansion t, normally of flexible [fabric] type. In some design, exhaust side silencer is located in the by stack upstream of the damper. Pinch Point and Approach Temperature These are two factors which affect the cost and effectiveness of WHRB. Pinch point is the differences between the gas temperature leaving the evaporator section of the system and the saturation temperature corresponding to the steam pressure in that section. Approach temperature is the difference between the saturation temperature of the fluid and the inlet temperature of the fluid . Figure-17 makes the definition more clear. These two factors and the desire on the part of the engineer to keep them as low as practical cause more concern to the system designer than any other factors in the design of WHRB.
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As a general rule, lowering the pinch point results in an increase in the total heat recovered in that section. However, lowering the pinch point will require more heat exchange surface and a subsequent increase in cost and gas side draft loss. Multiple computations are required to select the most economical optimum. Another possible penalty which must be evaluated in lowering . then pinch point excessively is unstable operation. The very low design pinch point may indicate more heat recovery but if all other factors are not realized, it may result in poorer performance when steam production is critical. Lowering the approach temperature can result in more steam production at that pressure level as with a lower pinch point, this can only be realized at a higher cost and same increase in draft loss. Higher approach temperature would result in design stability. The higher approach temperature would increase the surface in the evaporator section and assure a higher level of stability. Al low load the gas turbine exhaust temperature will help drop but mass flow of the gas may not drop signification depended on type of gas turbine and intent vane range. The quantity of steam generates in the evaporator will drop. The economizer, will see nearly the constant gas temperature. Conditions but will have to heat a reduced fed water flow. The reduction in mass flow of the gas is not great as the reduction in feed water flow. This could cause steaming and water hammer problem in the economizer.
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Path of Flow Fig. 17 In an economiser section a higher approach temperature will reduce the probability of steaming at lower loads or during. Start up a design feature that could prevent this problem of seaming at lower load is to located the feed regulating station on the downstream of the economizer rather then the traditional location upstream of the economizer . This will have the effect of maintaining an economizer
pressure higher then evaporator steam drum
pressure particularly during the economizer low flow situations when the value would be party closed with a corresponding high pressure drop.
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Types of Circulation Circulation of the water through the evaporator tubes can be forced utilizing circulation pumps or can be natural utilizing gravity and density difference between water and steam to obtain circulation. Both types of WHRB’ s are common with combined cycle power plants. Natural circulation boilers must have vertical or inclined to the vertical tubing and will normally have higher initial costs. Forced circulation WHRB’ s using a water/steam drum normally use horizontal tubing pump pressure to circulate the water/ steam through the evaporator tubes. The advantages with horizontal tube design is the lower cost of the tube bundles and potential for lower cost of section renewal if request in later years. However, due to horizontal tube design used with this type of boilers there will be lack of tolerance. Pressure drops through the horizontal evaporator tube banks must be balanced and often require restriction on some of the tubing in order to achieve this balance. This increases the pressure drop through the system and water tend to separate to separate easily leading to further problem and minimum velocity through the tubes must be maintained to avoid this separation. If separation does occur, the bottom of the tube will have a steam surface and heat transfer through the top of the tube will not be as effective as heat transfer through the bottom of the tube. The top of the tube will become hotter, setting up thermal stresses in the tube leading to sagging and in some cases out right tube failure due to overheating the tube. Thermal inertia of the forced circulation WHRB is less then the natural circulation WHRB which facilitates faster loading and unloading of the WHRB. The table below gives the summary of advantages and disadvantages associated with each type of design.
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Summary –Forced circulation Vs National circulation Function
forced
Circulation
Position Adv.
Natural
Circulation
Dis.
Position
Adv. Dis.
Circulation pump purch and inst. Costs REQ’D
X
NOT REQ’D
X
Circulation pump operating costs
REQ’D
X
NOT REQ’D
X
Balanced tube side press drop
REQ’D
X
NOT REQ’D
X
Potential for tube failure
HIGH
X
LOW
X
Potential for steam/ water separation HIGH
X
LOW
X
Initial cost
LOWER
X
HIGHER
X
Replacement cost
LOWER
X
HIGHER
X
Tolerance to changed conditions
LOWER
X
HIGHER
X
Sensitivity to adverse conditions
HIGHER
X
LOWER
X
Faster startup and shutdown
FASTER
X
SLOWER
X
(Less thermal inertia)
Unfired or fired Whrb As stated gas turbine at high excess air. Hence, the exhaust gases have very high oxygen content of the order of 16 to 20% which is sufficient to if any fuel firing desired in the WHRB or its duct. A WHRB designed to recover only the heat available in the exhaust gases is termed as unfired WHRB. Where are heat in the exhaust gases is supplemented
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by firing of additional fuel in the WHRB duct, the WHRB is termed as a fired WHRB. It must be recognized that the overall combined cycle plant efficiency will drop when duct firing due to the fact that Rankine cycle efficiency is lower than then combined cycle efficiency especially when the steam is used in a relatively low pressure induction type steam turbine. However the cost of installing a duct burner is extremely small and other equipment costs are nonexistent. Ducts burning can be advantageously used in combined cycle units where a need exists for peaking capacity. For evaluation of duct burner firing for peaking in a combined cycle value of additional KW produced by the steam turbine should be compared to the cost of generating the steam. Costs of additional steam turbine generator capacity should be added, if applicable. Choice on Condensates Pre-heater Decision about adding a condensate pre-heater is dependent on the being available in the back end of the WHRB,the requirement for heat to be added to the condensate return and economic justification. In most HRSG’s the back end heat is usually available and justification for the heater is based on need due to the temperature of the return condensate and economic justification. Economic justification for a condensate pre-heater should evaluate and compare all the costs associated with the installation of a condensate pre-heater against the value of the steam saved by the utilization of the condensate preheater. The steam saved by utilization of a condensate preheater will be the difference between the steam used by deaerator operating with the condensate preheater and without the condensate preheater. Without the condensate pre-heater additional deaerator steam would be used to heat the incoming condensate (feet water) to the saturation temperature required for dearerator operation.
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Condensate pre-heaters are subject to similar low load problems as experienced by the economizer. The problem is aggravated in the condensate pre-heater, however by a temperature rise in the exhaust gas at low load as the economizer will use less heat front the gas with the reduced
feed water flow. The
condensate pre-heater will therefore be subject to hotter gas together with reduced condensate flow which can cause severe steaming and steam hammer in the heater. In many cases the chosen solution to the condensate pre-heater low load problems is a planned by- of the condensate preheater during the infrequent low load operations permitting the condensate preheater torn day. Alternate solutions to this problem, dependent on the excepted frequency of low load operation would include running a full flow feed through the pre-heater at all times and dumping the surplus heated water to the condenser.
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9. Gas Turbine Generator Synchronous Generator The air cooled generator Type WY 18 L is of simple but robust construction. The generator and the gas turbine 13D are placed on a common and plain concrete foundation, with the same center line level for both the gas turbine and the generator rotor. Therefore, the space below the active part could be used to house the four cooler sections. The outline drawing shows the principle design and the main dimensions. Generator Design The generator is built in compliance with the latest standards of the IEC, ANSI and VDE, and is designed to operate continuously at rated load but also it withstand a wide range of abnormal operating conditions, including short circuits. Stator casing and Iron core The horizontally spilt stator casing of the active part consists of two welded sheet steel constructions, which are screwed together. Inside the casing sheet, steel ribs are arranged as guide plates for optimum cooling air distribution to the active sections. The four identical air recooling elements inserted into the casing below the active part can be withdrawn horizontally. The low-loss core segments are punched from silicon alloyed electric sheets. They are carefully deburred and are given several coats of a heat resistant varnish which can withstand temperatures much higher than the normal operating temperature. By this manner, the individual segments are dependably
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insulated from each other, so that even after many years; operations no short circuits or even hot sports can occur. After stacking the core segments outside the casing, vibrations and inseased temperatures are applied when the core is compressed between heavy pressplates. The press-plates are made of low-loss non-magnetic material and press by special fingers the whole cross section of the core. The resulting core remain rigid over its whole length, especially the highly stressed end parts, even if subjected to varying load conditions or daily multiple starts. Distance profits are welded to thicket segments, so that in constant distances radial slots are formed for flowing through of the cooling air. The would core will be inserted as a ready unit into the casting and will be welded with the caper part by use of adjusting pieces.
OUTLINE DRAWING OF ALTERNATOR Figure-18
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Stator-Winding Bars and Installation of Bars The mechanical energy generated by the turbine is converted to electrical energy by the generator and appears in the stator winding in the form of current and voltage. The winding is capable of withstanding all thermal and mechanical stress arise from normal operating or from external faults, with a generous factory of safety. The stator winding consists of individual bars installed in rectangular slots distributed uniformly along the inside circumference of the stator core. Each winding bars consists of solid conductors, insulated from each other. The conductors are first wound with resin-impregnated glass fiber, then assembled together and pressed in heated modulus to form compact bars. During bar assembly, the conductor’s positions in the bar are transposed, or twisted, so that each conductor occupies each possible position once in the straight part of the bar. Thus, the same amount of current appears in each couch and no unnecessary losses occur. The bars so formed are called Roentgen bars, after their inventor. The main bar insulation is built up following the Micadur (R) system, which was developed by BBC and which has been successfully used for many years. In this system, the bars are continuously wound, in an automatic tape winding machine, with the required number of layers of mica-coated glass-fabric tape.
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STARTOR SLOT CROSSECTION Fig.-19 The straight part of each bar is pressed to size, the bars then are dried in an oven, then they are impregnated in vacuum, by a special epoxy resin, and hardened in an oven by means of circulating air. The finished insulation complies with the Insulation Class F specifications of the IEC, ANSI and VDE standards.
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ENDWINDING FIG-20 Terminals The winding connections and the terminals are connected by means of flexible copper strips. The flexible strips absorb possible relative motions between casing and winding which may be caused by different heating of casing and winding. The terminals are designed to withstand forces in case of a short circuit at he terminals. The insulation is designed fir the 1.5-fold test voltage of the generator, and consists of impregnated mica-glass strips. Both ends of the terminals have two parallel connecting plates, inside the generator for connecting of the flexible strips, and outside for the connection to the bus bars to the star point, respectively.
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Rotor Body The rotor balances the torque of the gas turbine. It leads the magnetic flux, and carries the field winding and the necessary parts for conducting the field current. The rotor body is a single forging of heat-treated alloy steel having very good mechanical and magnetic properties. After heat treatment, the rotor body is tested using the most up-to-date methods to ensure that the required properties have been obtained. As an example, the rotor is subject to ultrasonic testing over its complete length and cross-section. Therefore, a central bore for testing purposes is not required. Longitudinal slots are milled in the two winding zones of the rotor surface to carry the field winding. The different slot shapes in the pole zone and winding cause different moments of inertia relative to the direct and the quadrature axis. In order to achieve uniform bending of the rotating rotor relative to these two axes, transverse slots are milled in the pole zones. This acts to reduce the amplitude of the double-frequency flexural vibration in a smooth-running rotor. The driven shaft end of the rotor is provided with an integrally-forged coupling flange for connecting the turbine shaft. At both ends of the rotor are seats for the fans and also journals for the bearings, the ends of the rotor body are machined to take the retaining rings. The nondriven shaft ends has a central bore to take the rotor filed current lead, while radial holes are drilled for the lead studs which connect the rotor leads with the field winding.
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Rotor Winding The rotor winding is composed of hollow conductors which carry the excitation current required to produce the armature field. The conductors are made of drawn silver containing oxygen-free copper having high-grade properties. In the active region, the hollow conductors are insulated from each other and from the rotor iron by Nomex, a polyamide paper, fulfilling class F requirements.
ROTOR WINDING FIGURE-21 The rotor winding is directly cooled by air howing from both ends of the rotor through all the conductors. At all the air inlets and outlets, the creepage (leakage)
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paths between the bare copper and the rotor mass are rather long. This ensures goods dielectric strength of the winding even after long operation time. The filed current is conducted from the slip rings to the rotor windings by means of two half-round copper lead bars fitted in the shaft bore. At one end of the lead bars, connection is made to the slip rings by two leads studs and flexible-plate connectors. At the other end, connection is made to the rotor winding by similar lead-studs and flexible-plate connectors located under the retaining ring. At each end, one stud is provided for each polarity. The stud is made of flexible-plate connections compensate all mechanical and thermal stresses. Retaining Ring Retaining rings which carry the centrifugal forces of the rotor end windings are fitted at each end of the active part of the rotor. Each retaining ring has a cylindrical section and disc-shaped cap end. The retaining rings are shrunk onto the rotor ends and maintained in their axial position by means of a bayonet attachments. A special key prevent any movements of the rings in the tangential direction under short circuit conditions. A special feature is the insensibility of the used steels against stress corrosion. Cooling System The axial fans at both ends of the motor provide for two cooling circuits. The air which cools the rotor enters the end-coil space between the end bells and the shaft. In the vicinity of the slots it flows through slits into the hollow conductors, where the air flow is being divided. One flow path is through the hollow conductors in the slots, entering the air-gap at the middle of the rotor through
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radial ages in the windings and slot wedges. The other one leads through the hollow conductors in the end-coils, finally leaving the space under the endbells through slots n the rotor body and entering the air-gap.
AIR FLOW SCHEME OF THE ALTERNATOR FIG-22 Some of the air which cools the stator is led straight into the air-gap. Together with the cooling air from the rotor end-coils, it leaves this region through radial ages in the frame. From there the air returns via the coolers to the fans. The rest of the air cools the stator end-coils and flows through axial channels into the second frame chamber. From there it flows through the laminations towards the air-gap where it splits up again. One half flows back through further slots into the first frame chamber, the other half mixes with the air coming from the rotor at the middle of the machine. From there the air es through the laminations to the third frame chamber, the coolers and aging to the fans. Each of the cooling elements consists of a finned tube bundle and two water chambers. The materials for the coolers will be selected depending on the quality of the water. The coolers can be cleaned easily with brushes.
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Bearings The generator is equipped with two journal bearings. The bearing at the driven end is part of the immediate block of the gas turbine. The bearing on the nondriven end is insulated to prevent shaft currents. The rotor is earthed via earthing brushes. The bearings are connected to the lub and jack in oil systems of the turbine. Slipring Unit The equipment for conducting the excitation current (direct current) into the rotating rotor in order to produce the magnetic field in the rotor winding is located in the slipring unit at the non-driven end of the generator. It consists mainly of two slip rings shrunk onto the shaft (but insulated from it), a set of carbon brushes in with the slip rings and carried in replaceable holders, and the brushholder s. The actual transfer of current takes place between the brush surfaces and the slipring surfaces. The constant pressure required between the surfaces is provided by rolled-strip springs on the brushes. The heat generated in the slip rings and brush gear is removed by a dual exhaust fan which draws cooling air through each side of the unit. During operation, the brush gear can be inspected through safety-glass windows provided in the casing. Temperature Monitoring Temperature sensors are provided in the stator winding and the cooling air circuit. The connecting wires to the sensors are brought out through bushings to the instruments terminal box.
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10. Excitation System General Description The synchronous generator is shunt excited. The power required for the excitation is taken from the generator terminals and fed to the field winding through the excitation transformer and the thyristor-controlled rectifier units. The thyristor-controlled by the voltage regulator through the grid control unit and pulse amplifier stages, so that the generator voltage is held almost constant from no load to rated load. A fully-controlled 3ph AC bridge circuit is used for the rectifier circuitry. This permits reversing of the polarity of the excitation voltage during rectifier operations ensuring rapid control during build-up or discharge of the field. A three phase AC auxiliary gird is required for field flashing for the generator excitation which is supplied via the transformer and the field flashing circuit breaker until the generator is excited to such an extent that the excitation equipment can operate automatically. When a fault occurs, the generator can be deenergized quickly with the field circuit breaker and the voltage dependent resistor. The short circuit device protects the excitation equipment and the thyristors from excess voltage by short circuit the field windings through the limiting resistor. Description of Operation Automatic Voltage regulator (A.V.R) Regulator rack
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The actual value and set point are compared in the Unitrol Section and the difference between the two is amplified in the control amplifier. The output voltage of the amplifier is limited to a value set by an internal potentiometer. The generator is controlled as follows below the cut-off frequency F. the control voltage is proportional to the frequency, and above this the voltage is held constant to a set value (see fig. 23)
Figure-23 The cut-off frequency can be adjusted by an internal potentiometer. The set point adjustment is made using a reference value transmitter Unitrol, which is installed in the regulatory rack. Adjusting elements allow the level and range of the voltage regulator to be selected (see fig. 24)
Figure 24
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Adjustable compound resistances are connected in the circuit to ensure the required reactive load distribution during parallel operation with other generators. The resistors allow the regulator drop to be changed. AP. I.D. filter and a differential amplifier are used for stabilizing, which permits optimum matching of the controlled generator. A power pack supplies a stabilized voltage to the set point potentiometers. Grid control unit automatic channel and manual channel the output signal of the regulator rack or manual control potentiometer is fed to the input of the grid control unit Unitrol together with a 3ph A.C. voltage from the filter Unitrol which is synchronous with and has the same phase as the supply for the power rectifier. From this, 6 phase voltages are formed, which are compared with the regulator or hand control signal (see Fig. 25). The differences is formed into square wave form voltages by an operational amplifier with controlled limits. Six trigger pulses are thus formed at 60 (electrical) intervals by the change in polarity.
Figure-25 The trigger pulses can be shifted depending on the size of the AC and DC voltage. For safety reasons, adjustable limits are provided for full rectifier or DCAC converter operation. The weak pulses of the grid control units 1001c-P are
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amplified in the pulse stages of Unitrol. Control relays in the unit allow the pulses to be suppressed, i.e. blocked, or to be reversed in DC-AC converter operation. Pulse end stages The output pulse of the grid control unit are fed to the parallel-connected pulse end stages of Unitrol. These end stages are designed to control up to a maximum of 8 thyristors in parallel per branch by means of the appropriate pulse gradient and power level, i.e. a 3ph AC bridge with up to 48 thyristors can be controlled. The pulse of every end stage can be suppressed by means of a blocking relay. The corresponding thyristors therefore remain isolated. Current rectifiers Three current rectifiers unit are fitted in section of the lead rectifier cabinet. The thyristors are directly fitted to the heat sinks. The current strips are connected to the DC AC busbars, the auxiliary and control cables are lead to the appropriate terminal blocks. Each unit is ventilated by its own fan. The fans are supplied through the fuses, which are directly connected to the secondary voltage of the excitation transformer, the transformer, and the protection circuit breaker. The air flow from the fans is monitored by a wind vane relay. Regulator limit rack Limit criteria All limits act equally in that they influence the voltage control and alter the excitation accordingly. The criteria are :
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•
Too large a stator current
•
Too large rotor current
•
Too larger a load angle for stability
In the over-excited range, the rotor or stator current limiter reduces the excitation and in the under-excited range the load angle limiter increases the excitation. The rotor or stator current limiter acts through a time delay and integrator to maintain both good regulation in the event of load changes, and also the dynamic behavior of the generator when intermittent grid faults occur. This allows the protective system to operate selectively. Load angle limiter Unitrol The load angle, the angle between the generator longitudinal axis the grid voltage vector, can be determined by forming a vector diagram of the generator and the external reactance. The generator voltage and current are fed to the input of the load angle control limiter. The quardrature reactance xq of the generator is the determining parameter, and can be adjusted between 0.5 and 3.0 p.u. While the external; reactance xe can be adjusted between 0 and 3.0 p.u. by means of potentiometer: The two voltages so formed enclose an angle between them which corresponds to the load angle. This angle is transformed in a transducer to a DC voltage signal. This angle is transformed in a transducer to a DC voltage signal. This signal is compared to an adjustable set point. When the measure value exceeds the set point, the limiter comes into operation and acts upon the voltage regulator. The excitation is then increased until the load angle corresponds to the adjusted set point. Despite increasing generator voltage, the voltage regulator remains ineffective.
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Rotor current limiter Unitrol This limiter protects the rotor winding from thermal overload. The excitation current is measure on the AC side at the supply to the controlled rectifier units, and converted into a current proportional DC voltage within the limiter. This is then compared with an adjustable set point, amplified and fed to the input of the voltage regulator. The limiter acts with a time delay, so that the dynamic control is not affected in cases of fault. Stator current limiter Unitrol The stator current is converted by a transducer into an equivalent DC voltage which is polarized positively or negatively according to the over-or under-excited operating condition. This forms the actual value for the regular in which the comparison with the adjustable set point is carried out the difference signal first amplified. The output of the amplifier for limiting the stator current influences the regulation of the voltage in such a way as to reduce he excitation. Temperature-dependent current limiter The temperature-dependent current limiter is used to protect the generator winding against thermal overload, should the ambient air temperature exceed 40 deg. C. The current limiter, by an appropriate reduction in excitation current, prevents the windings from overheating. On gas turbine systems, on which the limiter is used, the active power of the turbine decreases with increasing air intake temperature, and the limits given by the generator are always in the specified range. Thus any effect on the active power nominal value is not necessary. The current limiter includes supervision, which incase of irregularities such as nominal/actual deviations, gives a command signal to shut down the turboset.
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Basically the system consists of two units : (a)
Nominal value formation and nominal/actual value supervision of the stator current.
(b)
Nominal value formation and nominal/actual value supervision of rotor current.
Excitation the system consists of two units: Excitation with field flashing The supply for power rectifier is from the 3ph excitation transformer which receives its power from the generator. Before excitation takes place at rated speed, there is only the residual voltage of the generator available. This is insufficient for starting the excitation equipment. The thyristors of the controller remain blocked. An auxiliary voltage of 414 V, 50 Hz is available for field flashing, which supplies sufficient excitation current through the matching transformer and diode bridge for 35% of the generator voltage. The on command is given by the start-up automatic, or from the control room, via the decontic control. The command is only given when the criterion “ready for connection” is fulfilled. The field breaker is switched on. At 70% generator voltage the voltage relay switches the field flashing breaker off. Switching off must take place within 20s, otherwise the time supervision switches the field breaker off. The excitation process will be interrupted. De-excitation De-excitation can follow through the following criteria :
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(a)
By the command Excitation off via the or from the automation or from the control room.
(b)
By a fault within the excitation equipment.
(c)
By the generator protection.
The field breaker is immediately opened and the field discharges through the voltage dependent resistor. The excitation circuit must not be broken as long as current is flowing, since this can cause high voltages which would damage the thyristors or the rotor winding. When the filed breaker is switched off, a switches the resistors in, parallel with the rotor winding. A second breaks the supply from the rectifier. The resistor is sized so that the over voltage of the filed breaker. The resistor is designed as nonlinear and voltage-dependent, whose resistance decreases with increasing voltages. In this way the deenergizing time constant becomes shorter. The blocking voltage of the thyristors lines above this cut-off overvoltage. The overvoltage short-circuit device, parallel to the rotor winding, offers additional protection as undesired break in the rotor circuit occur during operation. When the short circuit device acts, the limiting resistor is switched in parallel to the rotor winding. It acts between the allowable switching voltage of the field breaker and the cut-off overvoltage, so that it is only activated when a fault occurs. Power supply Automatic Voltage Regulatory (AVR) Automatic voltage regulator and manual voltage control have two independent power supply units each, one AC-DC converter and one DC-AC converter. The AC-DC converters Unitrol are connected to the secondary side of the excitation transformer via transformer. The DC-AC converters Unitrol are fed from the 220
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V DC distribution AC-DC converter Unitrol connected to the 220 DC distribution serves for power supply for the protection. Control Voltage The 220 V DC supply voltage in the control cabinet is provided by a 220 V DC distribution Systems. The control voltage is monitored by, which gives a signal the voltage is low. Every function has its own control circuit, so that when failure occurs, only a part of the system is out of operation. When a failure occurs in the circuit it must be decided whether the excitation equipment can remain in operation or not, according to the importance of the faulty circuit. The faulty control circuit must be brought back into operation again as soon as possible. The protection switches are monitored by the built-in auxiliary s. When a protection switch cut out, the alarm “Protection switches Voltage” is illuminated in the control cabinet and in the electrical control room the excitation warning signal is given. Normally only a small amount of power is taken from the battery for the controls. Switching process require approx. 10 KW for a short period. Method of operation of the complete equipment When the generator is stationary the circuit diagram shows that field breaker is open. The generator must be brought up to rated speed before it can be put into operation. Above 95% rated speed, the excitation process can be started by switching on the field breaker. The generator is excited up to 35% rated voltage by the filed flashing. Above this value the electronics function reliably, and the regulator takes over the control of the generator voltage. At 70% generator voltage the field flashing is switches off by the field flashing breaker. The set point is compared to the actual value. During the excitation process the generator
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voltage is low. The control amplifier Unitrol 2010 gives a higher signal to the grid control unit Unitrol. At first a celling voltage of approximate. 40% of max. excitation voltage at rated generator voltage is present. The generator voltage and thereby the exciter voltage continually rise. As soon as the output voltage of the voltage transformer reaches the adjusted set point, the control pulses to the thyristors are pushed back until set point and actual generator voltage are equal. When manual control has been selected, then the firing angle of the thyristors can be adjusted with the hand potentiometer, which corresponds to a certain exciter voltage. The actual value of the generator voltage has no influence on the firing angle-in contrast to the automatic control-since with manual control, there is no closed control circuit. When the potentiometer is changed as a result. The changeover to manual channel can be made at any time. However the pulse difference between the output of the grid control unit automatic channel and the grid control unit manual channel must be zero. The pulse difference can be read off on the comparative voltmeter via the measurement Unitrol 0040, comparator Unitrol and measuring converter. The comparison is made in the control room via the contractor by adjusting the potentiometer. When zero voltages is shown on the comparative voltmeter, a smooth changeover can be made to manual control. Protection and Monitoring equipment The excitation equipment is monitored and protected by various protective relays, motor protection relays, and various monitoring equipment. The function description=MKC01…..MKC05 gives information about the monitoring equipment provided, and the result when a fault occurs.
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Alarms The signals for the excitation equipment are shown on the circuit diagrams. They are illuminated in bottom for acknowledgment, lamp test, functional test and lamp reset. Protection of the excitation equipment and the excitation transformer Electronic inverse time overcurrent relay type IKC 062 delay trip (change-over to manual), instantaneous trip. Electronic short circuit relay type Unitrol. Temperature protection with PTC-Resistors fitted to the excitation transformer.
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11. Gas Plant Operation Gas Turbine, WHRB, Steam Turbine Starting Modes Basic conditions for plant operation are as follows:•
Start up or shut down of G/T, WHRB and S/T of each module is performed separately from the other module (except for S/T gland steam back-up and heating steam back up systems).
•
Start up/shut down mode is selected freely form among those mentioned.
The start up/shut down procedure for WHRB and S/T here mainly describes operating procedure for G/T by- damper, WHRB inlet damper and remote operated valve necessary for start up and shut down from G/T exhaust gas ission to WHR till rated load operation of S/T. For detail operating procedure for G/T WHRB, S/T auxiliaries and remote operated valve following procedures are followed. Start up Mode The start up mode of G/T, S/T and WHRB shall be selected from among the following as a rule through various other start up modes are conceivable according to power demand and operating principle. Outline of each start up •
Normal start up mode This mode is two unit (2G/T’s + 2WHRB’s +1 S/T) combined cycle start up mode which, after starting up one each of G/T and WHRB, starts up the
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other G/T and WHRB and brings the output of G/T and S/T the target output of the module. In this mode, S/T is loaded with one each of G/T and WHRB in operation, and after the intial load is achieved, the pressure control of HP/LP by- valves of both WHRB’S is changed to common pressure control and then S/T is loaded up. The timing of starting up the other G/T and WHRB is left to the discretion of the operator, and S/T is kept stand by at the initial load until the pressure control of both WHRB’S is changed to the common pressure control. •
Rapid start up mode This is a mode of starting up both G/T’s and WHRB’S at the same time, changing the pressure control of HP/LP by- valves of both WHRB’S to the common pressure control, then loading up and brining G/T the output of and S/T to the target output of the module is achieved. This mode is used only for start up after right stop or hot start up (with vacuum). In other start up modes; in which the time from G/T start up to S/T loading is long, simultaneous startup is not made to reduce heat lose at start up.
•
G/T/WHRB additionally start up mode This is a mode of starting up the other G/T and WHRB one until combined cycle of G/T, WHRB and S/T is in operation. In this mode, the HP Steam pressure and temperature of the other G/T and WHRB are raised up those of G/T and WHRB in operation, and then the pressure control of HP/LP
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by- valves is changed to the common pressure control, and S/T is loaded up. •
Single G/T/WHRB start up mode This is a combined cycle start up mode of starting up S/T with only one each of G/T and WHRB in operation. The other G/T and WHRB remain stopped.
•
Individual pressure control Individual pressure control means pressure control made automatically by using HP/LP by- valves of both WHRB’s and detecting pressure before HP/LPCV so that HP/LP steam pressure of both WHRB’S will be the same and constant respectively.
•
Loading of S/T S/T loading rate after HP/LP by- are fully closed and HP/LPCV are fully opened is determined by the load change rate of G/T, namely S/T load change rate increases as G/T load change rate increase. (Except for G/T/WHRB additionally start up mode). S/T unloading rate too is determined by G/T load change rate under HP/LP by- valves are fully closed and HP/LPCV are fully opened.
STARTUP PROCEDURE •
Normal start up mode Cold start up
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The condition in which HP turbine rotor temperature is below 120 degC is called cold condition. In cold start up, the HP turbine rotor needs warming, which is performed to raise the rotor temperature above the transition temperature of the rotor material 120deg.C and prevent brittle fracture in high speed operation to achieve this heat sock is done by holding rotor speed at 2000 rpm during start up of the steam turbine. The time of heat soak varies with rotor temperature before steam ission. Heat soak time of 2 hours is given on the assumption that the rotor is at the ambient temperature. Heat soak is performed with one G/T in operation to reduce start up loss and the other G/T shall be stared up just before that soak is completed. At cold start up, WHRB outlet steam temperature must be below 430 C because of temperature limitation at steam ission to S/T. Therefore, WHRB inlet exhaust gas temperature must also be limited accordingly. WHRB and S/T shall be started up after G/T load is held at a load (about 30%) at which boiler outlet steam temperature can be kept below 4300 C. •
Start up conditions Turbine vacuum
Without vacuum
Gland steam supply conditions
2.5t/h x 2000 C x 3kg/cm2 above
Steam ission conditions
60t/h 3000 C-4300 C x 39kg/cm2
WHRB inlet damper initial opening
100%
Steam evaporation at G/T 30% load
25t/h
SH outlet below valve capacity
35t/h x 29kg/cm2
Time required for pressure
About 40 minutes
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Rise (0-29 kg/cm2)
(at G/T 30% load)
WHRB temperature rise rate
4002 C/h
S/T speed rate
300 rpm/min
S/T load up rate
•
(Before HP by- valve full close)
1%/min.
(After HP by- valve full close)
2.5%/min.
Start up procedure Preparation for start up -
Start up the following common equipment which is necessary for start up of G/T, S/T and WHRB. a)
Raw water pump system (clarifier, pretreatment plant and water treatment plant)
b)
DM water transfer pump system.
c)
Compressed air system.
d)
Compressed air system.
-
Fill up condenser and deaerator with water.
-
Start up circulating water system CWP Condenser-cooling tower CWP
-
Start up chemical injection system.
-
Start up seal air fan.
-
Make sure that all preparations for start up have been completed.
WHRB start up
-
Start up HP BFP and LP BFP
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All manual operation
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-
Start up boiler circulating pump
-
Make sure that by- damper is full open and WHRB inlet damper is full shut.
-
Start up G/T after all preparation for G/T start up have been complete.
-
Start turning S/T
-
Open HP/LP drum air vent valves and SH outlet air vent valves.
-
When WHRB inlet damper is open, automatically open HP steam pipe drain valve and LP steam pipe below valve.
-
After synchronizing G/T, fully open WHRB inlet damper. By- pre-heater and fully open HP/LP economizer re-circulation value.
-
Start up G/T up to 30% load and hold that load.
-
When HP steam pressure reaches 2 kg/cm2, automatically close HP/LP drum air vent valves and SH outlet air vent valve.
-
When HP steam pressure reaches 3kg/cm2 x 2000 C. Open the Hp steam stop valve and HP turbine inlet pipe drain valve to start warming of HP turbine inlet pipe.
-
After warming is over, start up gland condenser exhaust fan, and open gland steam stop valve to start gland steam supply
-
Start up vacuum pump after gland steam has been established.
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Until HP/LP by- valves are put into operation, open SH outlet blow valve, HP steam pipe drain valve and LP steam pipe blow valve, and steam is blown into the WHRB blow down tank and S/V flush pipe.
-
At that time, opening of SH outlet blow valve and LP steam blow valve is automatically adjusted according to HP/LP steam pressure at start up of WHRB.
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-
Opening HP steam pipe drain valve is automatically adjusted according to HP steam temperature at start up of WHRB.
-
After vacuum has been established (550 mm Hg), open turbine drain valves and open HP/LP by- valve inlet stop valves to put HP/LP turbine by system is individual pressure control.
-
Open HP by- valve to start individual pressure control so that HP steam pressure will be constant of 29 kg/cm2.
-
Open LP by- valve to start individual pressure constant so that LP steam pressure will be constant of 5kg/cm2.
-
When HP/LP by- valves are put in operation, fully close by damper.
-
After HP/LP by- valves have been put in operation and SH outlet blow valve HP steam pipe drain valve and LP steam pipe blow valve.
-
When LP by- valve is put in operation open LP steam stop valve and LP turbine inlet pipe valve and LP steam pipe blow valve.
-
When LP by- valve is put in operation open LP steam stop valve and LP turbine inlet pipe drain valve to start warming of LP turbine inlet pipe.
-
After warming is over, open deaerator heating steam supply valve to start heating of deaerator.
-
At the same time, open condenser pre-heater re-circulating valve by the specified degree (at gas firing only)
-
When boiler evaporation is established and drum level control is switched to 3-factor control, it rate to pre-heater and fully close HP/LP economizer re-circulation valves.
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•
S/T start up
-
Make sure that all preparations for S/T start up have been completed
-
Make sure of turbine steam ission conditions 29kg/cm2 x 3000 C (below 4300 C).
-
After steam ission conditions have been completed, HP/LP turbine inlet pipe drain valves are fully closed.
-
Start steam ission to turbine (DEH on)
-
Start accelerating. Accelerate at 300 rpm/min. up to target of 2000 rpm. Make rub checks as necessary during acceleration.
-
Make heat soak at 2000 rpm for 2 hours. (Heat stocks time depends on mismatch temperature difference of turbine)
-
Startup the other G/T just before heat sock is completed and hold it at 30% load, increase steam temperature and pressure of the other WHRB and when HP/LP steam pressure reached 27/cm2 and 4.5/cm2 respectively, put HP/LP by- valves in operation. (To start up the other G/T and WHRB, follow the startup procedure described earlier).
-
After heating soak, accelerate up to 3000 rpm/min.
-
Check that preparations of generator have been completed.
-
Loading.
-
Maintain 5% initial load for 10 minutes.
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•
Load up
-
Put HPCV and LPCV (Governor valve) in upstream pressure control and sequentially close HP/LP by- valves so that the S/T load up rate will be 1% min. at this time, HP/LPCV automatically following up in pressure control. As a result, steam goes to S/T.
-
When S/T load reaches 20% full close all turbine drain valves.
-
When HP by- valve is fully closed, shift HP by- valve to backup mode.
-
When LP by- valve is fully closed, shift LP by- valve to back up mode.
-
Fully close HP main steam stop valves for other WHRB adjusting temperature and pressure differences.
-
Fully close LP main steam stop valve for other WHRB, adjusting temperature and pressure differences, fully close LP turbine by valve according to the predetermined programme and then switch to back-up control
-
When HPLP by- valves are fully closed, adjust G/T output to achieve target plant output.
-
At this time, adjust G/T load up rate so that the S/T load up rate will be 2.5% min. (GT load up rate will be approx. 2.9% min.)
•
Warm Startup The condition in which HP turbine rotor temperature is between 1200 C and 1400 C is called warm condition.
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Because of thermal stress, heat soak from 0 to 40 minutes is required, depending on the steam turbine rotor temperature. Startup procedure is basically the same as is cold start up. When making a weekend shut down of the plant, if a normal shutdown is made, the steam turbine rotor can be held at a relatively high temperature, and so heat sock time can be shortened. If, rotor metal temperature is above 300 C, and heat sock is not required. Because of temperature limitation at steam ission to S/T, WHRB outlet steam temperature must be below 4300C. Therefore hold G/T load at a load (about 30%) at which WHRB outlet stream temperature can be kept below 4300 C, and then start up WHRB and S/T. •
Start up conditions Turbine vaccum
Without vacuum
Gland steam supply conditions
2.5t/h x 2000Cx.3kg/cm2 above 75t/hx3000C-4300 Cx29kg/cm2 (10% initial load)
WHRB inlet damper initial opening
50%
Steam evaporation at G/T 30% load.
25t/h/
SH outlet blow valve capacity
35t/h x 20 kg/cm
Time required for pressure rise at
About 25 minutes (at
G/T 30% load.
G/T 30% load)
WHRB temperature rise rate
4000 C/hr
S/T speed rate
300 rpm/min.
S/T load up rate (Before HP by-
2.5% min.
Valve full close)
3.0% min. (Corresponding to the C/T load up rate of 30%/min.)
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•
Star up Procedure
-
Preparation for Start up •
Start up the following common equipment which is necessary for start up of GT, S/T and WHRB.
•
a)
Service water pump system or clarified water pump system.
b)
Compressed air sytem.
Start up circulating water system-CWP-Condenser-Cooling Tower CWP)
•
•
Startup cooling water and condensate systems.
•
Start up chemical injection systems.
•
Start up seal air fan.
•
Make sure that all preparations for start up have been completed.
WHRB start up −
Start up HP BFP and LP BFP.
−
Fill up HP/LP DRUM with water up to normal water level.
−
Start up boiler circulating pump.
−
Keep WHRB inlet damper fully shut
−
G/T start up have been completed.
−
Start turning S/T
−
After G/T is loaded, open WHRB inlet damper upto 30% opening.
Note : *Manual operation
−
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Start up G/T up to 30% load and hold that load.
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−
Start drain blow Opening HP steam pipe drain valve and LP steam pipe blow valve.
−
When HP steam pressure reached 3kg/cm2 x 2200C. Open the HP turbine inlet pipe drain valve to start warming of HP turbine inlet pipe.
−
After warming is over start gland steam supply.
−
Start up vacuum pump after gland steam has been established.
−
Open SH outlet blow valve, HP steam pipe drain valve and LP steam pipe blow
−
Valve and steam is blown into the WHRB blow tank down tank and S/T flush pipe.
−
At that time, opening of SH outlet blow valve and LP steam blow valve is automatically adjusted according to HP/LP steam pressure at start up of WHRB.
−
HP steam pipe drain valve is automatically adjusted according to HP/LP steam temp. at start up of WHRB.
−
After vacuum has been established (550 mm Hg.), open turbine drain valves and open HP/LP by- valves inlet stop valves to put HP/LP turbine by- system in individual measure control mode.
−
Open HP by- valve to start individual pressure control so that HP steam pressure will be constant of 29kg/cm2g.
-
When HP/LP by- valves are put in operation, full open WHRB inlet damper and full close by- damper.
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-
After HP/LP by- valves have been put in operation, shut SH outlet blow valve, HP steam pipe drain valve and LP steam pipe blow valve.
-
After HP/LP by- valves have been put in operation, shut SH outlet blow valve, LP steam pipe blow valve.
-
When LP by- valve is put in operation, open LP steam stop valve and LP turbine inlet pipe drain valve to start warming of LP turbine inlet pipe.
-
After warming is over, open deaerator heating steam supply valve to start heating of deaerator. (Deaerator set pressure at 0.3 kg/cm2g)
−
At the same time, open condenser pre-heater re-circulating valve by the specified degree (at gas firming only).
•
S/T Start up
−
Make sure that all preparations for S/T start up have been completed.
−
Make sure of turbine steam ission conditions 29 kg/cm2g x 3000 C (below 43000C).
−
After steam ission conditions have been completed HP/LP turbine inlet pipe drain valves are fully closed.
−
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Start steam ission to turbine (DEH on).
115
−
Start accelerating. Accelerate at 300 rpm/min. upto target speed of 3000 rpm. Make rub checks as necessary during acceleration.
−
Start up the other G/T just before steam ission is completed, and hold it at 30% load, increase steam temperature and pressure of the other WHRB and when HP/LP by- valves in operation.
−
Loading.
−
Maintain 10% initial load for 10 minutes.
−
After initial load operation is over, open HP/LP steam stop valve of the other WHRB, and change pressure of the other WHRB and when HP/LP by- valves in operation.
−
Loading.
−
Maintain 10% initial load for 10 minutes.
−
After initial load operation is over, open HP/LP steam stop valve of the other WHRB, and change pressure control of HP/LP by valves to common pressure control.
•
Load up
−
Put HPCV (governor valve) in upstream pressure control sequentially close HP/LP by valves so that the S/T load up rate will be 2.5% min. at this time, HP/LPCV automatically following up in pressure control. As a result, steam goes to S/T.
−
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When S/T load reaches 20% full close all turbine drain valves.
116
−
When LP by valves is fully closed, shift LP by- valve to back up mode.
−
When HP by valves is fully closed, shift HP by- valve to back up mode.
−
After HPCV is fully opened, restore deaerator set pressure to normal pressure of 3kg/cm2, and put condensate pre-heater recirculation control back to automatic mode. (at gas firing only)
•
Rapid start up mode (Hot start up) The condition in which The HP turbine rotor temperature is above 4000C is called hot condition. In a hot start up, the turbine rotor temperatures is high enough, so the operating procedure required is to load and speed up the turbine in a shortest possible time using high temperatures team to minimize the mismatch temperature. Therefore hold G/T load at 40 MW at which this condition can be satisfied, and then start up WHRB and S/T. It is assumed that vacuum of condenser is maintained inlet condition.
•
Startup conditions Turbine vacuum
With vacuum
Gland steam supply conditions
Gland steam being supplied from WHRB
by
remaining
steam
pressure. Steam ission conditions
110t/h x 4000C above x 20 kg/cm2 (15% initial load)
WHRB inlet damper initial opening © PMI,NTPC
100% 117
Steam evaporation at G/T 50% load
25t/h
SH outlet blow valve capacity
35t/h x 29 kg/cm2
Time required for pressure rise
About 10 minutes
(10-2kg/cm2)
(at G/T 50% load)
WHRB temperature rise rate
4000 C/hr.
S/T speed rise rate
300 rpm/min.
S/T load up rate
2.5%/min.
(Before HP by- valve full close) (After HP by- valve full close) •
Start up procedure −
Preparation for start up Startup the following common equipment which is necessary for start up of GT, S/G and WHRB. a)
Raw water pump system clarifier, pretreatment plant and water treatment plants.
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b)
Service water pump system or clarified water pump system.
c)
DM water transfer pump system.
d)
Comp ressed system
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-
Start up chemical injection system.
-
Start up seal air fan.
-
Make sure that all preparations for start up have been completed.
−
WHRB startup
-
Start up HP BFP and LP BFP
-
Start up boiler circulating pump
-
Make sure the by- damper is full open and WHRB inlet damper is full shut.
-
Start up G/T After all preparations for G/T start up have been completed.
-
Make sure of S/T turning.
-
Open HP main steam stop valve and establish vacuum, supplying vacuum gland steam by remaining steam pressure of WHRB.
-
After G/T is loaded, open WHRB inlet damper upto 100% opening.
Note : * Manual Operation -
Start up G/T up to 40% load and hold that load.
-
At boiler start up fully open LP main steam stop valve and HP/LP by- stop valves in operation automatically by individual pressure control.
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-
At that time, opening of SH outlet blow valve and LP steam blow valve is automatically adjusted according to HP steam pressure at start up of WHRB.
-
Hp steam pipe drain valve main automatically adjusted according to HP steam temperature at start up of WHRB.
-
Open HP by- valve to start individual pressure control so that HP steam pressure will be constant of 2kg/cm2.
-
Open LP by- valve to start individual pressure control so that HP steam pressure will be constant of 5kg/cm2.
-
When HP/LP by- valves are put in operation, fully close by damper.
•
Open the turbine drain valve.
S/T start up -
Make sure that all preparations for S/T start up have been completed.
-
Make sure of turbine steam ission conditions 2 kg/cm2 x 4000C above.
-
After steam ission conditions have completed HP/LP turbine inlet pipe drain valves are fully closed.
-
Start steam ission in turbine.
-
Start accelerating. Accelerate at 300 rpm/min. upto target speed of 3000 rpm.
-
Check that preparations of generator have been completed.
-
Loading.
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•
-
Maintain 15% initial load.
-
After initial operation is over, full shut turbine drain valves.
Load up −
Put HPCV and LPCV (governor valve) in upstream pressure control and sequentially close HP/LP by- valves of both WHRB’S that the S/T load up rate will be SH/min. At this time HP/LP by- valves automatically following up in pressure control.
−
As a result steam goes to S/T.
−
When HP by valve is full shut, shift HP by- valve to backup mode.
−
When LP by valve is fully closed, shift LP by- valve to backup mode.
−
When HP/LP by- valves are fully closed, adjust G/T output to achieve target plant output.
−
At this time, G/T load up rate will be 3.6% min.
G/T, WHRB additionally start up mode While the plant is in partial load operation, the number of G/T’s in operation is changed in some cases to increase efficiency of plant operation. When starting up G/T and WHRB in such case, the start up procedure must be such that minimize the temperature difference between high pressure steam of the plant in operation and high pressure steam in G/T and WHRB to be started up.
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•
Start up conditions -
-
Conditions of G/T and WHRB in operation 1.
G/T load …………………………100%
2.
WHRB steam condition ………………..29kg/cm2x 4950C
3.
S/T load ……………….. about 40t
Condition of WHRB to be started up …………… Cold condition.
Startup procedure •
WHRB startup
-
Startup HP BFP and LP BFP.
-
Startup boiler circulating pump.
-
Make sure that by- damper is full open and WHRB inlet damper is full shut.
-
Startup G/T after all preparations for G/T startup has been completed.
-
Open HP/LP drum air vent valves and SH outlet air vent valve.
-
When WHRB inlet damper is open automatically open HP steam pipe drain valve LP steam pipe blow valve.
-
After G/ is loaded, open WHRB inlet damper up to full opening.
-
G/T is 40% load hold on.
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-
When HP steam pressure reaches 2kg/cm2g, automatically close HP/LP drum air vent valves, and SH outlet air vent valve.
-
Until HP/LP by- valves are put n operation, open SH outlet blow valve, HP steam pipe drain valve and LP steam pipe blow valve, and steam is blown not the WHRB blow down tank and ST flush pipe
-
At that time, opening of SH outlet blow valve and LP steam blow valve is automatically adjusted according to HP/LP steam pressure at startup of WHRB.
-
After HP steam temperature reaches above 4000C, open HP/LP by valve inlet stop valves to put HP/LP turbine by- system in individual pressure control mode.
-
Open HP by- valve to start individual pressure control so that HP steam pressure will be constant at 27kg/cm2.
-
Open LP by- valve to start individual pressure control so that LP steam pressure will be constant of 4.5kg/cm2.
-
When LP/HP by- valves are put in operation, fully close by damper.
-
After HP/LP by- valves have been put in operation, shut SH outlet flow valve,
-
HP steam pipe drain valve and LP steam pipe blow valve.
-
When HP by- valve is put in operation, equalize pressure and temperatures of both WHRB’s, open the HP steam stop valve.
-
When LP by- valve is put in operation, equalize pressure of both WHRB’s and open LP steam stop valve.
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•
Load up
−
Put LPCV (Governor valve) in upstream pressure control and sequentially close LP by- valve so that S/T load up rate will be 2.5% min.
−
When by- valve is fully closed, shift LP by- vale to backup made.
−
Put HPCV (governor valve) in upstream pressure control and sequentially close HP by- valve so that the S/T load up rate will be 2.5% min.
−
When HP by- valve is fully closed, shift HP by- valve to backup mode.
−
When HP/LP by- valves are fully closed, adjust G/T output to achieve target plant output.
WHRB SHUTDOWN PROCEDURE •
Normal shut down mode Normal shutdown is mode used as planned shut down of both WHRB’s. In this steam turbine has to the taken out first since we have to shutdown both WHRB’s. The following steps are adopted in WHRB shut down.
-
Start reducing GTs loads at the of 3.8% min.
-
Maintain GT load such that the HP steam tem. Is 4800C.
-
Put HP/LP turbine by- of both WHRB’s in individual mode.
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-
Bring down S/T load to 15% by opening the HP/LP by valve of both WHRB’s
-
Take LP steam of both WHRB’s in out of service.
-
Open steam turbine breaker and strip S/T manually.
-
Open the by- damper of both GTs.
-
When ST speed is less than 400 RPM. Close WHRB inlet damper of both WHRB’s.
-
Close the HP and LP Turbine by- stop valves of both WHRB’s after closing of the by- control valves.
-
Break condenser vacuum when condenser vacuum is less than100 mm Hg.
-
When the HP/LP drum pressure become less than2kg/cm2 open HP/LP drum air vents.
-
Stop one each CWP, LP & HP BFP.
-
Confirm that HP & LP economizer recalculation valve are opened and condenser preheater is by-ed.
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UNIT –I
UNIT-II
GT LOAD DOWN START
GT LOAD DOWN START
H P STEAM TEMP.
HP STEMA TEMP.
4800C
4800C
HOLD GT LOAD
HP HOLD GT LOAD
HP/LP TB BY
HP/LP TB BY
INDV. MODE
INDV. MODE
LP STEAM OUT OF SERVICE
HP STEAM OUT OF SERVICE
DEA HEATING STEAM VALVE CLOSED ST MAIN BRKAR OPEN AND ST TRIP
DEA HEATING STEAM VALVE CLOSED
GT BY DAMPER OPEN ST SPEED ^400RPM
GT BY DAMPER OPEN
WHRB INLET DAMPER CLOSE
WHRB INLET DAMPER CLOSE
HP/LP TURBINE BY CLOSE
HP/LP TURBINE CLOSE
BY
VALVE
CONDENSER VACUUM BREAK COND. VACCUUM <-100 mm of hg Gland steam Group off HP STOP V/V CLOSE
HP STOP V/V CLOSE
HP DRUM PR.<2kg/cm2
HP DRUM PR <2kg/cm2 open
open HP drum air vents
HP drum air vent
LP DRUM PR <2 kg/cm2 open
LP DRUM PR Less then <2 kg/cm2
LP drum air vent
LP drum air vents STOP ONE CWP ONE LP7 HP BFP
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WHRB out off shut down In this shut down only WHRB is taken out of service. ST will be in service with second
WHRB. Corresponding
GT
may be
stopped as
per
the
situations. The following steps are adopted in the shutdown. Reduce the GT load 30% of the corresponding WHRB which is to be taken out of service. -
Change HP and LP by- to individual mode.
-
Change LP turbine by- to main and open gradually (ST load down rate should be 2.5% min.
-
When LP boiler pressure, PB is less than LP turbine inlet pressure, close LP steam stop valve.
-
Change LP turbine by to individual mode.
-
Change HP turbine by to manual. Open gradually, ST load down rate 2.5%/min.
-
PB less than PT is when the shutdown boiler HP stream pressure is less than the turbine inlet HP steam pressure close HP stop valves.
-
Change HP turbine by to individual mode.
-
Open GT by damper.
-
After stabilizing HP/LP drum level close WHRB inlet damper.
-
GT shut down as per the situations.
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CONFIRM HP CV
CONFIRM LP CV
SET 29 KG/CM2
SET IN 5 KG/CM2
SHUT DOWN SIDE WHRB GT LOAD=30% CHANGE HP TB
CHANGE LP TB
BY TO IND. MODE
BY INDV. MODE CHANGE LP BY LOAD DOWN (ST) TO MANUAL & GRADUAL RATE 2.5 min. OPEN PB< PT PB: BOILER PRESSURE PT TB: INLET PRESSURE
LP STEAM STOP VALVE CHANGE HP TB BY INDIVIDUAL>MODE CHANGE HP TB BY TO MANUAL
ST load down rate
AND GRADUALLY OPEN
2.5%/min.
PB
PB BOILER PR PT TB INLET PR>
HP STEAM STOP VALVE CLOSE CHANGE HP TB BY IND MODE GT BY DAMPER OPEN STABILISE HP/LP DRUM LEVEL WHRB INLET DAMPER CLOSE GT SHUTDOWN IF REQUIRED
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MALFUNCTION AND REMEDIES ABNORMALITY
CAUSES
REMEDY
1. High Drum Level LP or HP drum
a. Malfunctioning of level controller
1. Change over the level Controller into manual Mode and drop the water Level.
b. Sudden increase in load
2. Re adjustable of the level Controller. 3. Closed the corresponding HP/LP Stop valve if both WHRBs are in service to avoid tripping of ST while avoid carry over of water to steam turbine.
2. Drop in drum level (LP drum or HP drum)
a. Malfunctioning of drum level controller
1. Change over the level Controller into manual Mode and normalise the drum level
b. Trouble of feed water pump
2. Start the standby feed water pump
c. In sufficient quantity of feed water
3. Re adjustable of level controller
d. Boiler tube leakage e. Sudden decrease of load 3. Too high temp. of flue gas
a. Too much of exhaust gas flow
1. Investigate the gas turbine
b. Too high temp. of feed water
2. Reduce the temp. of feed water
c. Fouling of heating surface
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SHUT DOWN PRODECURE -
Except in an emergency shut-down, load must be reduced gradually.
-
At approximately 15% of rated load, open all turbine drains.
-
When all load been removed, shut-down the unit by tripping either manually or remotely. This closes all the turbine HP steam and LP steam valves and disconnects the generator from the line.
-
Make sure that the auxiliary oil pumps starts when the bearing oil pressure drops below 0.85+0.05 kg/cm2.
-
Refer to the generator instruction book for information on the water supplied to the hydrogen coolers.
-
In order that the turning gear will be remotely engaged, be sure that the rotor speed is below the turning speed before the control switch is turned to the “START” position. This will also supply oil to the turning gear.
-
Shut-down the auxiliary oil pump and make sure that the turning gear oil pump starts when the bearing oil pressure drops below 0.75+0.05 kg/cm2.
-
Open the vacuum breaker when the turbine speed reaches about 400 rpm. Shut down the air removal equipment.
-
When the vacuum has reached zero shut-off sealing steam from the HP steam header to the gland steam control value and shut down the gland steam condenser exhauster.
-
Shut - down the condensate pumps.
-
Shut - down the circulating water pumps.
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-
Regulate water to the oil cooler to maintain the temp. of the oil leaving the coolers between 350 C and 380 C, if possible.
Loss of Electrical Load to the System In the event of a complete or partial loss of electrical load, the following procedure should be noted: -
Since the governor is in such a position that the no load speed is higher than 3,000 rpm, the speed will rise will suddenly at a rate proportional to the electrical load lost.
-
This sudden speed rise will cause the acceleration response feature of the governor momentarily to assume control of the turbine, and close the both HP and LP control valves rapidly.
-
With the both LP and HP control valves closed, the speed of the unit will decrease at a rate depending on the residual load left on the gtenerator.
-
If desired, the unit may now be resynchronized. If it is decided not to put the unit back on the line, the normal shut – down procedure outlined above should be followed.
-
If the cause of the load dump cannot be determined, resulting in a delay in resynchronizing, the unit should be tripped automatically after a specified time, the shut – down procedure described below under “Loss of Pressure or Temperature” should then be followed.
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Loss of Pressure or Temperature If the initial pressure regulating system is cut – out or inoperative the following procedure will apply: -
The operator will begin to reduce load in an attempt to maintain pressure above 80% of normal or temperature drop to not more than 830C.
-
If the pressure falls below 80% of normal or temperature drops 830C, reduce the load and trip the unit.
-
Place unit on turning gear, note power input to turning gear motor and listen for rubs. If everything is normal, follow standard starting procedure to bring unit up to speed and synchronize.
-
For the case where the pressure can be held above 80% of normal and temperature to less than 830C drop, bring the unit up in normal manner after temperature and pressure have been restored to normal.
-
If the unit trips from a known cause resulting from testing, etc. or if the vacuum is lost, let the unit come to rest and place on turning gear.
-
Check the rotor for truth. If everything is normal follow “Normal Starting Procedure”.
-
Where vacuum and speed can be held, reset overspeed trips and proceed with synchronizing the unit.
-
For the case where vacuum is lost and the low vacuum trip mechanism trips the load and unit, let the unit come to rest and place on turning gear.
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-
Clear vacuum trouble and follow “Normal Starting Procedure”.
Protective Devices Function
-
This unit is equipped with protective devices, which trip the unit in the event of low vacuum. Low bearing oil pressure and axial shift.
-
In addition, the unit can be tripped, either with the trip solenoid or with hand trip lever.
-
The situation can also arise where by the unit is inadvertently tripped in the process of testing the protective devices if the overspeed trip test lever is released prematurely.
-
In the event the unit is tripped as noted in above and the trouble has been recognized, cleared and the vacuum maintained, relatch the unit and proceed to synchronize.
-
If in the even of a trip and the vacuum is lost let the unit come to rest, following the normal shut-down sequence. After the trouble is corrected, restart the unit in accordance with the normal starting sequence.
Shut Down of Steam Turbine CHECK ST OIL GROUP GROUP GROUP ON ST TURNING GROUP GROUP GROUP ON ST DRAIN GROUP GROUP GROUP ON CONDENSATE PRE HEATER IN AUTO HP 2nd
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Economizer and LP economiser Recirculation valve on auto. 1st GT
2nd GT
GT Load Down from UCD or U
DT Load Down form UCD or U
If GT Load = 50%
If GT Load = 50%
Take HP TB PC coned.
Take HP TB PC coned
“INDIVID”
“INDIVID”
Take HT TB PC coned.
Take HP TB PC coned
“INDIVID”
“INDIVID” Check GT & ST should not be in APR
Decrease HPNTB P/C set
Decrease HP TB P/C SV to 10
2
Valve to 10kg/cm /(g)
kg/cm2 (g) gradually at the rate of
gradually at the rate of
let1.5 kb/cm2 min.
1.5 kg/cm2min Decrease LP RB P/C SV to 2kg/cm2
Decrease LP RB P/C SV to 2kg/cm2
If LPTB by open=50% Class LPSSV, IV
Close LPSSV IV check LP check
Check LP steam inlet D/V; IV open
LP steam inlet D/V; IV open
Continue GT Load decrease in unit mode or U up to 20% to 30% ST load 15%
open generator, breaker from UCB
HAND TRIP ST from U Check
(i) HPCV close, HPSV close (ii) HPCV close, HPSV close (iii) Turbine Drains open
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OPEN GT BYP ASS DAMPER
OPEN GY BYP ASS DAMPER HP and LP drum level will go down. Increase SV to + 50mm as after 10 min closing the inlet damper the drum Level will min. and go further down.
CLOSE WHRB INLET DAMPER
GO FOR GT ETOP CLOSE WHRB INLET DAMPER As gland steam will bead be available for 15 to 20 min
Close HP TB by, I/V
Close HP TB by, I/V
Close HP TB by, I/V
Close HP TB by, I/V If ST SPEED 400 RPM
Condenser Vacuum Group OFF
see (i) condenser vacuum pump stop (ii) Vacuum breaker open If condenser vacuum 100 of Hg
Gland steam Group Off
See (i) Gland steam condenser Exhaust fan stop (ii) Gland steam supply close
-
Close deaerator Heating steam supply
-
See turning motor run and turning gear engaged. (Check at local also)
-
Check condensate PH by B/P value open and inlet close.
-
HP and LP economizer re-circulation value open.
-
START TOP and STOP AOP
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Close HP SSV
135
-
Keep only one see each of the following auxiliaries, and stop if more than one running. i)
Circulating water pump CWP
For one module
ii)
HPO Boiler feed pump HPBSAP
One set
iii)
LP Boiler feed pump LP BFP
One set
iv)
Condensate extraction pump CEP
One set
Some Important Alarm/Trip Set Value •
Bearing Vibration Hi
HI 125 (P-P) Alarm
•
Bearing oil pressure low,
0.75+0.05 kg/cm2
Alarm low,
0.52 kg/cm2 Trip
Turbine speed High,
3240 RPM alarm HI HI,
•
3330 RPM Trip •
Condenser vacuum low,
635 mm Hg Alarm Low, 550 mm Hg Trip
•
Axial shift Trip
•
GT No load trip: if generator breaker of Both GT ( of the Module of which
5.6 kg/cm2
ST is opened then ST of that module will trip on “GT NO LOAD TRIP” •
HP main steam pressure high
> 70 kg/cm2
HP Main steam Temperature High
> 5250 C
HP main steam Temperature low
< 1500 C
HP exhaust Temperature High
< 1750 C
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•
•
•
•
•
•
•
LP main steam pressure high
> 7 kg/cm2
LP Main steam Temperature High
> 1750 C
LP main steam Temperature low
< 1500 C
LP exhaust Temperature High
> 800 C
Condenser Hot well level abnormal Low NWL
150mm
High NWL
+150 mm
ST Parameters ST Eccentricity High
0.0750mm
Axial Shift Low
1.100mm 2.10+2.0kg/cm2
High
+0.700mm
HP Differential expansion
LP Differential expansion
Low Low
-2.30mm
Low
-1.70mm
High
+4.70mm
High High
+5.70mm
Low Low
-2.30mm
Low
+2.90mm
High
+5.10mm
High High
+5.90mm
HP TB by spray water pressure low
9kg/cm2
-
HP by valve outlet temperature High
1900C
-
HP steam to condenser temperature High
2000 C
Thrust bearing temperature high
990C
-
1070 C
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Bearing temperature High
137
•
•
-
Lubricating Oil temperature High
500C
-
HP Oil pressure low
16kg/cm2
-
Main oil tank level low FL
+2165mm
Deaerator level Low
NWL 300mm
-
Low Low
NWL 60mm
-
High
WL+300mm
-
Deaerator pressure High
3.2kg/cm2
-
Condensate flow low
100t/hr
Oil pump auto start : -
Auxiliary oil pump auto start
0.85kg/cm2
-
Bearing oil pressure low alarm
0.75kg/cm2
-
Turing oil pump auto start
0.75kg/cm2
-
DC Emergency oil pump auto start
0.52kg/cm2
-
Bearing oil pressure Low Low Trip
0.52kg/cm2
ST Run Back Run Back on ST will operate automatically if ST load > 50% and its load will come down to 45% in the following cases.
i)
Out of the 2 running CEP one trips and 3rd one does not starts on auto.
ii)
Out of the 2 running HP BFP one trips and 3rd one does not starts on Auto.
iii)
Out of the 2 running LP BFP one trips and 3d one does not starts on Auto.
iv)
STG transformer winding temperature High
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Soaking Time in Warm Start (in minutes (M.T.) S=
Where
5 + 8T 50
=
9M} {P {3 x 2 + 14} 200
T
= steam inlet temperature in 0C
M
= 1st stage metal temperature in 0C
P
= Intel steam pressure in kg/cm2
For conservative time estimation last term can be neglected which is negative (and negligible) S
=
5 + 8T
=
9 M Minuets
50 Exhaust Hood Spray This spray will start if exhaust hood temp. 700C This will close when turbine load 15% and exhaust hood temp 700C Exhaust hood temp. high alarm at 800C Curtain spray Certain spray will start when CEP is running and any of HP & LP. TB By valve i/v and control valve open. It will close when HP & LP by control valve close System Alarm/Trips The following alarm./trips are provided to safe guard the operation of steam turbine. © PMI,NTPC
139
Main oil tank level high FL
+ 2565 mm
Main oil tank level low FL
+ 2165 mm
Thrust bearing wear alarm
2.10+0.2kg/cm2
Thrust bearing wear trip
5.3-5.6kg/cm
Bearing oil pressure low alarm
0.75+0.05 (bearing oil) kg/cm2
Bearing oil pressure low trip
0.52 kg/cm2
Turning oil pump auto start
0.75+0.05 (bearing oil) kg/cm2
Emergency oil pump auto start
0.65+kg/cm2
HP oil pressure low alarm
18.0 kg/cm2
Aux. oil pump auto start
0.85+0.05 (bearing oil) kg/cm2
Oil cooler outlet oil temp. high
500C at 900 rpm to rated 400C turning to 900 rpm
Axial shift alarm at
2.1+0.2kg/cm2 (trip control oil)
Turbine trip indication
3.0+0.2kg/cm2 (auto control oil)
Turbine reset indication
6.0+0.2 (auto stop oil)
Turbine trip test indication on
3.0+0.2kg/cm2 (at trip test)
Turing gear interlock
0.3kg/cm2 (bearing oil)
Journal bearing metal temp alarm
107o C
journal bearing metal trip
113oC
thrust bearing metal alarm
99oC
thrust bearing metal trip
107oC
bearing oil drain tem alarm
77oC
Main oil transfer pump takes suction from bottom of main oil tank and transfers the oil to the dirty oil storage tank. From dirty oil storage tank the dirty oil is fed to plant oil purifier and after purification oil is stored in clean oil storage tank. From clean oil storage tank the oil is transferred back to main oil tank by clean oil transfer pump. Main oil transfer pump, plant oil purifier, dirty oil storage tank clean oil storage tank and clean oil transfer pump are common for both modules.
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Turbine Gear Operation During Shut-Down In order to facilitate starting, the turning gear should be in operation and should preferably have been in operation through-out the shut-down period Even after the turbine is comparatively cool, this offers minimum time lost in starting and an assurance of safety in the even of any leakage of steam in to the turbine during the shut-down period. When it can be arranged without delaying work schedules, the turning gear and the oil circulation system should be kept in operation for not less then 48hours after shut-down. In the event that continuous rolling during shut-down is not practical, the turning gear should be started again at least an hour before rolling the turbine with steam. It should remain in operation sufficient length of time be fore itting steam to the turbine to allow the rotor to become straight with in the limits noted in the section on “Limit son operation. Recognizing that tha preceding schedule may not be practical in case repairs or adjustments are to be made to the turbine, the following schedules are recommended as the absolute minimum periods considered satisfactory under various circumstances. •
The turbine is to be dismantled. The turning gear should be kept in operation until the dismantling programme requires that it should be stopped.
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If an emergency necessities it, the turning gear may be shut down immediately. However, it must be recognized that this may subject the rotor to a severe bow. It must not be used for continuous rolling unless the bearing lubrication system is operating. Bearing oil circulation must be maintained after shut down to protect the bearing against overheating. The minimum circulation period should be 24 hours after shut-down. During this period oil temperature leaving the coolers should be held below 35oC. If possible •
Shut down of extensive repair or adjustment Both the turning and bearing oil circulation systems are to be kept in operation for a minimum of 12 hours; oil temperature from the cooler should be maintained below 35oC. if possible. Both the turning gear and the oil circulation may then be shutdown. If operation of the unit is to be resumed before the turbine is thoroughly cooled, the turning gear should be started 3-4 hours before rolling the unit with steam.
•
Shut-down for minor repair or adjustment Depending upon the nature of the work to be done, the following shedule may be adopted. Keep the turning gear and the oil circulation systems in operation for a minimum of 3 hours. Both systems may then be shut off for a period of
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not more than 15 minutes; if practical, however, oil circulations should be maintained. •
Immediately thereafter roll the turbine on turning gear for at least 2 hours. Both turning gear and oil circulation may then be stopped for not over 30 minutes; however. 15 minutes after stopping, the rotor is to be turned 180o with the gear. This 180o turn may be made without oil circulation, if avoidable.
•
Following the 30 minute shut down period as above, the turbine should again be rolled on turning gear with oil circulation for a minimum of 2 hours.
•
The system may then be shut down indefinitely, provided that at 30 minute intervals for the next 6 hours the rotor is turned 180o Deg. Turns may again be made without oil circulation if necessary.
•
In any case, if the turning gear has been disengaged, it should be put in operation a sufficient time before itting steam to the unit to allow the rotor to become straight within a limit of approximately 0.025mm on the truth indicators or within a limit of approximately 0.05mm on the eccentricity recorder indications.
System Alarms The following alarms and shutdown point are provided for gas turbine protection. -
Hand trip
-
Alarm/Shutdown
-
Fire
-
Alarm/Shutdown
-
Lube oil Pressure Low
-
Alarm
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143
-
Lube oil Temperature High
-
Alarm
-
Lube oil tank level Abnormal
-
Alarm/shutdown
-
Inlet Air Filter Abnormal
-
Alarm
-
Vibration high
-
Alarm
-
Vibration high
-
Alarm/shutdown
-
Exhaust gas temperature
-
Alarm/Shutdown
-
Gas Pressure Supply Pressure Low
-
Alarm/Shutdown
-
Liquid fuel Main Pump Trip
-
Alarm/Shut down
-
Gas Leakage
-
Alarm
-
Liquid Fuel Drain Tank Level Hi
-
Alarm
-
Over speed
-
Alarm/Shut down
-
Exhaust Gas Pressure Hi
-
Alarm
-
Exhaust Gas Pressure Hi
-
Alarm/Shut down
-
Flame Out
-
Alarm/Shut down
-
Flame Detector Abnormal
-
Alarm
-
Blade Path Temperature Spread Hi
-
Alarm
-
Disc Cavity Temperature Hi
-
Alarm
-
Turning Gear Trouble
-
Alarm
-
Bleed Value Abnormal
-
Alarm/Shut down
-
Acceleration Rates Insufficient
-
Alarm/Shut down
-
Auxiliary Motor Abnormal
-
Alarm
-
SW Position Wrong
-
Alarm
-
Inst. Air Pressure Low
-
Alarm
-
Electrical Fault
-
Alarm/Shut down
Emergency Operation The Gas Turbine System can be operated in an emergency during the following conditions: -
Operation of the turbine with a control or lube oil system leak.
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-
Immediately tighten nuts, bolts or fasteners, if leak cannot be stopped the gas turbine must be shut down and repairs made.
-
Operation with a turbine with a fuel leak.
-
If a fuel leak occurs prompt action must to taken to contain and repair the leak. If the leak cannot be controlled the gas turbine must be shut down.
-
Operation of a gas turbine with abnormal vibration.
-
Many factors affect vibration, the cause of any abnormal vibration must be found and corrected. Unsteady blade path temperature, high bearing temperatures, turbine rotor rubbing against the turbine casing, speed and load will all directly or indirectly affect the amount of vibration.
-
Alarm and trip devices will shut down the unit if vibration exceed 0.200 mm (Peak to Peak).
-
Operation of a turbine during compressor surging.
-
If the gas turbine speed exceed its specified low frequency operating range, compressor surging can occur. If surging occurs the unit must be shut down immediately and inspected for damage.
-
Operation of a gas turbine in a under frequency condition.
-
If the unit is to run in under frequency mode particular attention should be paid to turbine blade resonance and compressor surging. If the turbine compressor enters a surge condition it must be tripped immediately.
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Turbine blade resonance is produced by blade conditions above or below the blades normal operating frequency range. Turbine blade resonance is a main concern of low frequency operation. When operating below Hz the unit must be monitored for abnormal noise and vibration. If unit frequency falls below 47.5 Hz for more than 15 seconds or falls below 47.0 Hz it will automatically trip. System Trouble Shooting Dependable, economic operation over long periods of time is only possible if the equipment is maintained and operated according to the manufacturer’s instructions. If problems develop, they should be traced and corrected as soon as possible. The following is a list of potential problems, possible causes and suggested checks to be performed by the plant equipment operator. Potential Problem -
Gas turbine starting motor will not start when “GTSTART” push button is depressed.
Possible Causes
-
Unit auxiliaries not running, starting motor circuit not operating Properly.
Operator Action
-
check all switch positions at unit auxiliary motor control centers, check starting motors breaker for standing switch gear alarms at breaker.
Potential Problem -
Gas turbine trips at ignition and light off sequence.
Possible Causes
“Fuel On” – “FLM2,”FLM3” out of control sequence,
-
CSO and main fuel oil pump discharge pressures are
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146
out of adjustment (liquid fuel firing only), blade path temperatures during light off abnormal. Operator Action
-
Check “Fuel ON” –“FLM2 and FLM3” lamp sequence and compare to previous operation, check for any extreme difference between “FLM2” and “FLM3” lamp sequence, check CSO and main pump discharge pressure,
check
blade
path
temperature
spread. Abnormal Operation The response characteristics of turbine-generator units and their auxiliaries to power system emergencies and the procedures to follow during such periods are of particular interest to the operating utility. In general, question may arise concerning the following: -
Low-frequency operation
-
Maintaining low auxiliary load on the unit
-
Steam turbine motoring
Low Frequency Operation Limits for operating frequency and time at frequency are given for the protection of the turbine and generator during under-frequency over-frequency operation. For the turbine prevention of resonant operation of the tuned blades is the major consideration. Operating limits have been developed form engineering analysis coupled with laboratory testing and extensive field telemetry testing.
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Time limits are based on fatigue life consideration and have been apportioned to the more prevalent under-frequency operating mode. For specific blading styles, more conservative off-frequency operation limits have been imposed than previously published. Generator operating limitations are also presented which must be observed during off-frequency situations in order to prevent undesirable thermal excursions of generator components. The primary consideration in operation of a steam turbine under load at other than the synchronous frequency is the protection of the long tuned blading in the low pressure (L.P) turbine element. In the design and design verification process, great care is taken to properly tune the tapered and twisted blades that constitute the last several stages in the L.P. turbine element. Operation of these stages under load, at a speed that causes a coincidence of blading natural frequency in a lower mode and multiple of the actual turbine running speed frequency, will lead to blading fatigue damage and ultimately blading failures. Auxiliary Load Operation Most turbine generator units are designed to withstand complete isolation from the system and remain in service at no load or at an auxiliary load level. However, it should be recognized that some of the larger plants now in operation, and under construction, have plant control interlocks which automatically trip the complete plant in the event of load separation.
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The transient stress in the turbine rotor is a factor to be considered when suddenly dropping from full power out to auxiliary load and the subsequent rapid application of load after the connection to the system is reestablished. The immediate affect is an instantaneous drop in first stage temperature of about 120 degree c. This drop in stage temperature after initiation of the
produces a peakstress in 10to 15 minutes
transient which decays slowly to zero
in about
one
hour, if auxiliary load is maintained for an hour or more, the rotor is forcecooled to new equilibrium state. The subsequent reloading should then be performed at a moderate rate in order to avoid
a large thermal stress in the opposite direction. To minimize
the accumulation of rotor fatigue damage, it is recommended that auxiliary load operation following
an electrical out occur only when the
system
conditions make it absolutely necessary. Steam turbine motoring Motoring occurs when the steam supply to the turbine is shout while the generator is still On the line. Since there is
no steam energy available
to
supply the turbine-generator
losses, the generator will act as a synchronous motor and drive the turbine. Although this condition is generally
defined as generator motoring, the
protection is not for the generator, but to prevent overheating low pressure turbine or high pressure turbine blade. Monitoring can caused by closing steam valves without taking the unit off the line. This can be done by.
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-
Closing all governor valves during shut- down operation.
-
Tripping the turbine while not tripping the generator breaker.
Our general policy in connection with motoring of a turbine generator unit is as follows: -
Closing all governor valves during shut-down operation.
-
Tripping the turbine while not tripping the generator breaker.
-
Operation with high density steam entrapped within a high pressure casing may result in a rapid temperature rise of the steam and internal turbine parts.
-
A turbine- generator unit should not be motored for any extended period without ventilating steam being itted to all turbine elements.
-
It is recommended that such operation be limited to not less than one minute.
-
It is recommended that the opening of the generator circuit breakers for the purpose of avoiding or limiting a motoring condition be controlled by the closing of the main valves.
Interlock for turbine motoring is installed to protect turbines from the damaging effects caused by motoring and lack of ventilating system.
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Emergency Operations •
Black out (Grid Failure) Main Control Broad is connected to the grid with two 200 KV lines terminating 220KV and two 400 KV lines terminating at 400 KV. At Main Control Broad the interconnection between 220 KV and 400 KV Buses is through two ICTs each of 325 MVA capacity. Black out condition may occur when all the four lines trip or there is failure of supply at both 220 KVS/S and 400 KV S/S. In case both 220 KV lines trip and 220 KV side breakers of ICTs also get opened then a circuit called “House Load Circuit” gets triggered. This will cause tripping of both STs and desynchronization of all the GTs except one reselected GT which will supply House Load of Gas Plant. In all other conditions units at Gas Plant will trip on high frequency or low frequency (Grid sinking). In case of total power supply failure Gas Plant provision has been made of “Emergency Gas Turbine” (EGT) to run essential auxiliaries and to start one Gas Turbine. One “Emergency D.G.Set” has also been provided to keep the units on turning gear in case of non-availability of EGT.
Following operation shall be carried out in the condition of black out (i.c.,when all the efforts to save one of the GTs for House Load have failed ):-
Ensure that DC Lub. Oil pumps and DC Seal Oil Pumps have started in auto, if not start manually.
-
Ensure the starting of EGT in Auto/ Manual.
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-
Charge Station 6.6 KV SW/GR (one bus only) from EGT and charge auxiliary transformers one by one. Put on sufficient lights
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Start cooling Water Circulating Pumps (110 KW) for GTs and then one IAC.
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Ensure starting of AC Lub. Oil pumps of GTs and TG Oil Pumps of STs. Then stop DCLub. Oil Pumps. Start AC Seal Oil Pumps of all the units and then stop DC Seal Oil Pumps.
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Ensure starting of turning gear of all the units.
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In case EGT is not available then “Emergency D.G. Set shall be started. Other restorations are to be done only after power Supply is made available from the grid. IOCC and 220 KV S/S shall be ed for restoration of grid supply, so that start up power can be made available to the Gas Plant.
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Start Raw Water Pumps and Clarified Water Pumps.
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Start6 on PAC.
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Line up one of the Gas Turbines for start up.
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Start the selected Gas Turbine and keeping both 220 KV lines and ICTs out of service charge one 220 KV Bus (In case Grid supply has not been restored till that time).
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Take one Station Transformer into service (which can feed to the other dead 6.6 KV Bus)and transfer station auxiliaries load to this transformer.
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Following shall be attempted only if Grid Supply has not been normalized. -
After getting a load of –7 MW on gas turbine, by starting station auxiliaries, one of the 220 KV lines shall be charged from Gas Plant end with proper communication to 220 KV S/S and they shall be advised to load the line at the rate of –5 MW/minute.
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Start one more gas turbine and keep it ready for synchronization. This units can also be brought on bar if the previously running gas turbine has crossed a load of 20 MW. Then further load can be increased at the rate of 10 MW/minute.
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After the grid condition are normalised EGS can be stopped. Get the other 220 KV line charged from the grid end and synchronize at Plant end. Similarly synchronize the 400 KV lines, Bring the combined cycle into service.
Tripping of one of the 220 KV lines(Station Load full) Normally when both the 220/400 KV ICTs are available, tripping of the 220KV lines shall not pose much problem. However it shall be ensured that the other 220 KV lines and ICTs are not getting overloaded. To reduce the station load one of the gas turbines can be desynchronized temporarily. If the overloading is minor, then station load can be reduced slowly. In case only one ICT is in service then changes of overloading 220 KV lines/ICT are high and one or two gas turbines can be desynchronized temporarily, depending on the amount of overload, to control the station the station load. After ascertaining the healthiness of the line units can be normalized.
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Tripping of both 220 KV lines (Station Load Full) Normally when both ICTs are available tripping of both 220 KV lines will overload the ICTs slightly. The amount of overloading will depend on the reactive power flow through ICTs. Depending on requirement, one of the gas turbines can be desynchronized temporarily or the load on all the units can be reduced gradually to a safe level. However , if only one ICTs is available at the time of tripping of both the 220 KV lines, immediately two GTs shall be desynchronized, load on remaining GTs shall be reduced and load on STs shall be reduced by operating HP/LP by valves. If fast action is taken ICT will also trip on over load causing “House Load Circuit” triggering. Another course of action can be, opening of 220 KV Bus coupler breaker. This action may however trip the units connected to the bus, where 220 KV lines were connected earlier. After ascertaining the healthiness of the line units can be normalized. Tripping of one of the ICT s {Station Load full} Same action as in above point no. 2 will have to be taken depending on the loading of remaining ICT and 220 K V LINES. Tripping of both ICT s {Station load full} Same action as above will have to be taken to avoid overloading of the 220 K V lines. High Frequency Operation At a Frequency of 51.2 Hz there is provision for GT s to run back to 60 MW. In case Frequency remains above 51.5 Hz for more than 2 minutes both GTs of a module will get automatically desynchronized
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NO load ST Trip”. Before this happens efforts can be made to bring down the frequency by desynchronizing of the GTs manually and reducing be made load on remaining GT. Simultaneously IOCC shall be ed
to control the
frequency. During this process HP/LP stop values of the out of service WHRB shall be kept closed to a void tripping of ST on “WHRB HP/LP Drum level High “. If the frequency comes down the GT shall be synchronized and WHRB shall be taken into service immediately to avoid drop in outlet temperatures of WHRB. If all the efforts fail the remaining GT will also get desynchronized and ST will trip on “ GT No Load ST Trip”. During this time efforts shall be made to keep the WHRB s in steady operating condition by controlling the drums level and operating the HP/LP by valves suitably. When frequency comes, GT s can be synchronized and ST rolling can started to bring ST into service .
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12. Protection And Interlock General Philosophy Regarding Protection Electrical Equipment All electrical equipment must be protected against internal and external faults, as well as abnormal conditions which may endanger the equipment or the system. Appropriate relays and devices have to be provided, to detect the faults and potentially dangerous conditions and to isolate the concerned equipment cost the earliest in order to minimize the damage. The basic requirements of sensitivity and speed have to be met by provisions of carefully selected relays and relaying schemes, since failure of a protection operate cannot be ruled out, and it need lead to damage of costly equipment, it is necessary to provide back up in the form of redundancy of relays, local or upstream back up relays etc. however, continuity of power generation is also of utmost importance and it dictates that the risk of tripping (due to mal-operation of the protection) when a tripping is not really necessary be minimized. This risk is related directly to the number of relays and other elements in the protection scheme must be made as simple as possible. Redundancy and back ups must also be minimised and tendency of over protection checked. The protective relays must be connected to automatically isolate the endangered equipments if the damage is imminent or short circuit has already occurred. However, in cases where the equipment or system is not immediately endangered and can continue in service for some more time, the protective devices should be connected to initiate only an alarm. This would enable the plant operator to take corrective steps and prevent a tripping or to prepare the outage of an equipment (by changing over the stand by provisions or bringing down the unit load).
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It is also important that the control room operator is not burdened or confused with too many alarms. The alarm facial on the unit control board (UCB) are therefore to be used for annunciating only those abnormal conditions for which the UCB operator has to take some distinct preventive or restorative action. It is generally adequate and/or with hand reset flags on protective and auxiliary relays s or switchgear. Protection of Generator & General Bus Against Bus Duct Against internal Fault The generator bring very costly and critical items in a power plant, should be protected by duplicate protections. Phase to Phase Faults are to be covered by differential relays 87G and 87GT. In case of CGB scheme 87 GT covers up to CGB generator is not protected by87GT. In such a case voltage controlled over current protection is provided as back up. The stators earth faults would be covered by 64 G1 & 64G2. the earth faults in the stators winding close to natural are generally remove, however the chances of such faults can not be ruled out. It is therefore proposed to have at least one stators earth fault relay to detect faults anywhere in stators winding including neutral terminal. The relay 64G1 therefore, should be capable of detecting earth faults anywhere in the stators winding but 64 G2 may cover only 95% of the stators winding. There are various type of relays available which operate on different principles to detect earth faults close to neutral (64G1). One of the type has there elements built in. One element detects rise of the fundamental frequency voltage across the neutral grounding resistor and protects the generator winding about 95% (relay element similar to 64G2). Second element detects collapse of third harmonic voltage present on neutral and thereby detects the earth fault close to neutral. Third element which supervises
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the voltages, prevents the tripping command to exist during generator shutdown period. ASEA relay type RAGEA operates on as principle. Relays type PVMM, manufactured by English Electric Co., works on a more or less similar principle, except that the second element compares the third harmonic voltage element protects the entire generator stators winding except for a blind zone in the middle of the winding. Another principle to inject a low frequency signal (ariybd .01 pu) which biases the generator neutral point to earth. Through a minimal amount of current flows continuously due to generator and lead capacitance, any drastic increase in the current indicates the existence of a ground fault. BBC relay type GIX 103 ir GIX 104 & SIEMENS relay type 7UE22 are examples of relays working on this principle. Any one the above described type of relays can be considered for 100% stators earth fault protection. The relays are time delayed by about one second to prevent mal operation during transformer KHV side ground faults currents is limited to about 10A. The earth fault relay 62 G4 shall be voltage operated inverse time relay connected across the open delta winding of an auxiliary voltage transformer 9 (VT) on generator terminal. The generator bus ducts has an isolated phase construction throughout and hence cannot have a direct phase to phase fault. Their coverage by one 87GT (and partly by 87G also in case of GCB scheme) for short circuits is therefore considered sufficient. Earth faults in the generator bus ducts are however covered by two protections 64G and 64 G2. The configuration transverse differentials protection (50G) is proposed to be provided for generator internal fault protection.
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In case generators have only three terminals brought out in line as well as neutral sides (even if the stators winding is double star) it would not be possible to apply the conventional transverse differential protection for internal faults. The inter turn fault protection is not considered very essential since such faults are very unlikely in single turn coil construction adopted for the large generators. For earth fault protection of the generator rotor circuit a two stage relays (64F) working on the principle if insulation resistance
measurement of rotor is
provided. The first stage of the relay shall be connected for alarm in UCB. Further fall in insulation resistance of the rotor circuit initiate operation of the relay second stage which is recommended to trip the unit. However it is recommended that the machine be shut down for detection of fault location and rectification as early as possible on occurrence of first stage alarm because a second earth fault at another location could be quite harmful. Protection of Generator Transformer & EHV connection Provisions of duplicate instantaneous (main) protection is mandatory for all 220 KV/400KV elements. Accordingly the overhead EHV connections between generator transformer and switchyard would be covered by two differential protection 87GT and 87Tl each of would detect all type of short circuits. 87GT and 87LT would also cover HV side earth faults, HV/LV winding would be covered by generator earth fault relays 64G1 and 64G2. In addition to the above differential protections, the generator transformer would have the usual Buchholz relay (63T), winding temperature indicator and oil temperature indicator, each of these would have alarm and trip s. Winding temperature indicators would be connected to as repeater dial on the unit control board, and would also be used for transformer cooler control.
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Transformer would also have oil level indicator with alarm , and devices to detect abnormal, ties in coolers control. Protection of Generator and Generator Transformer Against External Faults and Abnormal Operating Conditions As the large-generator have a very limited capacity to withstand and negative sequence currents (caused by unbalanced loading, opening of phase, and uncleared system fault etc.) a precise negative sequence protection 46 G must be provided. It should have an 12 element to give a time delay alarm in case of a persistent negative sequence current exceeding the generator continuous 12 capability and 12t unit to initiate generator tripping before generator 12 2t capability limit has been reached. While the generator has a certain overload capacity, continued operation at a load higher than its rated current can lead to overheating and consequent insulation failure. The protection against stators over heating would be provided by the generator temperature monitoring system. No protection or separate temperature monitoring for bus ducts is necessary since they would have a naturally cooled rating slightly higher than the generator rating. A three-phase single zone impedance relay (21G) is to be provided for the back up protection of generator against external three phase and phase to phase faults in the EHV system, which may be hanging on due to failure of their own system primary protection, The zone of 21G should be extend beyond the EHV switchyard. It should be connected to trip the generator after a time delay of 1.0 to 1.5 seconds so that the generator is tripped only if the EHV line protection has not cleared the fault even in second zone.
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The back up protection against resisting earth faults in the EHV system is provided by an over current relay 51 NT connected to the current transformer (CTs) in generator transformer HV neutral. Local breaker backup (or breaker failure) protection 50Z is to be provided for the generator breaker EHV as well as GCB). It would facilitate fast back up isolation of faults not cleared by the instantaneous primary protection. Loss of excitation protection 40G is provided as per universally accepted practice. In case steam turbines ratings high (<=200MW), a Pole slipping protection is also provided to cover the protection against instability in EHV system. The generator can develop high over voltages in the even of mal operation of automatics voltage regulator, or a load thrown off while generator excitation is under manual control. An over voltage relay 59G set at 110% is to be provided to detect its condition and give an alarm in case it persists for 2-3 seconds. The severe over voltage would be covered by over fluxing protection which is explained in next clause. Over fluxing of the generator transformer and unit transformer can occur due to over voltage on generator terminals, or due to excitation application while turbo generator is at lower speed. Its persistence can cause gradual over heating and damage to the generator and transformer. An over fluxing protection 99T is to be provided to protect the generator and transformer against the condition. The generators to be tripped if voltage/frequency remains above 1.2 p.u. for about 2-3 seconds. Time delays necessary in order to allow AVR to correct the situation. In case of GCB scheme separate over fluxing relays need to be provided for generator and generator transformer.
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Motoring of the turbine generator can occur due to storage of steam/fuel supply. While it does not affect the generator, it can cause gradual over heating of turbine blade. The low forward power relay 37GA along with a time delay is used for detecting this conditions. While reserve power relays have conventionally been used for this protection, a low forward power relay (LFPR) is proposed to be used since both are equally effective for this purpose. In fact LFPR is better, since turbine blade over heating would occur even if the unit is in generating mode at a load at which LFPR operates about 0.5% of rated output. Any other electrical protection which have to be provided for turbine safety is the under frequency protection. The turbine manufacturers normally stipulate stringent limitations regarding under frequency operation of turbine, and it would be safer to trip the unit whenever the frequency tends to drop to a value dangerous for turbine operation. A multi stage relay is proposed to be used for under frequency protection (81G) the first stage of the relay gives an alarm in unit control board indicating that the system is operating in an under frequency regime. The operator should inform the Regional Load Dispatch Centre regarding this so that they can take timely action by way of load shedding to improve the frequency. The second stage will stage will again give an alarm permit operation for a set time after which the machine will be tripped. The third stage when reached will instantaneously trip the machine. These levels of operation are normally defined by the turbine manufacturers. Relay are also available which have df/dt functions. However it would also be beneficial to work out am overall operation strategy, during under frequency conditions, in association with the Region Load Dispatch Centre and other utilities in the region.
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Monitoring of generator VT fuses would also be provided to give an alarm in UCB, in the even of fuse failure. VT fuse failure may maloperate back up impendence stators earth fault (64G2), low forward relays and under frequency relays therefore these protections would be blocked on VY fuse failure. Besides the above electrical protections, the generator and its auxiliary systems (hydrogen cooling, bearing lubrications, seal oil, gas etc.) would be protected in accordance with manufacturers recommendation. Fire protection system shall be provided for generator transformer and its shall be give for class A tripping. Generator have certain MW/MVAR capabilities which are indicated in capability curve. The operation beyond its capability is harmful to the generator. In order to give a clear picture to the operator about the actual operation of the generator with references to its capability, it is proposed that DDC MIS should have a facility to show the actual operating point to generator plotted on the generator capability curve. Protection of Unit Auxiliary Transformer The unit auxiliary would have separate differential protection (87UT) as their main electrical protection against phase-to-phase faults. 87UT shall also have set instantaneous element. Earth fault bt their HV winding be covered by 64G1 and 64G2 and the earth faults in each LV winding shall be covered by restricted earth fault protection 64 RUT. Back up to 64RUT and HT system earth faults shall be provided by earth fault 51 NUT connected to a CT in transformer neutral. Backup to 87 UT for transformer faults and to HT incoming breaker protections for HT system faults would be provided by overcurrent relays 51UT with a definite time delay to coordinate with down stream protection. Besides the above electrical protections, the transformers would have the usual protective devices like
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buchholz relay with alarm and trip s, winding and oil temperature indicators with alarm and devices to detect cooler abnormality. Winding temperature indicators would have a repeater dial on UCB and will also be used for cooler control. The H.T. bus ducts between the unit auxiliary transformer and the associated H.T. Switchgears would also be protected by the transformer protections 87UT, 51UT, 64NUT. Unit auxiliary transformer is so designed that they would not normally get overloaded. However, winding temperature indicator (WTI) and oil temperature indicator (OTI) are provided alongwith alarm to detect any overloading and annunciate. Fire protection system shall be provided for unit auxiliary transformer and its shall be given to class A tripping. Generator Tripping Logic Scheme without GCB In the absence of any circuit breaker on generator on terminals, the generator and associated transformer become one integral unit as far as their protection in concerned. In case of short circuit anywhere in the generator bus ducts, generator transformer, EHV connections upto the circuit breakers, unit auxiliary transformer and H.T. bus ducts it is necessary to immediate isolate and H.T. unit incoming breaker instantaneously. This is commonly known as class A tripping and the following protections must be connected to initiate the same: 87G, 87GT, 87LT, 87UT, 63UT, 95G, 51UT, 64G1, 64G2, 64RUT, and 51NUT. Tripping of the circuit breakers as mentioned above would suffice for electrical isolation of the fault, the tripping of the turbine is not necessary. However, the electrical fault which caused the tripping would require investigation, and tripping
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of the turbine would be required subsequently in any case. Therefore, there is no point in trying to keep the turbine in operation after a class trip and it can also be tripped alongwith the circuit breakers in case of class A trips. Besides the risk of dangerous turbine over speeding upon disconnection of generator from the grid is higher when turbine is not tripped. This risk is considerably reduced when turbine is tripped simultaneously with circuit breakers. Scheme with GCB In case generator or circuit breakers are provided on generator terminals, EHV and 6.6 KV auxiliary breakers should not be tripped for generator faults/trouble since 6.6 KV supply needs to be retained for running of steam turbine. It is therefore, required that following protections need only be connected to tripping of generator circuit breaker, field breaker and turbine (class A1): 87G, 95G, 64G1, 64G2. However, the protections which covers generator transformer, UAT’s and associated systems beyond GCB upto EHV breaker such as: 87G, 87UT, 63UT, 51UT, 64RUT, 51NUT need to be connected to trip EHV breaker, field brenker, UAT LV brenker and turbine alongwith GCB (Class A2). Protections 64G, 21G, and 51NUT require isolation of the generator from a persisiting external fault, for which tripping of the EHV breakers alone would normally be sufficient. However, the fact that these ultimate back, up time delayed protections have operated indicates a serious failure of EHV switchgear or relays, since they would operate only if all the normal back ups EHV system (i.e. second main protection, second zone of distance relays and local breaker) has already failed to open (after having received a tripping command from the bus bar differential protection or local breaker back up), in which case there would be no point in giving another tripping impulse to the same breaker.
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Considering all these aspects, it is proposed that the back up protections under consideration (46G, 21G & 51NT) be connected to initiate class A/Class A2 trip. In case of over-fluxing, ideally only the generator excitation need to be rapidly cut back. Since this is not feasible when AVR is not functioning properly (which is evident from the very fact that 99T has operated), tripping of the excitation becomes necessary. However, this would lead to loss of excitation condition unless the generator is completely tripped at the same time. Therefore, in effect, over-fluxing protection (99T also have to be connected to initiate tripping of all generator circuit breakers. Considering the fact that serious abnormality in the excitation system is indicated, and generator back in service, it is proposed that turbine also be tripped simultaneously. 99T would therefore initiate class A/class A2 trip. The generator would work as an induction machine and there is a loss of excitation. The turbine generator speed would rise, the MW output would come down due to governor action and the generator would start drawing a large MAVR from the grid. While the generator can sustain this condition for a short time at much reduced active load, it is nevertheless to keep such an abnormal condition in which severe torque oscillations are produced, current are induced in the rotor core and overheating takes place. Since the protection operation indicates major failure in excitation systems protection should also be connected to class A/class A2 trip. Pole slipping of provided on steam turbine proposed to be connected for class A tripping since it is a condition in which turbine generator may have suffered mechanical damage and the unit has to be brought back only after careful checking.
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Unit Tripping Logic As mentioned earlier, most of the electrical protection of generator transformer unit have to connected for class A mode of tripping, where all circuit breakers and turbine are tripped simultaneously, as soon as the fault has been detected by protective relay. The electrical fault in the generator transformer are comparatively rare, and most of the unit tripping take place due to other causes generally related to problems in boiler, turbine take place due to other causes generally related to problems in boiler, turbine and their auxiliaries. For such problems, it is not necessary to trip the generator breakers instantaneously. Infact, it is desirable to delay their tripping, as explained below: In a class A trip, the turbine-generator gets instantaneously disconnected the power system, stops delivering power and is then free to accelerate. The turbine emergency stop valves take a finite time to close and some steam enters the turbine even after initiation of a trip. This steam alongwith the steam which is already present in the turbine expands to energy in accelerating the turbine. There after the turbine would start slowing down if the steam Valves have closed fully and there is a case all the valves have not closed fully and there is a leakage through any of the valves, the turbine may not slow down but start accelerating further. The dangerous possibility is now widely recognized and a sequential trip (also known as class B trip) the turbine valves are allowed to close first. When the steam inside the turbine has exhausted, the power output of the turbine generator comes to zero. This condition is verified by sensitive LFP, and then only the generator circuit breakers are tripped. This kit is insured that the turbine does not over speed. Another advantage of class B trip is that grid suffers the loss of generation as a cramp down over a few seconds, and not as a step. Two LFPR are provided to provide backup to each for the purpose.
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However in case of Gas turbines, since a large compressor load is connected on it’s shaft such dangerous over speeds are not expected and as such no necessity of all the protection are connected to trip electrical breakers as well as turbine simultaneously. The trip s of WTI and OTI of generator transformer (49T), the excitation system troubles if not urgent, and all abnormalities in generator auxiliary system (hydrogen cooling, bearing lubrications, seal oil, gas etc.) which require unit tripping should follow class B mode. Excitation system troubles which require immediate tripping of excitation would initiate class A tripping. In OTI of the unit transformer would be connected for tripping only associated H.T. circuit breakers (but not de-enrgising the transformer. Two separate hand reset lockout relays 86G and 186G would be provided for class A/Class A1/Class A2 tripping. They would operate in parallel but on separately fused 220VDC supplies, the various protections would be distributed on these two DC supplies such that even if one of the supplies, faults. The VT supplies, DC supplies and healthiness of lockout relays would also be monitored. Protection of Station Transformer The station transformers would have differential protection (87) as main electrical protection against phase to phase faults, 87 shall have one instantaneous high set element. Since EHV system is solidly grounded, the HV winding earth fault shall also be covered by 897. However, a restricted earth fault protection. Back up to incoming breaker protection for H.T. system faults would be provided by definite time delay over current relay (51) on all three phases on HV side of transformer. For earth faults in LV winding, restricted earth fault relays (64RST) shall be provided, Backup to this and H.T. system, earth fault shall be provided by a
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definite time delay really (51N) connected to a CT in transformer neutral, which shall be connected to trip EHV breaker. Besides the above electrical protection, the transformer would have the usual protective devices like Buchholz relay with alarm and trip s, winding and oil temperature indicators with alarm and the , oil level, indicator with alarm and devices to detect cooler abnormality. Winding temperature, indicator would have a repeater dial in central control room and also be used for cooler control. Station transformer are so designed that they would not normally get overloaded. However, WTI & OTI are provided alongwith alarm in order to detect any over loading and annunciate. Fire protection system shall be provided for station transformers and its shall be connected to trip its EHV breaker. Local breaker back up (or breaker failure) protection 50Z is proposed to be provided for the EHV breaker in the HV side of the station transformer. Protection of H.T. Motors Very divergent practices are adopted by different organizations in this respect. On one extreme, CTs are provided on only two phases and just one single element relay is provided for short circuit protection. Overload protection is provided only for those motors which have a distinct possibility of getting overloaded and not for every motor. The other extreme, CTs are provided on all phases as well as on the cable, separate short circuit elements are provided for each phase, overload protection is provided on all the phases as a general rule (whether the motor has a possibility of getting overloaded or not), stalling
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protection is provided for all motors, and a sensitive earth fault protection is provided using the core balance CT on the cable. A moderate approach should however be adopted. Low resistance natural grounding is being adopted for the HT system. CTs are to be provided on all phases, Proposed provision of relays would be as described below, the guiding philosophy being provision of some back up but a voidance of superfluous elements. For example, only balanced three phase overloading is expected for which provision of overload relays on one phase would be sufficient. However, for backing up against relay failure, overload relays proposed to be installed on two phases. Protection of overload relay on the third phase is considered absolutely redundant and has therefore been avoided. Selection of the types relays would be based on their reliability record and ability to work satisfactory under the prevailing environmental conditions. For protection against phase fault, all motors up to 2000KV rating should have instantaneous high set over current relay (50) having low transient over reach to avoid mal operation due to DC component in motor starting current on all phases. The motor rated above 2000KW need to be protected by high speed differential relay (87), which would be connected to H.T. switchgear CTs on one side and the CTs on motors natural bushings on the other side in order to have better sensitive of the protection. Provisions of relays on all phases would give inherent instantaneous back up for relay failure. For protection against earth faults, all HT motors should have sensitive earth fault relay (560N) connected o core balance (zero sequence) current transformer to be put in H.T. switchgear. CBCT is recommended in order to have better sensitivity of the protection for the cases where earth fault current is linked (low resistance or high resistance grounding).
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For overload protection, all motors would have inverse time over current relays (51) on two phases, and their Characteristics would be adjusted selected to lie between the hot withstand curve and the starting curve of the motors. The stalling or locked rotor protection will be provided only for those motors which do not have sufficient gaps between their hot withstand and starting curves or which have a distinct possibility of stalling. It will comprise of an over current relay (50/LR) having plan adjustable definite time delay, which would operate in conjunction with speed switch with adjustable setting on motor shaft. One more over current relay (50/OL) will be provided for overload alarm, for all motors, it would alert the operator in advance, so that corrective action may be taken or preparations made before a motor is tripped out by the over load relays (51) Provision of 51 on two phases and 50/OL on the third phase would provide inherent back up against relay failure as well protection against faults near the motor neutral which may not be detected by the other protections. Single phasing protection is not necessary for motors controlled by circuit breakers having a common operating mechanism for the three poles. Negative sequence current protection is also not considered necessary for power plant auxiliary motors since the supply voltage would always be balance in the close proximity of generators. The large motors lasted above 1000KW would be having winding temperature monitoring, with automatic scanning by the embedded RTDs. These would be connected for alarm only winding temperature monitoring is not considered necessary for smaller motor, which would normally be self cooled with little possibility to overheating as long as they are not overloaded. However, adequate monitoring of the cooling system shall be provided where water coolers are involved.
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The proposed provision of CTs and connection of relays for the motors rated up to 2000KW and for motor above 2000KW are shown in fig. 3&4 of annex-C respectively in enclosed sketch. Composite relays are also available which have all the features as mentioned above. These relay are static and/or microprocessor based having built in on line testing facilities, and have thermal memory for overload protection of motor. Such relays, when employed, should meet the environmental conditions at their location. Transient overreach requirement for short circuit protection and the initial cost are some of the additional factors needing attention. Protection of Auxiliary Transformers (6.6 KV/0.433KV) The H.T. feeders to auxiliary transformers would have instantaneous high set overcurrent relays (50) on all phase for protection against phase to phase fault. The LT transformers rated above 5 MVA, if any, shall be provided with differential protection instead of short circuit protection (50). Definite time delayed low set overcurrent relays (51) would be provided on all phases for protection against LV side faults. The latter would also provide backup protection against faults in the LV system. Inverse time overcurrent relays are not proposed to be used since time varies with the fault current and low intensity faults are cleared after a considerable time delay. Use of overcurrent relays with a definite adjustable time delay for 51 would provide clearance of all type of faults in less than one second (Ref. Annexure-D). For protection against HV side earth faults, all LT transformers would have an instantaneous earth fault relay connected to core balance CTs for all the LT transformers. For protection against LV side earth faults, 6.6KV/0.433KV transformer would produce current of the same magnitude as the faults (since the neutral is solidly
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grounded) and would recover by overcurrent relays 51 mentioned in cl. No. 11.1.0. for the protection against earth faults inside the LV winding, a definite time delayed earth fault relay is proposed which should be connected to a CT in transformer LV neutral side. This earth faults relay shall be fully coordinated with maximum fuse resting of outgoing motor feeders. Besides the above electrical relays, the liquid filled LT transformers would have Buchhloz relays (63) with alarm and trip s, winding and/or oil temperature indicators with alarm and trip s. Dry type indoor transformers would have only winding temperature indicator, with necessary s for alarm and trip. The LT transformers normally so sized that they are not over loaded. However, winding temperature and oil temperature are monitored and alarms are given for temperature high in case of any overheating. Protection of LT Motors The 415 V motors rated above 125 KW would be controlled through circuit breakers. They would be protected against phase-to-phase faults either with instantaneous short circuit released in the circuit breakers, or with instantaneous high-set over current relays (50), provided on all phases. For a sensitive earth fault protection these motors would have instantaneous overcurrent relay (50) in the residual CT connection. For overload protection, the breaker controlled motors would have inverse time overcurrent relay (51) on two phases for tripping and a definite time overcurrent relay (50/OL) on the third phase for alarm. The arrangement would be similar to that described earlier for the H.T. motors. The smaller 415 V motors (rated upto 125 KW) would be controlled by ors. They would be protected against phase-to-phase and earth faults by
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HRC fuses. The fuse would be selected as not to be affected by repeated motor starting. The overload protection would be provided with three phase thermal (bilmetallic) overload relays. Single phasing protection should be provided for all or controlled motors which are protected by fuses. It would be ensure that this protection is not too sensitive to negative sequence current, and is adequately stable during motor starting, normal voltage unbalance etc. Undervoltage protection is proposed to be provided on all the H.T. and L.T. breaker controlled motors in order to avoid spurious tripping of reverse supply breaker (due to drawl) of starting current of all the motors connected on particular bus), when it is closed on a dead bus. The under voltage protection would be connected on bus VT and would trip all the motors connected on the bus VT and would trip all the motors connected on the bus after a time delay. Time delay is provided in order to allow the changeover to take place on up stream switchgear and voltage dip due to the fault anywhere in the system. Protection of Auxiliary Supply System For phase to phase faults on H.T. bus and back up protection for outgoing feeder on these buses, a definite time delay over current relay need to be provided in all phases on H.T. tie and incoming breakers. Earth faults on H.T. bus and backup to all outgoing feeders on unit/station bus would be covered by a definite time delay overcurrent relay need to be provided in all phases on H.T. tie and incoming breakers. For phase to phase faults on 415V incomes & buscoupler breakers. Tie to 415V MCC’s/boards, if controlled by breakers, shall also have short circuit release fitted in the breaker. The releases shall be definite time delayed magnetic/solid
© PMI,NTPC
174
state type. Definite time delayed over current relays as real so acceptable in place of releases, Earth fault current in the 415V system would produce currents on same magnitude as for phase to phase faults, and therefore would be covered by phase faults relays. Earth fault protection therefore is not necessary to be provided in incomes, buscouplers and tie feeders to MCC/board. Time graded protect as detailed above, results in a very high operating times (about 1.0 sec.) for relays located at income. Through switchgear have one second through faults withstand capability, they can not be expected to withstand internal faults for such long time without serious damage. It is therefore recommended to adopt a reverse blocking principle to cut down the grading times. For this arrangement all the relays as described above located on ties, bus coupler, income, shall have instantaneous (with a short delay of about 100as) and delayed (coordinated with downstream) stages with the instantaneous stage of up stream blocked by all the down stream protection current element. With this arrangement a faults last any point in the system can be detected within 100m.sec. Protection of Emergency Diesel/Gas Turbine for Black Start Facility For phase to phase fault protection three phase high impedance differential protection should be provided for these generators. This protection shall be extended upto 6.6KV breaker so as to protect 6.6KV cables also, For6.6 KV bus phase to phase faults and back pup to out-going feeders protection, voltage controlled overcurrent relays should also be provided. Voltages control over current relays are recommended for back protecting because fault current form generator would reduce considerably owing to its very high steady state impedance. Since emergency diesel generator neutral is kept undergrounded, in a single earth fault only capacitive current would blow. Earth fault in this case shall be
© PMI,NTPC
175
detected by the voltage relay provided in the neutral VT which shall be connected for an alarm in the central control room. Above arrangement would facilitate the diesel generator operation under emergency conditions even if a simple earth fault exists in the system. Though necessary interlocks shall be provided to avoid motoring of the diesel generators, possibility of such occurrence cannot be ruled lout. Single phase reverse power relays are therefore proposed to be provided for diesel generators. Protection, described above shall be connected to trip 6.6 KV breaker as well as diesel engine fuel solenoid valve since only tripping of 6.6KV cable fault. Interlock shall be provided so as too allow the engine to be restarted only after manual resetting of tripping relay located at diesel generator control . The reverse power relay shall be connected only to trip diesel generator 6.6KV breaker.
© PMI,NTPC
176
13. Spare Parts Management Spare parts management is an important aspect for trouble free operation and minimum breakdowns or shut down time in a plant. For optimizing power generation, availability of required number of quality spare parts is of paramount importance.
While quality is normally ensured, availability is mostly poor
because of inadequate planning, Selection of spares in right quantity is equally important, otherwise the plant may have high inventory of spares, still may not have a needed spares. In case of NTPC; due to diversity of equipment and use of highly abrasive coal, a large variety of spares are to be stocked. Unavailability of even a small item could cause a total or partial unavailability of the units, resulting into huge loss of generation and consequential industrial loss. The process of spare parts management is governed by number of factors, e.g. maintenance planning equipment history and behaviors, spare parts planning, forecasting, procurement policy and procedures etc. Efforts have been made here to dwell upon some of the important aspect and present them as a recommendatory system of NTPC power stations which may be taken as general guidelines. The stress is on the maintenance planning, spare forecasting, planning, stocking and inventory control. Procurement of Initial Spares A provision equivalent to 4% of the equipment cost is made towards the purchae of initial spares in our feasibility reports and detailed project reports. These initial spares include consumable spare parts, unit replacements and insurance spare. The spare parts are recommended by Operation Services based on past O & M experience,
manufactures
recommendation
and
potential
indigenous
development of such items.
© PMI,NTPC
177
The provision of 4% of equipment cost of initial spares have been found to be inadequate and as a result, the quantity of initial spares are restricted to the provisions made by CEA, Even though the prices quoted by vendors for mandatory and recommended spares are very competitive since they are subjected to bid evaluation were are not able to procure sufficient quantity of initial spares due to financial constraints. Mandatory and Recommended Spares During initial periods, the spares parts were procured based on the recommendations of the manufacturers who were quoting very exorbitant prices as their virtual monopoly over these
prices which were not subjected to any
evaluation. In NTPC, to obtain, more competitive prices of spare parts, it was considered desirable to prepare a list of spare parts which will normally be required and procured along with the main equipment and are independent of equipment order. These lists include some of he high value insurance items and unit replacement assemblies. Such spares which are included in technical specifications as part of supply are known as Mandatory Spares. The prices of these spares have to be competitive as they are subjected to evaluation. Once the order is placed on the successful bidder, he is asked to submit a detailed list of spares which will be required for day-to-day operation and maintenance of initial period of 3 years as well as any other special and high valued items not included in mandatory spares, the list of recommended spares including mostly fast moving consumable type of items like bearing, liners, gaskets, HRC fuses, contractors etc. This list is subjected too rigorous scrutiny and spare parts 3 year O & M are recommended
© PMI,NTPC
178
Since the total cost of mandatory and recommended spares has to be within the provisions of 4% of the total equipment cost as per CEA guideline, this causes resource constraints which result in either reduction of quantity of spares of enhanced expenditure on spare parts. Basic for Procurement of Initial Spares The initial spare included essential insurance items, unit replacement and consumable of 3 years. The period of 3 years has been chose because of non availability of O & M personnel at site at the time of placement of order for main equipment and long lead time associated with imported spare parts. Also no consumption is worked out on the basis of past experience of similar equipment and the manufactures recommendations. Any period less than 3 years of procurement of initial spares could lead to undesirable situation of stock out. The system of thorough review of manufacture’s recommendation has helped NTPC to avoid the common pitfalls of lending up with the high inventories of those items which are normally not required and not having the items, unavailability of which could case total or partial outage of the equipment. While deciding upon the adequate quantities of consumables spares of 3 years consumption, the following kept in view:-
-
Anticipated or known failure rate
-
Succeptability of he items to loss/ damage
-
Self Life
-
Manufacture’s recommendation
-
Interchangeability with other similar spares
© PMI,NTPC
179
-
Development potential
-
Operating condition and environment
-
Total installed quantity (Population)
-
Accessibility of skills, tools, time and other facilities of repair
-
Wear life of components
-
Sub assembly replacement and repair at later date
-
Consumption rate
-
Fast technology developments of items of imported origin specially C&I items The above considerations have helped in proofing down the quantities of various slow moving items. It is observed that space parts recommended by vendor needs to be reviewed judiciously, so as to optimize the recommended list of spares of procurement in respect of number and type of item and such review has helped in cutting down our inventory at least by 20-30% of the original recommendation. The actual values of spares for all the packages considered together as per past experience, works out to be in the range of 5.5 to 6% for 200MV stages and similar trend is emerging for 500 MW sets where relatively fewer orders have been placed so far. Since the initial spares that are being ordered includes Insurance Spares, unit assemblies and consumable spares for 3years, it has not been possible to limit the value of initial spares within the stipulated provision of 4% given by CEA. NTPC being committed to high plant load factor and considering the isolated location long lead time, multiplicity of make and type of equipment,
© PMI,NTPC
180
sophisticated control & instrumentation and operational aids. The spares ordered initially exceed the provision to have a sufficient stock of spares, to avoid shortages and stock-out situations. Based on the above experience & judicious review of initial spare parts requirement, it has been proposed to CEA that the provision for initial spares procurement should be as follows for both 200 MW & 500 units. Mech.
Elect.
C& I
% to total
Packages
Package
Package
Equipment
Cost Indigenous Plants
6%
4%
8-10%
6%
Imported Plants
6-8%
4-5%
12-15%
8%
The matter is under consideration with CEA and it is hoped that as a special case of NTPC, the provision of initial spares may be suitably enhanced. Spare Parts Planning, Forecasting & Procurement In order to ensure availability of spares at the minimum inventory carrying cost, a systematic approach is necessary to be evolved on forecasting planning and procurement of spares. The main objectives of this approach are as follows:
-
To ensure availability of spare parts whenever needed so that plant availability is not affected.
-
To achieve optimum inventory turnover into i.e., consumption and purchase of spare parts be so matched that capital blocked in inventory is minimum.
-
To procure the spare parts at the lowest individual and lowest total cycle cost with consistency is quality.
© PMI,NTPC
181
Maintenance Planning Spare parts planning is dependent on the maintenance planning function. The behavior pattern/ history of the equipment is available from the past operational feed back which helps in preparing maintenance and capital overhauling schedules. Requirement of spares for all type of maintenance. i.e. preventive, break down and capital repair & overhauling, can be planned accordingly in the following manner. Preventive Maintenance Scheduling of preventive maintenance of various equipment in the power station is done as per the norms worked out on the basis of experience of power station maintenance engineers or as per recommendations of the manufacturers. A detailed weekly preventive maintenance programme for all the equipment is finalized in advance and is strictly adhered to by the maintenance personnel. Certain fast wearing type of items like glands, gaskets, packing etc. are necessarily to be changed during these preventive maintenance and hence can be planned for availability. Capital Repair & Overhauls During capital repairs and major overhauls, many items are required to be changed depending upon the wear and tear, failure and past history of the equipment. Theses items are normally long lead items and hence a comprehensive planning for such spares should be done at least 2-3 years in advance to meet the overhauling requirements. 5year rolling plans for stations should be prepared, updated and reviewed every year for the purpose of
© PMI,NTPC
182
ascertaining the need of spares, depending upon the plan condition and equipment behaviour. Break-down Maintenance It is very different to predict the requirement of spares for such jobs as the pattern of breakdown occurrence is uncertain. However based on regular condition monitoring predictions can be made Regarding residual life of the equipment. Accordingly, requirement of spares can be worked out for future period depending upon past history and failures. Spare planning & Inventory Control Maintenance Planning will indicate the schedule of maintenance and quantity of spares required to meet schedules. These quantities are to be cumulated for total quantity of spares required to meet the schedules. These quantities are to be cumulated for total quantity and analysed for deciding the order quantities keeping the current stocks, quantity due in against pending orders and lead period. The inventory for the consumable spares should be such that it meets the management objectives of no stock outs and minimum inventory holding. Since there is always some gap between the time when order is placed and the spare is actually reviewed, the role of spares planning is very vital. The planning cycle and inventory level fixation depends upon various factors. Location of the plant
The location of the plant effects the lead time & local source development. NTPC plants located at remote place have to carry high inventory of spares.
© PMI,NTPC
183
Nature of spares
Proprietary items or imported spares which calls for cumbersome import procedure, have high lead periods and should be planned well in advance.
Lead Time
The time elapsed between indent raising and availability of spare part for use is an important factor which governs the stocking policy: This time comprises internal lead time of indent processing & placement
order,
apart
from
the
manufacturer’s delivery period. Equipment history and
The equipment behaviour and failure rate usage rate determines the usage rate. It provides the guidelines for inventory stocks. However, usage rate varies with the age of plant.
Organization and Budgeting
Spare
parts
planning
set
up
for
identification Budgeting and placement of
orders
for
spares
help
in
curtailinistrative and procedural delays.
Similarly
adequate
Budget
provisions and availability of funds for spares leads to optimum inventory stocks.
© PMI,NTPC
184
Analysis and Classification For the purpose of planning, it is necessary to conduct an analysis of spares with respect to consumption; critically movement and their availability etc. ∗
F.S.N. Analysis
The spares are classified into Fast, Slow and non moving categories based on the movement rate.
∗
Fast Moving
Items which have moved at least once in 1-2 years
Slow Moving
Items which moved at least once in 2-4 years.
Non Moving
Items which have not even once in 4-5 years.
Type of Spares: Insurance Spares
The spares which normally not required but are vital for the equipment as in case of any miss hope or accidents non availability of these items will lead to long out ages. For example turbine rotors exciters etc.
Unit Replacement
In order to reduce equipment down time complete assembly is replaced. This assembly/sub assembly can be repaired later on and can be used again.
Consumable Spares These are fast wearing out items and have reduced life. These spares are replaced. ∗
ABC Analysis
Based on the annual consumption or usage values, spares are replaced a new when found worn out.
Non-Moving Items
© PMI,NTPC
185
A.
Contributing to 60-80% of total spares consumption. Industrial annual consumption Of more than 50 thousand.
B.
Covering about 25% items contributing to 15-30% of total usage value. Industrial consumtion between
C.
5000-50000/-.
Annual usage value of each spares less than 5000/-.
Criticality Analysis
The spares categories according to their criticality of usage In the plan.
Vital
Spares which cause complete stoppage of plant and service risk in operation are vital like control value speed governors load limiters.
Essential
In absence of such spares, the plant can run but perhaps at substandard performance level like spares of ID fan, mills etc.
Desirable
These are the spares whose non-availability may not Adversely affect the plant performance but may increase down time later on.
SDE Analysis
Categorization depending upon sources and leadtime of procurement:
Scarce
Normally proprietary or imported spares with long lead time.
Difficult
Special efforts are to be put in the for procurement of these items. They are high tech. Items.
© PMI,NTPC
186
Easy
These items can be procured from standard suppliers in a short lead time.
Inventory Control To meet the objective of spares planning, inventory levels should be so optimised that stock out as well out stocking are minimum. The inventory cost include carrying cost and ordering cost. Optimisation is to be achieved so as to minimise the total cost by controlling how much to order and when . To achieve this, it is necessary to classify the spares into above categories such as ABC, VED, and FSN and expected life. On the basis of these classification , Inventory levels safety stocks & recording level can be decided for each and every items. Based on the VBC and ABC analysis, a confidence level matrix have been adopted to determine the safety stock of
the spares.
Confidence matrix V E D
A 85% K=1 75% K=0.75 70% K=0.7
B 95% K=1.7 85% K=1.0 76% K=0.75
C 99% K=2.5 95% K=1.7 80% K=0.8
Periodic Review System The spare parts planning and forecasting is proposed to be done on fixed review period basis. It is envisaged that all the consumables spares will be reviewed only once in a years. A calendar for the review of spares have been prepared which provides review of spares segregated into seven packages such as SG, TG, Elect., C&I, Off-sites etc. Apart from these, high consumption critical spares
© PMI,NTPC
187
of Bowl mills, coal handling plant and ash handling plants will be reviewed on quarterly basis. The maximum inventory level has been fixed on the basis of consumption rate, lead time and safety stock. 1.
2.
Average consumption during lead period
Average consumption during review period A.
total population ------------------------- x Review per Expected life 1.5 years –2 years 1 year
Review period
=
1year
Safety stock =KM = face or depending upon ABC /VED analysis = consumption during lead period
maximum Level = lead period consumption stock
5.
x Lead period
= =
Safety stock where K M 4.
=
total population ______________ Expected life
Lead period i) BHEL spares ii) Other than BHER
B. 3.
=
+
review period Safety Stock
Reorder quantity Reorder quantity
© PMI,NTPC
=
Maximum level –(present stock –quantity due).
188
N.B in case the present stock is less than lead period consumption , recorder quantity should be taken equal to review period consumption only. The identification of consumable spares, their classificatication schwas VED, ABC, FSN and expected life havebeenentrusted to site Ormond the fixation of maximum in vent ore safety stock etc. have been entrusted to Operation Services. The periodic annual review of spare will be carried out by Site O&M Planning &Materials Deptt. The objective of the spare development is to make a systematic effect to develop the spare parts indigenously or in house for all categories of spares whether Indian or Imported so as to achieve the following objectives. −
Cost reduction
−
Alternative source
−
Import substitution
−
Availability
−
Reduction in Lead time
It has been proposed that items which are common to 2 or more than projects or which have a annual consumption value of more than 10 lakhs will be taken up for developments from Corporate Centre while the items having less consumption value or particular to individual projects will be taken up for development by project itself. The responsibility of preparation of drawings and technical specification will be O&M planning at Sites and COS at Corporate Centre while replacement of order will be the responsibility of site materials Deptt. And Materials department at projects and Coordinate Centre respectively. Once the source standardisation is done, projects can place order on these parties for their annual requirements.
© PMI,NTPC
189
Already the jobs of spare parts developments has been taken up by all the projects & coordinate Centre with encouraging results and the number of spare parts developed will increase with the availability of drawings and material specification of spares which is a very elaborate process. Vender development The spares being im ported by the plant can be boradly as -
Spares for which there is not enough capacity witmn the country though the required specifications are available
-
Spares for which there no specification is available but capacity is a vailable within the country (most of these are propriety nature spares imported as per suppliers part numbers).
A study conducted shows that the category(b) constitutes about 80% of the total import budget of spare and thus requires special attention towards import substitution drive. We had developed in our project many of the spares which would lead to foreign exchange savings to the tune of several lack per annum. The interesting feature of this development management is that ones specifications of an item is established, the item will go out of the import-list for all time to come leading to saving foreign exchange over a number of years. Dependence on continued import and indigenous proprietary procurement is not only expensive but also very time consuming and leads to long delivery per period and available inventory as well. The development spares required continuous efforts right from identification of spares for development to the trial runs, assessment of performance of the spares supplied by the vendors and
© PMI,NTPC
190
accordingly enlisting successful vendors for such development items and continue to explore new competent sources. Identification for Spares for Development Spare parts be identified for development purpose on the following criteria: (a)
Spare of imported origin.
(b)
Spare of indigenous proprietary origins. − Spare of vital/ essential in nature. Long/ irregular delivery period from original equipment manufacturer/ Oes and not of high tech-components. − Spare common among various NTPC projects. − Spares generally bought out items and only assembled by OEM/OES. − High consumption spares. − High value spares and not of very sophisticated technology involved.
Identification of Sources − Press ments, Exhibitions, State wise yellow pages, DGTD. Manufactures/ trade and NSIC directories and s/ other projects past experience. − Introductory letters received from vendors and ments published in leading newspaper journals. − Information obtained from DGTD regarding items licensed for same/ allied items.
© PMI,NTPC
191
− Magazines like Industrial /Electronic product finders, Computers Today’s, Industrial Equipment News, National Product News, Telemetric India, etc. Registration of Vendors − The vendors shall be required to furnish the details such as technological expertise, financial soundness, manufacturing/ testing facilities and past performance with regard to supply and quantity of same/similar items in the prescribed Performa. − Screening committee consisting of representatives from Finance, Material, O&M, QA not below the level of Manager/ Supdt. At Site/OS will finalize the list of verdors for undertaking the development of items, based on their capability and capacity. Suppliers work may be visited if necessary. − Vendors ed with DGTD/NSIC and known to NTPC site/OS need not be reassessed Enquiries Enquiries can be floated for spare development to all the ed vendors for similar items. In case more than one vendor is not available in the approved vendor list for a particular item, enquiry can also be floated even to single vendor with approval. Placement of Development Order − The negotiation/Tender committee (as per DOP) places the development orders. The original price or spares/ items, estimated costs, specific investment and infrastructure by the vendors will be considered
© PMI,NTPC
192
− The committee can propose to issue more than one PO against single item in case of multiple offers (all being L1). − Negotiation committee can recommend issuing of more purchase order for single items at different rates to different vendors.
© PMI,NTPC
193
14. Date Sheet for Gas Projects S. NO.
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
a) Net plant capacity, Base load (MW) at design condition (i.e. 270C ambient temp. 60xRM, 320C CW temp. at site elevation)
413.3
652.56
645. 36
817.48
648.6
b) Combined Cycle net heat rate at base load design condition of 15 kg/sec. per Gas (K cal/KWhr.)
1796
1796
1771
1748
2.
Fuel & its Source
Natural Gas tapped from HBI pipeline
Natural Gas tapped from HPI pipeline
Natural Gas from BHEL pipeline.
3.
Gas Requirement
4.
Water Source
5.
Condenser cooling system
2.25 MCMD (Annual Average) Etawah branch canal Closed circuit
6.
Mode of operation of plant
1.75 MCMD (Annual Average) Kota right main canal Hybrid (Closed circuit/once through) Base load and/or two shift or peaking duty depending on technical and/ economic considerations
Natural Gas /NGL Natural Gas is supplied From HBJ 2.25 MCMD (Annual Average) Hazira Branch canal Closed Circuit
Base load and/or two shift or peaking duty depending on technical and/ economic consideration
Base load and/or shift or peaking duty depending on technical and/ economic consideration
7.
Main Plant awarded on
MHI Japan on 03.09.90
8.
Orientation of GT Exhaust
GEC-Alsthom on 05.03.90 Axial
1.
DESCRIPTION
© PMI,NTPC
ABB, West on 20.8.87 Axial
Axial
1834 (This figures are with DM Water injection turbine) Natural Gas From Gandhar Gas Basin.
3MCMD (Annual Average) Net Canal Closed Circuit
1.5MCMD (Annual Average) Wareada river Closed Circuit
Base load and/or Two shift operation or two shfit or peaking duty depending on technical and/ economic consideration. Siemen AG, West Marubani, Corporation on 26.3.92 Axial Axial
194
S. NO.
DESCRIPTION
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
270C asb. Temp. 60% RH 253 M above seal level, 320C CW temp. ABB, 98.9 89.25
270C asb. Tmep. 60% 270C asb. Tmep. 60% RH, 140M above sea 60% RH, 5.5 M above Level, 320C CW temp. sea level, 320C CW temp. MHI, Japan Eropean Gas Turbine 121.51 116.9 111.76 105.62
270C asb. Tmep 60% RH 210 M above sea 320C CW temp. Siemens AG, . 142.42 131.31
2770
2623
2697
2692
2617
31
32.78
31.9
31.95
32.8
6.76
8.2
7.8
9.6
9.4
3000 12.4 370 505
3000 13 412 513
3000 12 388 540
3000 10.2 462 560
3000 14.3 500 525
GAS TURBINE AND AUXILIARIES 9.
Design Conditions
10.
Manufacturer
11. 12.
ISO base rating at Net O/C base output of GT at design condition (MW) Net O/C heat rate at base load and design conditions (K cal/Kg. Hr.) Net efficiency of GT (%) (O/C, at base load, design conditions) Natural gas consumption (O/C, base load design condition kg/sec, assuming cal value of gas as 8700 K cal/m3 and density of gas As 0.856 kg/m3 Turbine speed (rpm) Compression Air bass flow (O/C base Exhaust gas temp. (0C)
13. 14. 15.
16. 17. 18. 19.
© PMI,NTPC
270C asb. Tmep. 60%RH, 23.25M above seal level, 320C temp. ABB, 147.8 131.36
195
S. NO.
DESCRIPTION
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
20.
Ignition of speed of the shaft (RPM)
600
600
510
400
600
21.
Self sustaining speed of the shaft
1500
2100
1950
2100
2000
22.
start-up mechanism
frequency Converter
Starting motor
Frequency Converter
Frequency converter
Starting motor
DESIGN FEATURES 23. 24. 25. 26. 27. 28.
No. of compressor stages No. of turbine stages No. of air cooled stage i) Guide ii) Rotating blades GT Generator Rating MVA GT Generator Cooling by TYPE OF AIR FILTER Type of Air Rating MVA system.
18 5
19 4
17 3
16 4
21 5
2 3 125 Air
2 2 141.08 Hydrogen
2 2 171.875 Air
2 2 165.5 Air
3 3
Self cleaning Cartridge type
Self cleaning Cartridge type
Self cleaning Cartridge type
Self cleaning Cartridge type
Self cleaning Cartridge type
Diverter type
Diverter Louver
Diverter type
Diverter Louver
Diverter type
Forced circulation/ Dual Pressure
Forced circulation/ Dual Pressure
Forced circulation/ Dual Pressure
Forced circulation/ Dual Pressure
Air
BY DAMPER 29.
Type WHRB
30.
Type
© PMI,NTPC
Natural circulation/ Dual Pressure
196
S. NO. 31.
DESCRIPTION
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
Manufacturer
Wanger Biro/Austria
MHI. Japan
CMI. Belgium
BHEL, India (with Wagener Biro design)
a) HP Super heater Out let temp. (0C) Outlet pressure (bar)
485 63.97
495 60
520 70
530 65
504.5 76.9
b) LP Drum/Super heater Out temp. (0C) Outlet Pressure
210 5.91 bar
158.1 7 kg/cm2
193.2 7 bar
200 5.7 bar
220 6.1 bar
32.
STEAM TURBINE Make
ABB, Germnay
MHI, Japan
ALSTHOM,
BHEL, India
ABB,
33.
Type
Tandem compound Dual pressure Condensing type
Tandem compound Dual pressure Condensing type
Tandem compound Dual pressure Condensing type
Tandem compound Tandem compound Dual pressure Dual pressure Condensing type Condensing type
34.
Mode of Operation
Sliding Pressure
Sliding Pressure
Sliding Pressure
Sliding Pressure
35.
HP CYLINDER
Single flow 25 Stages
Single flow 25 stages (Impulse)
Single flow 13 Stages Impulse
Double flow 24 Stage (Reaction)
Single flow Single flow
36.
LP CYCLINDER
Double Flow 6 stage (Reaction)
Double Flow 6 stages (Reaction)
Double Flow 5 stage (Reaction)
Double Flow 7 stage (Reaction)
Double Flow
37.
Rated Speed
3000rpqm
3000rpm
3000rpm
3000rpm
3000rpm
38.
Based load out put at
153.28 MW
109.3MW
116.1 MW
154.51 MW
227 MW
© PMI,NTPC
KHI, Japan (with Henry vogt design)
Sliding Pressure
197
S. NO.
DESCRIPTION
ANTA
AURIYA
KAWAS
DADARI
GANDHAR
39.
Make
ABB
MHI
DELASWAER
BHEL
ABB
40.
Condenser back pr. At load and design condition.
100 bar
72mm hg abs
92 a bar
0.1122 kg cm2
100a bar
41.
Cooling water flow
29700 M3/hr
23340M3/hr
23061 M3/hr
25000 M3/hr
37728 M3/hr
42.
Design Cooling water temp.
320C
320C
320C
320C
320C
one (1)
one (1)
one (1)
one (1)
one (1)
SULZER
BHEL
3x50%
3x50%
DEAERATOR 43.
No. of Deaerator/module FEED WATER PUMP A.
HP FEED WATER WATERPUMP
44.
MAKE
SULZER
45.
No. of HP feed water
3x50%
TORISHIMA JAPAN 3x50%
46.
Capacity
307.3m3/hr
210 T/hr
182.2 M3/hr
290 M3/hr
47.
TDM
94.3 bar
80 kg/cm2
110 bar
107 bar
TORIHIMA JAPAN 3x50%
SULZER
KSB
3x50%
3x50%
B.
LP FEED WATER PUMP
48.
MAKE
SULZER
49.
No. of LP feed water
3x50%
© PMI,NTPC
3x50%
198
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