Modulating Control(system Description) 242j1c

4-} TOSHIBA

3. APPLICATION SOFTWARE

3.3 Contro l System

CONTENTS PART: 1 DISTRIBUTED CONTROL AND INFORMATION SYSTEM 3 . APPLICATION SOFTWARE (CONT'D) 3.3. Control System 3.3.1. Generals of C60 Software

3.3.2. Modulating Control 3.2.1 DCIS Modulating Controls System Description 3 . 3.3. TOSMAP-AT / D40 Operation Manual 3.3.1 TOSMAP-AT/D40 Operating Instruction (6F2B0022)

BANG PAKONG POWER STATION UNITS 3 & 4 DCIS MODULATING CONTROLS SYSTEM DESCRIPTION

SYSDESCR.DOC Rev 0 2Sep91

Author: R J McDermott

CONTENTS INTRODUCTION ..... ..... ........ . .......

1

1 1.1 1.2 1.3 1.4 1.5

TRACKING & INITIALIZATION . . . . . . . . . . . . . . .. Type A Control Drives Type B 1 Control Drives Type B2 Control Drives Type C Control Drives Cascade Controls

2

2 2.1 2.2 2.3

TRANSMITTER DEVIATION SYSTEM . . . . . . . . . . . Single Measurement Dual Measurement Triple Measurement

4

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7

UNIT MASTER . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coordinated Controls - Introduction Required Output Computation Operating Modes Runback System Pressure Set Point Governor Control Firing Rate Demand

6

4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

Affi FLOW CONTROL . ......... ... ... . ...... Process Measurements Air Demand Excess Air Controls Air Flow Controller Tracking FD Fan Stall Air Heater Cold End Temperature Windbox Air Dampers

17

5 5.1 5.2 5.3 5.4

FUEL FLOW CONTROL . . . . . . . . . . . . . . .. .. ... Fuel Measurements Fuel Demand Fuel Controllers Fuel-Air Deviation Monitor

22

II

25

6.1 6.2 6.3

FURNACE PRESSURE . ... . .... .. . ....... ... ID Fan Speed ID Fan Inlet Dampers Implosion Protection

7 7.1 7.2

STEAM TEMPERATURES . . . . . . . . . . . . . . . . . . . . Main Steam Temperature Reheat Steam Temperature

28

8 8.1 8.2 8.3 8.4

FUEL OIL PUMPS .... ... .... . ...... .. ..... Fuel Oil Temperature Fuel Oil Header Pressure Fuel Oil Heater Steam Pressure Fuel Oil Transfer Pump Pressure

33

9 9.1 9.2

FEEDWATER .. . . . . ..... . . . . . . . . . . . . . . . ... Drum Level Feed Pump Minimum Flow

34

10 10.1 10.2 10.3 10.4 10.5 10.6 10.7

CONDENSATE & DEAERATOR ..... ....... .. . . Deaerator Level Deaerator High Level Deaerator Pressure Deaerator Temperature Condenser Level Condensate Recirculation Condensate Pumps Recirculation

38

11 11.1 11.2 11.3 11.4

FEEDWATER HEATERS ..... ... .. ........ . . . Feed water Heaters Level LP Heaters Drains Tank Level LP Heaters Drains Pump Recirculation HP Heaters Drains Pump Recirculation

41

12 12.1 12.2 12.3 12.4

MITSCELLANEOUS ........... .. ....... . .. . . Seal Steam Pressure Seal Steam Temperature Closed Cycle Cooling Water Temp Auxiliary Steam Pressure

43

13 SIMPLE 1NDEPENDENT LOOPS . . . . . . . . . . . . . . . . . .

44

6

BANG PAKONG UNITS 3 & 4 DCIS MODULATING CONTROLS SYSTEM DESCRIPTION INTRODUCTION The purpose of this document is to assist in the understanding of the design principles used for the modulating controls. The document should be read in conjunction with the following: (a)

Toshiba Drawing 7M1Z0218 "Modulating Control Block (Functional) Diagrams". The sheet numbers referred to in the following text relate to these diagrams.

(b)

Toshiba descriptive literature for C60 controllers and 870 computing and display system.

(c)

Black & Veatch International Project 14383 Bang Pakong Thermal Plant Unit 3 Piping and Instrumentation Diagrams.

2

1

TRACKING AND INITIALIZATION

Changes in operating mode are "bumpless". This is achieved by automatically initializing the selected signal to be equal to the downstream signal prior to transfer. This principle is applied to automatic-manual selections of control drives, cascade controls and boiler master operating mode changes. The simplest case is a controller with a single control drive. When control is manual, the controller output is made to track the manually set position demand. When control is automatic, the manual setter tracks the output to the control drive from the controller. Where multiple drives are used with a single controller, the tracking signal depends on the control drive configuration. Sheets 17, 18 and 19 show the standard tracking systems used to initialize automatic-manual transfers for various configurations of multiple control drives. The control drive tracking systems which follow these standard systems are not shown on the functional block diagrams. Tracking systems which differ from the standard are shown on the relevant functional diagram. 1.1

TYPE A

Refer to Sheet 17. This system applies to dual drives which control auxiliary plant with less than 100% capacity where both drives are normally in automatic [e.g.FD fans]. A bias setter allows changes to the relative loading; these changes are introduced gradually by using a delay function. Each drive has ~ separate auto-manual sub-window. Loop gain is constant for one or two drives in automatic. If one drive is auto and the other manual, the auto drive compensates for manual operation of the other. For example, increasing the manual drive output will decrease the auto drive the same amount without waiting for a change in the controlled process. The average control drive position is used for controller tracking. Feedforward signals, if used, are added to the controller output. It follows that, in tracking mode, the feedforward must be subtracted from the tracking input to the controller. 1.2

TYPE 81

Refer Sheet 18. This system applies to dual drives operated from a single automanual sub-window; it follows that both drives must be in automatic or manual .. These usually operate in the "split-control" configuration [e.g.Auxiliary St.ea.m Pressure]. The controller tracks the common manual demand signal to the two drives.

3 1.3

TYPE 82

Refer to Sheet 18. This system applies to dual 100% capacity drives which have individual auto-manual subwindows. Only one is permitted to be in automatic; the other is available as a standby. With both drives in manual, the controller tracks drive A unless drive B is selected to auto. A short time delay on "B Auto" ensures that the tracking signal from B is established before transfer to auto takes place. 1.4

TYPE C

Refer to Sheet 19. This system is used for configurations of more than two drives where any number may be in automatic [e.g. Condensate Pumps]. Loop gain is kept constant by modifying the controller error to be inversely proportional to the number of drives in automatic. The controller output tracks the first drive to be selected to automatic. [Default is drive A.] A short time delay before transferring to automatic operation ensures that the tracking signal is established. The track signal to the remaining drives on manual comprises the controller output plus the difference between the controller output and the actual position. The difference signal is transferred via the "track" input of an integrator. After selection to automatic, this difference signal at the integrator output is connected in reverse to the integrator input and slowly decays to zero. This decay is slower than the response of the control loop so disturbance to the process is minimal. 1.5

CASCADE CONTROLS

For a simple cascade loop, the primary controller tracks the secondary controller process variable when not auto. This forces the secondary controller error to zero for bumpless transfer. Tracking signals for cascade controls must include the reverse of any calculations applied to the forward path. For example, feedforward signals added to the primary controller output must be subtracted from the track signal. Similarly, multipliers become divisors and the inverse of any function generators in the primary controller output path must be applied to the track signal. Because of these complications, the tracking system for each cascade control is fully shown on the appropriate functional diagram.

4

2

TRANSMITTER DEVIATION SYSTEM

All process transmitters used for modulating control functions are checked by the Transmitter Deviation System. H an abnormal measurement condition is detected, all dependent control loops are tripped to manual control. There are three transmitter configurations: single measurement, dual measurement and 2. ") :.>-'! 'L'" triple measurement. The details are shown on Sheets _.21, ..2-3 and 24. These details apply to all relevant applications. The functional diagrams for specific applications show only a simplified version comprising signal comparison and resulting input to the auto permit logic. Transmitter deviations which affect fuel, air or governor control trip the coordinated control system to Manual mode as well as tripping the directly affected loop. The coordinated loops are also monitored by the fuel-air deviation system, refer to Section 5.3. 2.1

SINGLE MEASUREMENT }./)

Refer Sheet 21. In this case, the signal is checked to ensure that it is within the normal range with a tolerance of 5%. If it outside this range, an alarm is initiated and any control loops significantly affected by this signal are transferred to NOT AUTO status. 2~

DUALMEASUREMENT _a

Refer Sheet 23. The dual measurements are compared and, if they disagree by more than a preset amount [typically 3%], an alarm is initiated and affected loops are transferred to Not Auto status. The individual measurements are also checked for in range, if outside by more than 5% an alarm is initiated. A CRT subwindow is provided for each transmitter pair. This enables the operator to monitor each input and select one of the pair for control. Logic prevents selection of an out-of-range transmitter. H a deviation occurs, the operator selects the good transmitter and disables the logic signal which trips the relevant control loops. An alarm reminds the operator that the monitor is disabled. 2.3

TRIPLE MEASUREMENT

Refer Sheet 25. The median value for the three signals is derived. If any of the three disagree with the median an alarm is initiated and the relevant controls are tripped to Not Auto status. The individual inputs are also checked for inrange, if outside by more than 5% an alarm is initiated.

5

A CRT sub-window is provided for each triple measurement. This enables the operator to monitor all inputs and select the median or any one of the three inputs. Logic prevents the selection of an out-of-range transmitter. If a deviation occurs, the operator selects a good transmitter and disables the control trip. An alarm reminds the operator that the trip is disabled.

6

3

UNIT MASTER

3.1

COORDINATED CONTROLS -INTRODUCTION

The fundamental requirement for coordinated boiler-turbine controls is to automatically balance the boiler energy production against the prevailing energy demand of the turbo-generator. The energy transfer is effected by the flow of superheated steam from the boiler to the turbine where the heat energy of the steam converted into mechanical work. The rate of transfer of energy between the boiler and the turbine can be expressed in of energy rates as follows: (i)

Et = (Ef + Ew + Er)- Eb- Es' [MW]

Where: Et = Main and reheat steam to turbine Ef = Fuel to boiler Ew =Feedwater to boiler Es'= Change in boiler stored energy Er =Cold reheat steam to boiler Eb

=Boiler losses

For small to moderate load changes it can be assumed that Ew, Er and Eb are proportional to boiler output. Simplifying (i): (ii)

Et =K(Ef- Es')

When boiler and turbine are in balance, the rate of stored energy change is zero; i.e. Es' = 0 [Pressure steady]. Turbine input Et is controlled by the turbine throttle valve through the governor and Ef is controlled by varying the firing rate. Es' is a function of the prevailing out of balance between boiler and turbine. At higher loads and pressures, the steady state stored energy increases and additional fuel is needed until the required level is reached. The converse is true for falling loads. Temporary overfiring or under-firing is required to accommodate this.

7 The coordinated control system keeps the balance between boiler and turbine over the normal load range, accommodates stored energy requirements, controls pressure and temperature and ensures that the unit is kept within auxiliary plant limitations. 3.2

REQUIRED OUTPUT COMPUTATION

The Unit Master Display [CRT] provides the following facilities and indications on sub-windows: SWl

TARGET LOAD SE'ITER ACTUAL OUTPUT LOAD CONTROL MODE [LOCAUADS]

SW2

LOAD RATE SETTER

SW3

MAXIMUM LIMIT SETTER

SW4

MINIMUM LIMIT SEITER

SW5

PRESSURE SET POINT ACTUAL PRESSURE PRESSURE MODE [FIXED/SLIDING]

SW6

[SPARE]

SW7

GOVERNOR CONTROL [AUTO/MANUAL]

SWB

MODE SELECTION [CO, BF, BI, MAN]

Refer to Sheets 26 and 27 in conjunction with the following text. The target load is either set locally by the plant operator [Master Display Subwindow 1] or controlled from the Automatic Dispatch System [ADS]. When in ADS mode, incoming raise/lower pulses are integrated by the ADS Servo to form the target load signal. {A facility for an alternative analog ADS target is provided; a software switch enables the appropriate signal to be selected.} The ADS target is prevented from moving faster than the current rate-of-change setting. This feature ensures immediate response to reversals in ADS demand. The Target Load is subjected to various limiting actions on magnitude and rateof-change to form the REQUffiED OUTPUT. [RO] This signal is the basic demand for fuel, air and turbine governor.

8 The target is constrained by the maximum and minimum limit settings. These are mainly used to keep ADS control within current plant capability. The target load is also limited to the capacity of the auxiliary plant in service [Target Maximum]. When the unit is tripped, the minimum limit is set to zero. The target load tracks a load index when the coordinated loops are not in one of the automatic modes. If MAN or BI mode is pre-selected, target load tracks fuel flow. IfBF or CO mode is pre-selected, target load tracks unit MW output. The selected signal provides a reference for system balancing when changing to an automatic mode. Changes in the target load are subjected to a rate-of-change limiting as set by the operator. This operator selected rate will be over-ridden if it is higher than the current turbine rate limit setting. If a load runback is required because of an auxiliary plant trip, a fast runback rate will be selected [See Section 3.4]. A fast rate is also selected when RO tracking is required. A further constraint on Required Output [RO] is imposed by the Unit Capability Monitor. This checks the process deviation for the major flow loops [fuel, air, governor, feedwater and condensate]. Should the deviation exceed a certain threshold value, the RO is blocked from moving in a direction which would increase the error. This feature prevents mismatch of flow loops caused by poor transient response and limiting or failure of regulating devices. If the process deviations persist for longer than a preset time, the Required Output is adjusted up or down so as to eliminate the deviation. This is called Runup-Rundown action. Under steady state conditions with system frequency at 50Hz, the Required Output normally equals Target Load as set by the operator or ADS, provided that there are no plant limitations. When the system frequency deviates from 50Hz, the turbine governor takes corrective action by increasing or decreasing load to contribute to the frequency regulation of the interconnected power system. The required output must reflect this adjustment otherwise the controls would see a generation error and remove the unit's contribution. The frequency bias component of RO models the governor action from frequency deviations. Tuning setter AOl is adjusted as a function of governor droop setting. [A nominal 4% droop would produce 30MW/0.1Hz at rated pressure of 170 Bar.]

Required Output

Governor

Steam Flow

Fuel Valves

F D Fans

Blade Pitch

~

Feedwater Pumps

I D Fans

'if Feed Flow

Condensate Pumps

Figure 3.2 Propagation of R 0 Signal to Flow Control Loops

9 The Required Output forms the basic demand for fuel, air and governor as well as providing the load index for pressure set point computation. The required output is propagated indirectly to provide a feedforward demand signal to furnace pressure, feedwater and condensate controls. This is shown in block diagram form on Figure 3.2 3.3

OPERATING MODES

The required boiler-turbine balance can be achieved in several ways. The control system provides a choice of operating strategies for the co-ordination of the turbine governor, which sets the energy demand rate, and the boiler fuel/air inputs, which provide the required rate of energy production to match the demand. These different methods of operation are called SYSTEM MODES. Refer to sheets 35 and 38. 3.3.1

Coordinated [CO]

In this mode the boiler inputs and the turbine governor respond to the Required Output signal [RO]. This is either set by the operator or the automatic despatch system [refer Section 3.2]. Steady state boiler/turbine co-ordination is achieved by the use of the common RO signal to set boiler inputs and turbine demand. Refer to Fig. 3.3.1. The Required Output to fuel and air is modified by dynamic compensation signals which provide for the ensuing changes in stored energy when load and/or pressure are changed. This is effected by overfiring or underfiring as appropriate. Any residual unbalance is reflected by pressure changes as stored energy accomodates the unbalance. The RO to fuel and air is modified by a pressure controller to eliminate pressure deviations. The Required Output also provides the basic demand to the governor controller which regulates turbine energy input. The RO signal is modified by the MW controller so as to achieve the required steady state MW output. The Coordinated Mode is the normal method of operation.

ADS Setters

.. ~

t Target Load

,, .

..

Limits

Auxiliary Plant

,

...

Rate

, Capability

....-...--Flow Deviations

, Frequency Bias

Generation Correction

Dynamic Compensation

, Pressure Correction

, Pressure Deviation Block Excess Air Correction

,, Governor Control

Fuel Control

Fieu.re 3.3.1 Coordinated Mode

Air Control

Setters

0

Target

~xPS

Turbine Demand

PT

r

0

Auxiliary Plant

Limits

Dynamic Compensation

,,

0

Pressure Correction

Rate

Flow -Deviations

Capability

Excess Air Compensation

Frequency Bias

Generation Correction

Pressure Deviation Block

,, Governor Control

Fuel Control

(Auto Optional)

Figure 3.3.2 Boiler Follow Mode

Air Control

10 3.3.2

Boiler Follow [BF]

This mode allows for co ordination of boiler-turbine control with or without the governor on automatic. The coordinating signal is provided by throttle valve pressure ratio compensated for pressure set point [Pl!Pt*Ps]; this forms the basic demand to fuel and air, replacing Required Output. Refer to Fig. 3.3.2. Dynamic feedforward and pressure correction are provided as in Coordinated Mode. The governor, if selected to automatic, controls MW from the RO signal as for CO mode. Capability limiting is also effective when the governor is on auto. The boiler follow mode allows for responsive control when the governor unavailable for auto operation. [Manual control ofMW.] 3.3.3

IS

Base Input - Turbine Follow [BI]

Boiler energy input (fuel and air) is determined by Required Output [3.2] only. Frequency bias compensation to RO is not applied in this mode. Dynamic compensation and pressure correction are not applied to boiler inputs. Refer to Fig. 3.3.3 The turbine governor, if selected to auto, controls pressure before the throttle valve by regulating the throttle valve position. The turbine thus follows boiler input energy and maintains the set pressure. The resulting MW will be approximately equal to RO, depending on fuel heating value calibration. If the governor is not auto and the throttle valve is fixed, the steady state turbine output will follow boiler input energy, the MW will be approximately equal to RO and the pressure will be proportional to RO. Pressure can be modified by changing the throttle valve position manually. This will cause temporary disturbance to MW and steam temperature.

The Base Input-Turbine Follow mode is used when stable boiler operation is required. If a runback occurs in CO or BF mode, control mode is automatically transferred to BI mode. 3.3.4

Manual [MAN]

Governor manual and fuel manual; air auto (optionally). The target load tracks fuel flow to provide RO initial status proportional to boiler output. [The rate setter is by-ed.] With air on auto, the air demand thus follows fuel flow. Manual mode is normally used at sta.r t up and synchronizing until stable firing conditions are achieved.

~

<>

Ope rat or Sett ings-

..

Target Load

• a

Limits

• ..

Rate

Capability

-

Plant Max

-

Run back

"'------Flow Deviation

Pressure Correction

'

Pressure Deviation Block

Excess Air Correction 1

Governor Control

Fuel Control

(Auto Optional)

Figure 3.3.3 Base Input Mode

Air Control

11

Control system faults such as transmitter deviation will trip the selected mode to manual with air also manual. 3.3.5

Mode Selection

Fuel and governor control can only have auto status if one of the three automatic modes is operative. The term "Auto Permit" refers to pre-conditions which must be satisfied prior to automatic operation. The permissives which must be satisfied for each mode before auto operation is enabled are:

(a)

Coordinated [CO] Not Manual mode * FW on Auto * Steam Temperature on Auto * Air Auto Permit *Fuel Auto Permit* Governor Auto Permit * CO selected.

(b)

Boiler Follow [BF] Not Manual mode* Air Auto Permit* Fuel Auto Permit * BF selected. [Governor Auto optional.]

(c)

Base Input [BI] Not Manual mode* Air Auto Permit* Fuel Auto Permit * BI selected. [Governor Auto optional.]

(d)

Manual [MAN] No permits. [Air Auto optional.]

Following the selection of a mode, the process deviations for fuel, air and governor loops are forced to zero to ensure bumpless transfer. A back-calculation produces a tracking signal which is used to initialize the appropriate upper level controllers [Pressure, Oxygen, MW] at values which force the fuel, air and governor demand signals to be equal and opposite to the prevailing process variable. [Refer to Section 1 "Tracking and Initialization."] The system logic checks that permissives are met and that deviations for fuel, air and governor are approximately zero for 5 seconds before "Mode Auto" status is implemented. This is to ensure tracking is complete and bumpless transfer ensues. After balance check, Mode Auto status allows pre-selected coordinated loops (fuel, air, governor) to go to auto status. If a mode permit is lost, auto control is suspended and an alarm initiated.

12 3.4

RUNBACK SYSTEM

Refer to Sheet 29. The auxiliary plant capacity is computed from in-service status and plant rating for each type of auxiliary. For example, one motor driven feed pump plus one turbine driven feed pump would provide a nominal capacity of 450 MW. The system selects the lowest calculated value from feed pumps, circulation pumps, condensate pumps, FD fans and ID fans as the auxiliary plant capacity. The required output computation selects the lower of this value and the target load setting [Refer 3.3]. Tuning setters A02 to AlO allow the nominal maximum output for each type of auxiliary to be set. The appropriate fast Runback-Rate is selected if a runback is required to match Required Output to plant capacity following an auxiliary trip. The requirement for runback action is determined by Target Maximum being less than the prevailing Required Output. [Sheet 26]. When this occurs, the controls are transferred to BI mode prior to runback action being initiated. Each auxiliary plant group has a preset runback rate. The selected rate is determined by the group which limits the unit capacity to less than the prevailing required output. For example, consider the case mentioned in the previous paragraph at a load of 430 MW if the motor driven pump trips. The pump capacity is now 360 MW and the target load will reduce to this value. The runback system will select the pump runback rate which overrides the operator rate setting until the required output decreases to 360 MW. In the case of a multiple trip, the system will choose the lowest target and the highest rate. It should be noted that if an auxiliary trip results in a maximum target greater than the prevailing required output then no action results. This would be the case in the above example if the load was 300 MW before the pump trip. Tuning setters All to A14 provide the runback rates for each auxiliary type.

3.5

PRESSURE SET POINT

Refer to Sheet 32. The required output [RO] is used as the load index for development of the set point for sliding pressure operation as determined by the turbine manufacturer. A function generator [F(x)-04] computes the pressure set point from the prevailing RO. Tuning setter Al9 allows for adjustment of the maximum pressure.

13

If sliding pressure mode is not selected, the fixed pressure set point is set at the master display. Sliding pressure operation is available in CO and BF modes only. The fixed pressure set point tracks actual pressure when in sliding pressure or if an automatic mode not selected. On transfer to BI mode the pressure set point is held at the pressure existing at transfer. The rate of change of pressure set point is limited. The limit [% per min] is set by tuning setter A23. 3.6

GOVERNOR CONTROL

Refer to Sheet 35. The RO forms the basic demand signal to the governor system. The action of the modifying controllers for 'MW and pressure as well as the process depend on the selected operating mode; [See below]. The governor controller output is subject to directional blocking from pressure deviations. If the pressure is greater than set point by a preset amount, then the governor is prevented from decreasing. Likewise, increase is blocked on low pressure deviation. The governor controller output is transmitted via the auto/manual subwindow to a pulse converter. This compares the controller output with the calculated throttle valve position [Pl!Pt] and generates raise or lower pulses. The raise/lower pulses are integrated by the turbine governor system to form the load reference. The governor controls are operated differently depending on whether coordinated, boiler follow or base input-turbine follow mode is selected. [Refer Section 3.2 for discussion on mode selection.] (a)

Co-ordinated Mode:

In this mode the RO is the common demand signal to both turbine governor and boiler inputs; this common signal provides the required boiler-turbine co-ordination.

The process to the governor controller is turbine first stage pressure; this is an index of turbine energy input and is closely proportional to steady state MW output. The generation controller corrects for any residual difference between RO and the actual 'MW output after steam has ed through the reheater and downstream turbine stages. The generation controller adds a trimming signal to the RO.

14

(b)

Boller Follow Mode

In this mode the governor controls MW output, either as determined by the RO signal or from manual adjustment of the governor setting. The boiler inputs follow the resulting turbine energy demand [Sheet 38]. The turbine energy demand signal is calculated from the ratio of first stage to throttle pressure multiplied by the pressure set point. This signal coordinates the boiler and turbine with or without the governor on automatic. When the governor is on automatic it acts as described for co-ordinated mode [3.6.2]. If the governor is not on automatic, the boiler inputs still follow the turbine energy demand which results from the manual setting of the governor. [Boiler Follow -Governor Manual.] (c)

Base Input- Turbine Follow Mode

In this mode,the boiler inputs follow RO and the governor controls pressure. The RO signal forms the common demand to the boiler inputs and the governor. The process to the governor controller is effective throttle valve position, compensated for pressure set point [Pl/Pt X Ps]. This signal is independent of pressure and steam flow variations. The boiler fuel input is fixed at a value proportional to the RO, but the resulting MW is subject to variations in fuel heating value and unit efficiency. Pressure is maintained by the governor pressure controller [reverse acting] which adds a trimming signal to the governor demand. For example, if the pressure is increasing above set point, the controller will cause the throttle valve to open. This will cause an increase in MW and a decrease in pressure until balance is restored. The turbine thus follows the prevailing boiler energy input. The generation controller is not used in this mode. If the governor is not selected to auto, the MW will be approximately

equal to the RO as above, but the pressure will be proportional to RO and inversely proportional to throttle valve opening. Manual operation of the governor changes steady state pressure. [Base Input/Gov Manual] Such operation causes transient changes to MW output and steam temperature.

15 3. 7

FIRING RATE DEMAND

Refer to Sheet 38. The basic firing rate demand computation is dependent on the current operating mode. This demand is transmitted in parallel to the air and fuel control sub-loops.

(a)

Co-ordinated Mode In this mode, the governor is required to be on automatic controlling MW to equal the prevailing Required Output [RO] signal. The basic firing rate demand is also equal to Required Output. To this is added the following modifiers:

Heat Rate Correction This function compensates for the increase in unit heat rate as load decreases. At lower load, proportionately more fuel is required because of the lower efficiency.

(i)

(ii)

RO Rate

This compensates for the change in boiler stored energy at different load levels. The component of firing rate to accomodate stored energy change is proportional to both the firing rate demand and the rate of change of firing rate demand. The amount of RO Rate feedforward is set by tuning setter A25.

Pressure Rate This compensates for the change in stored energy due to different boiler pressures. The component of firing rate to accomodate pressure changes is proportional to the rate of change of pressure set point. This component is introduced in sliding pressure mode only. The amount of Pressure Rate feedforward is set by tuning setter A26.

(ill)

(iv) Pressure Correction The pressure correction controller responds to pressure error and its output recalibrates the steady-state firing rate demand to achieve zero pressure error; [Boiler-turbine balanced].

16

(b)

Boiler Follow Mode The basic firing rate demand is turbine energy demand, computed by Pl/Pt x Ps, where Pl is turbine first stage pressure, Pt is pressure before throttle valves and Ps is pressure set point. This signal replaces RO in boiler follow mode. The signal modifiers are the same as for co-ordinated mode as described in 3.7.1 (b),(c),(d). Tuning setter A24 calibrates the turbine energy demand signal. In this mode, the unit MW output may be automatically controlled by the governor to equal RO or be set manually.

(c)

Base Input· Turbine Follow Mode. In this mode the basic firing rate demand is simply Required Output [RO]. The modifiers for heat rate, dynamic compensation and pressure correction are not applied. Pressure control is executed by the governor controller if the governor is selected to automatic.

17

4

AIR FLOW CONTROL

Combustion air is supplied by two axial-flow forced draft [FD] fans. The fan output is controlled by varying the pitch angle of the fan impeller blades. Refer to Sheets 41-50 for analog logic and Sheets 432-436 for digital logic for the air flow control system. The FD fans operate in balanced draft configuration with the ID fans. [Section 6]. 4.1

PROCESS MEASUREMENTS

The measurement systems for combustion air flow, flue gas oxygen [02], and carbon monoxide [CO] are shown on Sheets 41 and 44. For 02 and CO measurements, selecting Sensor 1 actually selects the average of 1 and 2, provided that there is no transmitter deviation condition. This arrangement provides a more representative measurement. The air flow is measured by summing the flows through the two FD fans. The measurements are compensated for air density from the air temperature. A triple measurement system with median selection is provided [Section 2.3]. Tuning setter A28 calibrates the selected signal against the air demand. 4.1

PROCESS MEASUREMENTS

The measurement systems for flue gas oxygen, carbon monoxide and combustion air are shown on Sheets 41 and 44. For oxygen and carbon monoxide, selecting Sensor 1 selects the average provided that there is no transmitter deviation. This gives a more representitive reading. The air flow is measured by summing the flows through each of the two FD fans. A triple measurement system with median selection is provided. Tuning setter A28 calibrates the selected signal against the air demand [4.2]. 4.2

AIR DEMAND

The firing rate demand forms the basic demand for air flow. [Sheet 50.] This signal is subject to the following modifiers:

18

(a)

Lead/Lag

The purpose of this function is to ensure that the air flow is always in excess of requirements when the firing rate is being changed. The lead/lag function accomodates the different transient response characteristics of the fuel and air systems. For firing rate demand increases, a "lead" signal is applied when a positive rate of change is detected. This forces a higher rate of change to the air demand. Conversely, for firing rate demand decreases, the air demand is subject to "lag". This delays the reduction of the air demand relative to the fuel. (b)

Excess Air Correction:

In order to ensure complete combustion, the amount of air supplied needs to be in excess of that theoretically required to burn all the fuel. This additional component is called "excess air". The required excess air for a given fuel and load is calculated by the boiler manufacturer. [Refer 4.3.] (c)

Minimum Air Flow:

The air demand is subject to a minimum limit [normally 30%] and a fuel cross limit. The cross limit prevents a serious deficiency of air for the current fuel flow. Setter A27 is adjusted to ensure this limit action does not affect normal operation. The resulting control signal is the Air Demand. The selected air flow signal is subtracted from the demand to form the air error to the air flow controller. A high air error blocks further increase in RO. The error must be initialized to within +-2% of zero before air auto is permitted. 4.3

EXCESS AIR CONTROL

The amount of excess air can be determined by measuring the oxygen [02] in flue gas. A function generator [Fx-13] calculates the base oxygen set point as a function of firing demand; this function is based on boiler performance data. The carbon monoxide [CO] concentration is used to determine the optimum excess air for maximum boiler efficiency. The desired CO level is maintained by the CO controller. The computed base oxygen set point is corrected by the CO controller output. This correction signal is limited to +-2% 02. The base set point plus correction is the oxygen set point.

19

The desired percentage of excess air is calculated from the 02 set point by function generator F(x)-15. The percent excess air multiplied by the firing rate demand calculates the absolute amount of excess air. This is then added to the basic air demand as a feedforward. In order to obtain the exact oxygen content, the desired oxygen concentration is compared with the measured value and the resulting error is applied to the oxygen controller. The controller output trims the excess air demand feedforward to obtain the required value of oxygen. The 02 trim signal is limited to +-3%. 4.4

AIR FLOW CONTROLLER

The Air Flow Controller positions the pitch angle control drives so as to reduce the air flow error to zero. The control drives have auto/manual selection, position bias and equalizing control. Tuning setter A29 adjusts the amount of direct demand feedforward to the air control drives. The operation of the dual drive configuration [Type A] is described in Section 1.1. Air flow control pre-selected to automatic is a required auto permit for CO, BF and BI control system modes [Section 3.3]. The air flow may be selected to automatic in Manual mode; the basic demand is derived from total fuel flow. [Refer Sheet 26]. This method is normally only used at start-up to stabilize air flow at 30%. It is a prerequisite that furnace pressure is on automatic before auto air flow control is permitted. 4.5

TRACKING

If the air controller is not auto, the oxygen controller tracks a back calculation that forces the air error to zero. [Refer to Section 1.5.] The back calculation includes the inverse of F(x)-15. This ensures bumpless transfer when air control is transferred to automatic. To facilitate this initialization, the oxygen controller output must be available to the air demand computation when air is not auto. The manual adjustment of the oxygen trim signal is therefore only permitted

when air control is on auto.

20 4.6

FD FAN STALL

Axial flow fans can "stall" under certain operating situations. This condition is a function of the fan blade angle and air velocity through the fan. It can occur if a fan is operated at high head and low flow. This situation can be caused by restrictions in the flow path or by unbalanced parallel operation of two fans. Stalling causes severe vibrations to the fan and ducting and a sharp drop in fan output. Sheet 46 shows the system provided to warn the operator that operation is close to stall point. The volumetric flow is calculated for each fan and a function generator calculates the maximum safe pressure for the prevailing flow. This calculated value is compared to the actual pressure and an alarm is initiated if it is higher than the maximum safe value. 4.7

AIR HEATERS COLD END TEMPERATURE

Flue gas from the furnace is used to heat the incoming combustion air in two rotary regenerative air heaters. To avoid plugging and corrosion from sulphur products, it is essential to operate the cold end of the heaters above the acid dewpoint temperature. The heater cold end temperature is defined as the average of the air inlet temperature and the flue gas outlet temperature. The cold end temperature is controlled by pre-heating the air from the FD fans with hot water from the deaerator. Water to the two heat exchangers is supplied by three pumps. Two valves associated with each heater control the relative amounts of water returning to the deaerator and recirculating through the pumps. The water flow is relatively constant, its temperature is determined by the proportion of hot water from the deaerator to recycled water. The two valves work from a common signal but in opposite directions. The cold end temperature is calculated from the average of three thermocouples for each measurement as shown on Sheet 171. This is compared to the temperature set point and the resulting error is applied to the cold end temperature controller, [Sheet 175]. The controller output positions control drives; for low cold end temperature the proportion of recycled water is decreased, the water temperature increases which increases the amount of combustion air preheat. The opposite occurs for high cold end temperature. Auto/manual selection and set point adjustment is made at the appropriate CRT subwindow.

21 4.8

WINDBOX AIR DAMPERS

After leaving the air heaters, the combustion air is distributed to the furnace from the furnace windbox through windbox air dampers. These dampers are of two types; Auxiliary Air Dampers and Fuel Air Dampers. Refer to Sheets 121127 and 521-530. 4.8.1

Auxiliary Air Dampers

The Auxiliary Air Dampers are controlled to maintain the required differential pressure from the windbox to the furnace. The differential pressure set point is computed from steam flow by function generator F(x)-44. The auxiliary air is itted above and below the active burners. The controlled pressure ensures adequate air velocity. The selection of which elevations are active is executed by the burner management system. A single controller and associated auto/manual station operates all elevations of dampers. 4.8.2

Fuel Air Dampers

The Fuel Air Dampers control the flow of air around each burner. The opening is calculated as a function of burner pressure; F(x)-42 for fuel gas and F(x)-43 for fuel oil. The selection for gas or oil is made by the burner management system. The dampers for idle elevations are closed by the BMS. Elevation 1 is arranged to permit firing of single burners. all other elevations require a minimum of two [opposite] burners. Warm up oil is fired on elevation 1. When warm-up oil is used, the elevation 1 damper opening is fixed by tuning setter A70.

22

5

FUEL FLOW CONTROL

Refer to Sheets 53 - 62 for analog signals and Sheets 440 - 444 for digital logic. The boiler can produce rated output firing natural gas or fuel oil or combinations of both fuels. Dual100% capacity control valves are provided for both fuels. 5.1

FUEL METERING

(a)

Gas Flow Dual, 100% capacity metering systems are provided for gas flow; under normal conditions only one system is in service. Each metering system comprises a flow orifice, dual differential pressure transmitters, a pressure transmitter and dual temperature transmitters. A tuning setter A52 allows site adjustment of the specific gravity. From these inputs the volumetric flow is calculated at standard conditions [273.18 deg Kelvin, 1.0133 Bar Abs]. Tuning setter A48 calibrates the gas flow to match the firing rate demand in per unit values.

(b)

OilFlow The main [heavy] fuel oil flow is calculated from fuel oil to burners (+), return oil from burners (-) and warm up oil (+). The signals are modified to a common scale before the c6mputation and tuning setter A49 calibrates the total to equivalent per unit mass flow. The ignitor [light] oil flow is also metered and converted to mass flow by A47.

The three fuel measurements; gas, main oil and ignitor oil, are converted to equivalent heat flow by tuning setters A38, A39 and A37. These setters provide the facility to adjust for changes in fuel heating values. Ignitor oil is added to fuel oil to give total oil heat flow. Gas heat flow is added to the oil to give total fuel heat flow. 5.2

FUEL DEMAND

The fuel demand is computed by the coordinated control system from required output [CO or BI modes] or turbine demand [BF mode]. Refer to Section 3.7, Firing Rate Demand. The fuel demand is cross limited with the metered air flow, the lower being selected. This is to prevent significant mismatch between air and fuel. [Fuel demand>> air flow.] Tuning setter A30 adjusts the air flow signal so that it is normally not selected.

23 The fuel demand is apportioned to gas and oil fuel according to the Oil Ratio setting by the operator. The oil demand is calculated from fuel demand times the ratio setting. The gas demand is calculated from total fuel demand less the oil demand. 5.2.1

Combined Firing

When combination firing is being used, both fuels may be on automatic control or one fuel on auto and the other on manual. The control system accomodates all configurations of automatic operation. Transfer from one configuration to another is bumpless.

(a)

Both Fuels Auto The required proportion of oil fuel is set by the operator by adjustment of the ratio setter. The oil demand is computed from the total demand times the oil fuel ratio. The remaining demand is assigned to the gas fuel. The oil fuel ratio servo is only available to the operator when both fuels are on auto. If, when firing both fuels, one fuel becomes limited and the flow error exceeds a preset threshold deviation, [because of insufficient burners in service or any other reason], this error is added to the other fuel control error. Should both fuels become limited, the ensuing total fuel deviation will block further changes in total demand in the direction which would increase the deviation.

(b)

One Fuel Auto, the Other Manual In this case, the oil ratio setter tracks the proportion of oil flow of total fuel. The flow error of the fuel that is not-auto is added to the auto fuel error. Manual change to the not-auto fuel flow causes the error signal to the auto fuel to change an equal and opposite amount. The total fuel flow thus remains constant.

(c)

Both Fuels Manual The co-ordinated control system will be in the "Mode Not-Auto" status. The ratio setter tracks oil flow, thus balancing the oil fuel sub-loop. Prior to "Mode Auto" status, the total fuel demand equals total fuel flow and the gas fuel sub-loop will also be balanced, as follows:

24

Total Demand =Total Fuel Oil Demand =Oil Flow Gas Demand =[Total Demand - Oil Demand] =Gas Flow 5.2.2

Single Fuel Firing

In this case the non-fired fuel flow will be zero and the oil ratio set ter tracking will run to either zero (oil off) or 100% (oil on). The fired fuel receives the total fuel demand. 5~

FUELCONTROLLERS

There is a separate controller for gas and oil fuel. Dual 100% valves are provided for each fuel; only one of the pair is permitted on automatic at the same time. [Refer Section 1.3]. The controller output position the selected gas and oil control valves. The gas flow and oil flow errors are modified if necessary to hold the pressures between required high and low limits; in accordance with NFPA 85B and 85D. Tuning setters A34, A35, A36 and A41 set the minimum and maximum header pressures. 5.4

FUEL·AIR DEVIATION MONITOR

Firing conditions which lead to a situation where there is insufficient air to bum the fuel are potentially hazardous. This condition can be caused by incorrect manual operation of fuel and air or control system faults. An independent control system supervises the fuel-air ratio. Two levels of abnormal fuel-air ratio are detected; "Fuel High" and "Fuel Very High". The Fuel High condition, after a short delay, trips the coordinated controls to Manual mode and independently trips both fuel and air to manual. If further deviation occurs, the Fuel Very High condition initiates fuel firing rate cutback. The cutback action continues until the Fuel High condition resets. Refer to Sheet 586 for logic.

25

6

FURNACE PRESSURE

The supply of combustion air and the removal of the products of combustion is carried out by the forced draft and induced draft fans working in balanced draft configuration. The work is shared between the two sets of fans. The pressure inside the furnace is controlled to be slightly negative at all loads; this ensures the designed balance between FD and ID fans is maintained. It also prevents the leakage of extremely hot furnace gases to the boiler external area through casing and duct leaks. Changes in both FD and ID fan output affect both air flow and furnace pressure; however it is now standard practice to control air flow primarily by the FD fans and furnace pressure by the ID fans. This is in accordance with the NFPA code. To minimize furnace pressure deviations on load changes, the ID fans output follows the FD fans and the furnace pressure control trims the residual unbalance by further adjustment of the ID fans output. In effect, the air flow is controlled by both FD and ID fans in parallel. This is true also when the FD fans are in manual, provided that the ID fans are auto. The ID fans at Bang Pakong 3 & 4 can be regulated by either changing the fan speed through a variable speed coupling or by inlet damper control [refer 6.1, 6.2]. A triple measurement system as described in Section 2.3 is used for the measurement of furnace pressure. The control logic for the furnace pressure control is shown on Sheets 109- 117 [analog] and 513- 517 [digital]. 6.1

ID FAN SPEED

The variable speed feature of the ID fans enables the fan to operate nearer to optimum conditions over a wide load range which reduces losses and consequently improves efficiency. The response of speed control is somewhat slower than the FD fan blade pitch control because of the need to change the rotational inertia. The inlet dampers are used as the controlling device for furnace pressure because of the better transient response. The ID fan speed PID controller follows a set point computed from the total FD fan pitch position demand by the function generator F(x)-40. The process for the speed control loop is the total ID fan speed regulator control drive demand. The total ID fan speed thus tracks the total FD fan blade pitch and maintains balanced operation.

26 The two ID fan control drives are arranged in as dual drives, type A as described in Section 1.1. Demand feedforward direct to the control drives is adjusted by tuning setter A51. This improves transient response. The ID fan variable speed couplings are controlled by a local loop which adjusts ID fan speed to match the speed demand from the DCIS. 6.2

ID FAN INLET DAMPERS

The final furnace pressure is controlled by the ID fan inlet dampers. The ID fan speed is controlled to a value which places the inlet dampers at ·a steady state opening of about 60% over the normal load range with two-fan operation. On a load change, the ID fan speed tracks the FD fans and the ID damper controls correct furnace pressure transients. The median furnace pressure signal is compared to the operator set point and the error is attenuated for small deviations by function generator F(x)-39. [Furnace pressure signals tend to be very noisy.] This has same effect as reducing PID controller gain and prevents unnecessary controller action and wear to mechanical components. For larger deviations, the error is not attenuated. The controller is reverse acting. Feedforward from the difference between total FD pitch position and total ID fan speed demand is applied. Normally this calculation produces zero feedforward; the exception being when the speed control is on manual or operating in the flat part of the speed demand function generator. The amount of feedforward is adjusted by tuning setter A50. The damper control drives are in dual configuration, type A, as described in Section 1.1. 6.3

IMPLOSION PROTECTION

Two basic mechanisms can cause a negative pressure excursion of sufficient magnitude to cause structural damage to the furnace and ducting. Firstly, control malfunction or operator error can cause high suction to be applied with restricted air path into the furnace, e.g. FD dampers closed. The second mechanism is the result of rapid temperature drop in furnace gas temperature following a termination of fuel input. The temperature drop causes a decrease in furnace pressure which may be sufficient to implode the furnace.

27 The control system includes the following protective features against implosions in accordance with NFPA Code 85G: (a)

Triple furnace pressure measurement system.

(b)

Feedforward from FD fan pitch position demand.

(c)

Directional blocking of both FD and ID fans if an abnormal furnace pressure error occurs. For example, an abnormally low pressure causes blocking of ID increase and FD decrease.

(d)

A Master Fuel trip [MFf] initiates an override which reduces the ID fan inlet vanes to a preset proportion of its prevailing value. The override then decays over a number of seconds and allows furnace pressure control to resume. Tuning setter A90 adjusts the proportional transient reduction applied to the inlet dampers control drives when MFT occurs. The override is effective in both manual and automatic control.

(e)

FD Fan Stall alarm is provided to warn against the possibility of uncontrolled air flow changes caused by a fan stall condition.

28

7

STEAM TEMPERATURE

Control of main steam and reheat steam temperatures is effected by a combination of furnace gas recirculation, tilting burners and spray desuperheating. The gas recirculation and tilting burners both affect the relative distribution of heat between evaporative and superheating elements. The primary temperature control is by gas recirculation; the tilting burners are regulated to a pre-programmed position that is a function of load and proportion of oil firing. The gas recirculation flow and tilt position affect both main and reheat steam temperatures. The gas recirculation flow is regulated to control reheat temperature. The main steam superheater is designed to absorb sufficient excess heat to require desuperheating over the normal load range. This enables the main steam temperature to be controlled by desuperheating. For the main steam, there are two stages of spraywater desuperheating; these follow the primary and secondary superheaters. Spray desuperheating is also fitted at the reheater inlet for emergency use. The steam temperature controls are shown on Sheets 77- 98 [Analog] and 471- 499 [Digital]. 7.1

MAIN STEAM TEMPERATURE

The main steam temperature is controlled in two stages; secondary superheater outlet and tertiary [final] stage outlet. The desuperheaters are located before the The arrangement of superheaters and secondary and tertiary stages. desuperheaters is shown on Fig 7.1. The time constants associated with steam temperature controls are significant. Both control systems are arranged in a cascade configuration where the inner loop controls desuperheater outlet temperature. This assists in stabilizing the outlet temperatures against disturbances such as spray pressure fluctuations and burner changes. To further improve the dynamic response and stability, a number of anticipatory [feedforward] signals are applied; the objective being to reduce the amount of correction required by the process. All temperature and pressure sensors used for the steam temperature controls are duplicated with transmitter deviation monitoring logic as described in section

2.2.

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29 7 .1.1

Feedforward Signals

The basic feedforward to each system is a calculated set point for desuperheater outlet temperature [inner loop] for the prevailing load. This set point represents the expected steady state value for sliding pressure operation with gas firing and includes the expected amount of gas recirculation and excess air. The feedforward is added to the primary controller output; the controller modifies the feedforward to achieve the required outlet temperature. Other feedforwards are added to compensate for dynamic conditions and different operating conditions, such as fixed pressure. (a)

Drum Pressure

Increase in drum pressure reduces the enthalpy of the saturated steam and creates higher mass flow for the same firing rate. This results in a drop in steam temperature. Function generator F (x)-69 computes the steady-state drum pressure from steam flow for sliding pressure operation. The deviation of actual pressure from the computed value is added to the feedforward summer. High pressure causes increase in the desuperheater outlet set point to compensate for steam temperature drop. Tuning setter A65 calibrates the level of feedforward. (b)

Air Flow

Overfiring and underfiring on load changes alters the relationship between heat input and cooling steam through the superheaters and causes steam temperature variations. The relationship between steam flow and air flow is subject to transient change; this is used to generate a feedforward signal. Increases in air flow [firing rate] relative to the steam flow increases temperature; the feedforward decreases the desuperheater outlet set point. Tuning setter A68 calibrates the feedforward. This input also compensates for changes in excess air. (c)

Pressure Set Point

Changes in pressure set point require over/under firing which affects steam temperature. The effect is proportional to the rate of change of pressure. Increasing pressure set point causes a transient increase in steam temperature. The feedforward reduces the desuperheater outlet set point by an amount proportional to the pressure set point rate. Tuning setter A65 calibrates the feedforward signal.

30 (d)

Gas Recirculation

The amount of gas recirculation varies from the predicted value because of factors such as furnace fouling. The difference between actual and calculated gas recirculation generates a feedforward signal. Increasing gas recirculation increases the steam temperature; the feedforward decreases the desuperheater outlet set point. Tuning setter A61 calibrates the feedforward signal. 7.1.2

Secondary Superheater Outlet Temperature

The steam temperature in each of the two links from the secondary superheater outlet header has its own control system. There are four desuperheaters between the primary and secondary superheaters. Each of the two secondary superheater outlet temperature controllers operate in cascade configuration with the associated pair of desuperheater spray controllers which regulate desuperheater outlet temperature. The arrangement is shown on Fig. 7.1.2. The outlet links make a cross-over; hence Link A temperature is controlled by desuperheaters C and D and Link B is controlled by A and B, [Refer Fig. 7.1]. The secondary superheater outlet controllers set point is calculated as a function of steam flow [F(x)-36]. The calculated set point may be replaced by an operator setting. The common set point is compared with each of the two secondary outlet temperatures and the resulting error is applied to the appropriate controller. Each controller output modifies the feedforward signal [7 .1.1] to form the set points to the spray controllers. The feedforward comprises the basic set point computed by F(x)-14 and the dynamic components set by tuning setter A66. The setpoint to the spray controllers is auctioneered against the calculated saturation temperature [F(x)-33] plus margin, the higher being selected. Each desuperheater is equipped with two 100% capacity spray valves. Only one valve is permitted on automatic operation at the same time. Refer Section 1.3, Type B-2. 7.1.3

Final Superheater Outlet Temperature

The steam temperature from each of the two tertiary [final] superheater outlets has its own control system. There are two desuperheaters between the secondary and final superheaters. Each of the two final superheater outlet temperature controllers operate in cascade configurat-ion with the associated desuperheater spray controllers which regulate desuperheater outlet temperature. The arrangement is shown in simplified form on Fig. 7.1.3.

SECONDARY SUPERHEATER OU1LET SET POINT

, LINK A STMTEMP ,

LINK2A CONTROLLER

LINKB STMTEMP

LINK2B CONTROLLER FEEDFORWARD

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Figure 7•1•3 Final Superheater Outlet Temperature Control

31

The final superheater outlet temperature is set by the operator. The common set point is compared to each of the two final outlet temperatures and the resulting error is applied to the appropriate controller. The controller output modifies the feedforward signal [7 .1.1] to form the set points to the spray controllers. The feedforward comprises the basic set point computed by F(x)-20 and the dynamic components set by tuning setter A67. The set point to the spray controllers is auctioneered against the calculated saturation temperature [F(x)-34] plus margin, the higher being selected. Each desuperheater is equipped with two 100% capacity spray valves. Only one valve is permitted on automatic operation at the same time. Refer Section 1.3, Type B-2. 7.2

REHEAT STEAM TEMPERATURE

Reheater outlet steam temperature can be controlled by regulating gas recirculation flow, burner tilt angle or spraywater attemporation. The primary control method is gas recirculation; spray attemporation is used only if the gas recirculation system is unable to maintain control for any reason. [Introducing spraywater into the reheat stage lowers the unit efficiency.] Burner tilts are only used to compensate for the different characteristics of oil firing versus gas. The reheat temperature controls are shown on sheets 92 - 98 [analog] and 490 - 499 [digital]. 7.2.1

Gas Recirculation

The reheater steam outlet temperature is normally controlled by regulating the gas recirculation flow. Increased mass flow over the convective surfaces increases the heat absorption. Changes in gas recirculation also affect the main steam temperature, see Section 7 .1.1. The average reheater temperature is compared to the set point and the resulting error is auctioneered against the fan motor current deviation before being applied to the GR Fan controller. If either motor current exceeds the maximum set point, the controller acts to reduce the current rather than control temperature. The amount of gas recirculation flow is controlled by positioning the inlet vanes on the two GR Fans. A feedforward signal from function generator F(x)-32 is added to the controller output. This computes the expected GR Fan vane position as a function of load for gas firing. Function generator F(x)-32 addB a modifying signal proportional to the amount of oil firing. The control drives for these fans are fans conform to Type A as described in section 1.1.

32 7 .2.2

Reheat Sprays

The reheat spray controller set point is the GR Fan set point with a bias added. Under normal operation, the spray controller sees a low temperature and holds the valves closed. If the temperature increases above the set point by an 8.Iilount greater than the bias, the spray controller becomes active. The bias is removed if the GR Fans are not auto. The spray valves are in two pairs; each pair is configured as Type B-2, Section 1.1.

7.2.3

Burner Tilts

The burner tilt position follows a load program developed by F (x)-41. This program is modified by F(x)-45 when oil is fired, the modifier being proportional to the oil ratio.

33

8

FUEL OIL PUMPS

8.1

FUEL OIL TEMPERATURE

There are two oil heating systems, each with its own control system. Refer to sheets 133, 135, 533, 535. The base set point for the fuel oil temperature controller is set manually and is biased by the fuel oil viscosity controller. The bias range is limited to +1-5 deg. The temperature is controlled by regulating the flow of heating steam to the oil heater. A feedforward signal proportional to the oil flow is added to the temperature controller output. tuning setter A 71 calibrates the feedforward signal. 8.2

FUEL OIL HEADER PRESSURE

The fuel oil pressure is maintained by recirculating oil from the fuel oil pumps discharge header back to the storage tanks. Refer to sheets 139 and 539. Two parallel control valves are operated in split control configuration from a single controller and subwindow, Type B-1, Section 1.1. The control is reverse acting; increasing pressure causes the valves to open. 8.3

FUEL OIL HEATER STEAM PRESSURE

Steam is available to the oil heaters from two sources; IP extraction to deaerator and cold reheat. The oil heater steam pressure controller regulates the extraction steam and cold reheat pressure control valves in split control configuration to maintain the set value. Refer Sheets 143, 543 and Section 1.1 type B-1. 8.4

FUEL OIL TRANSFER PUMP PRESSURE

The fuel oil pressure from the transfer pumps is maintained by recirculating oil from the transfer pumps discharge header back to the storage tanks. Refer to sheets 147 and 547. Two parallel control valves are operated in split control configuration from a single controller and subwindow, Type B-1, Section 1.1. The control is reverse acting; increasing pressure causes the valves to open.

34

9

FEEDWATER

Boiler feedwater is supplied from a pumping system comprising two 60% capacity turbine driven pumps and two 15% motor driven pumps. Control of pump output is by varying pump speed. The pump turbines are equipped with variable speed governors and the constant speed motors are connected to their pumps by a variable speed hydraulic coupling. The motor driven pumps are normally used for start up and low load operation. Because of the differing capacities and response characteristics, the controls only permit one type of pump to be on automatic operation at the same time. Refer to Sheets 65 - 75 [analog] and 448 - 468 [digital].

9.1

DRUM LEVEL

The drum level control system comprises a three-element cascade configuration. The steam flow from the boiler is used as the basic demand to the feedwater flow controller, thus balancing the water input to the steam output. Imbalances caused by transient conditions and metering errors result in changes to the boiler drum level. A separate drum level controller adds a correcting signal to the feed water demand to keep the level at the set value. Steam flow increases cause a temporary drop in drum pressure. This lowers the boiling temperature and increases the volume of steam bubbles in the water which, in tum, causes a transient drum level increase. Introducing feedwater immediately to match the steam flow aggravates the level deviation. To overcome this effect, the demand from steam flow is modified to first decrease and then increase as a time lag function to the final value. The reverse occurs on a load decrease. 9.1.1

Process Measurements

The three measurements required for the system are steam flow, feedwater flow and drum level.

35 (a)

Steam Flow

The steam flow is measured inferentially from turbine first stage pressure. This provides a linear signal proportional to steam flow over the normal operating range. The tuning setter ABO calibrates the· first stage pressure to represent percentage steam flow. The calibration is subjected to transient offsets from cycle changes such as feedwater heaters out of service. (b)

Feedwater Flow

The feedwater flow is metered by differential pressure transmitters from an orifice. To this is added the superheater desuperheater spray flow. The flow calculations include correction for water density. The density is calculated from feedwater temperature by function generator F (x)-68. To improve accuracy at low loads, a low-flow transmitter is provided. This has a range of30% of the high-range transmitter. The transfer point is 25%. Function F(x)-37 biases the high range output down below 25% and F(x)-38 biases the low range down above 25% maximum flow. The transfer is made by a high select function. Tuning setter A91 calibrates the percent feedwater flow. (c)

Drum Level

Triple redundant drum level transmitters are provided at each end of the boiler drum [refer Section 2.3]. The median is selected from each set and the average calculated to form the process variable to the drum level controller. The drum level measurements are compensated for the effect of water and steam densities. The densities are computed from the prevailing steam pressure. The compensations are executed in functions F(x)-21 to 26. 9.1.2

Motor Driven Pumps

The feedwater flow error is applied to the motor driven pump controller if one or both are selected to automatic operation. The controller output forms the position demand to the two control drives for the hydraulic couplings. These drives are arranged in a modified form of Type A [Section L ll

36 Feedwater pumps will only deliver water to the drum when the pumping head is sufficient to overcome the drum pressure and the system friction losses. Function generator F(x)-2 calculates the minimum pumping speed from drum pressure and adds this to the speed controller output. The controller output is therefore always effective and the loop gain is independent of pressure. The control drive equalizing, biasing and tracking system recognizes only speed demands above the pumping threshold. A pump placed on auto will run up to the minimum pumping speed if it is below this value. 9.1.3

Turbine Driven Pumps

The feedwater flow error is applied to the turbine driven pump controller if one or both are selected to automatic operation. The controller output forms the speed demand to the two turbine speed governors. These outputs are arranged in a modified form of Type A [Section 1.1]. The mjnimum speed feature is applied similar to the motor pumps as described in the preceding section [9.1.2]. The minimum pumping speed for the turbine pumps is computed by F(x)-03. The manual speed demand outputs to the governors track the governor internal speed reference until the permit for remote auto mode is given by the governor logic. 9.2

FEED PUMP MINIMUM FLOW

Feedpumps require a minimum flow to avoid overheating. When the demand for feed water is low, the required flow is maintained by recirculating sufficient feedwater back to the deaerator. The total pump flow is the ·sum of the flows to the HP heaters and the recirculated flow. The control systems for feedpump minimum flow is shown on Sheets 334, 340 [analog] and Sheets 734 - 743 [digital]. The setpoints for minimum flow is fixed and cannot be changed by the operator. Manual operation of the recirculation control valves is not permitted; if manual is selected, the relevant recirculation control valve goes to the fully open position. This prevents pump damage from incorrect operation. The recirculation control valves can be subject to high pressure drops. Operating them at small openings for extended periods can cause "wire-drawing" damage to the valve trim. The control system adds a minimum flow bias to the set point when the valve position demand is greater than zero. The bias is removed when the valve flow is less than the minimum flow bias.

37

Under normal pump operating conditions the pump flow exceeds the minimum. The integral action of the recirculation flow controllers then holds the valves closed.

38 10

CONDENSATE & DEAERATOR

The condensate system comprises the condenser, three condensate pumps, gland condenser, LP heaters, deaerator storage tank and the condensate storage tank. Condensate is pumped from the condenser through the gland condenser and heaters to the deaerator tank. The dearator tank level is controlled by regulation the incoming condensate flow rate. The condenser level is maintained by transfer of condensate to or from the condensate storage tank. 10.1

DEAERATOR LEVEL

The deaerator level controls are shown on Sheets 105,109 [analog] and 501- 510 [digital]. A three element cascade control system is provided. The flow from the deaerator tank to the feedpumps is the basic demand to the condensate flow controller. This demand is corrected by the deaerator level controller. The deaerator level measured by triple-redundant transmitters with median selection. [Refer section 2.3.] Tuning setter A89 calibrates the feedwater flow signal against the corresponding condensate flow. The output of the three condensate pumps is regulated by pump speed using hydraulic couplings. The three control drives are configured as Type C, as described in Section 1.4. The flow controller matches the incoming flow to the outgoing flow to the feedpumps. Any imbalance creates a level error. The level controller continuously calibrates the demand from feedwater flow and maintains the level set point. A minimum condensate pressure is required to maintain the suction head for the boiler feed pumps seal water pumps. Low condensate pressure overrides the flow error. This prevents the condensate pumps from falling below the speed required to maintain sufficient suction head to the seal pumps. [This override only operates at start up; the condensate pressure is more than adequate under normal operating conditions.] 10.2

DEAERATOR HIGH LEVEL

The high level controller provides an alternative emergency control which prevents the level exceeding the safe maximum, [Sheet 269]. Condensate is dumped directly to the condenser through a pair of regulating valves in split control configuration. The high set point is set above the normal level; under normal operating conditions the integral action of the high level controller holds the valves closed. This loop must always be available for automatic operation; no manual control facilities are provided.

39 10.3

DEAERATOR PRESSURE

The deaerator pressure is normally maintained by regulating auxiliary steam to the deaerator. Cold reheat steam provides an alternative source; the auxiliary steam and cold reheat valves are arranged to operate from a single pressure controller in split control configuration. [Refer to Sheet 321.] The set point is fixed and no manual control facilities are provided. The loop is always on automatic. 10.4

DEAERATOR TEMPERATURE

The deaerator water temperature is measured at two points and averaged. The deaerator temperature controller maintains the temperature of the water by regulating the flow of auxiliary steam to the deaerator sparging nozzle header. Under normal operating conditions, the temperature is maintained by the flow of extraction steam to the deaerator-heater and the auxiliary steam control valve is closed by controller integral action. [Refer to Sheets 325, 725.] 10.5

CONDENSER LEVEL

The condenser hot well receives the condensed steam from the LP turbine stages. The condensate inflow is proportional to unit load, less losses such as continuous blowdown and leaks. The outflow is controlled by the deaerator level control and is proportional to unit load. The condenser and deaerator levels are interactive and respond in opposite directions to condensate pump speed changes. The condenser level is controlled by exchanging water with the condensate storage tank which acts as a buffer. Refer to Sheets 265 [analog] and 665 [digital]. If the level is below the set point, water is added to the condenser via the normal and emergency make up valves in split control configuration. If the level is high, water is diverted to the condensate storage tank from the condensate pumps discharge through the dump valve; this causes a reduction in flow to the deaerator which is compensated by increased pump speed and consequent lowering of the condenser level. Separate controllers are provided for the make up and dump valves. To avoid interaction, the controllers are arranged for proportional action only. At zero error, all valves are closed. For high level, the dump valve opens; for low level, the make up valve opens. Under steady load conditions, the make up valve will normally be open to compensate for system losses. The condell5at.e Bt.orage Lank

accomodates changes in system water volume caused by temperature changes at different loads.

40 10.6

CONDENSATE RECIRCULATION

The condensate recirculation system maintains a minimum flow through the gland condenser at low loads. Refer to sheets 249, 649. The additional condensate required for minimum flow is obtained by regulating the recirculation of condensate from downstream of the gland condenser back to the condenser. At high loads, the controller integral action holds the recirculation valve closed. 10.7

CONDENSATE PUMPS RECIRCULATION

The condensate pumps recirculation controls ensure that a minimum flow is ing through the pumps. This avoids overheating at low loads. Refer to Sheets 253, 653,657,661. The requirement for recirculation valve opening for each pump is computed from its net flow to the gland condenser. When a threshold flow is reached [flow decreasing], the function generator F(x)-59,60,61 ramps the valve open. To avoid hunting, the demand has a fast open, slow close characteristic. No manual control facility is provided. If manual is selected, the valves run to the fully open position.

41

11

FEEDWATER HEATERS

The feed heating system comprises three high pressure heaters [8, 7 ,6], a heater-deaerator [5], three low pressure heaters [4,3,2], a pair of condenser neck LP heaters [lA,lB] and an LP drains tank. The HP heater drains cascade to the deaerator and the LP heaters cascade to the LP drains tank. LP drains are returned to the condensate system ahead of LP2 by a pair of LP drains pumps. A pair of HP drains pumps lifts drains from HP6 to the deaerator at reduced loads when there is insufficient head from the extraction steam pressure at HP6. 11.1

FEEDWATER HEATERS LEVEL

The following description applies to LP2, LP3, LP4, HP6, HP7 and HP8. Refer to Sheets 283- 313. Each heater is fitted with a normal cascade drains control valve and an alternative emergency drain to the condenser. The normal and emergency valves have separate level controllers and transmitters; the set point of the emergency controller is set slightly higher than the normal controller. [The controllers are reverse acting, increasing level opens the valve.] Under normal conditions, the level is controlled by the normal valve to its set point and the emergency valve is held closed by controller integral action. [The emergency controller sees a low level.] If the normal controller fails to keep its set point and the level approaches the emergency set point, the emergency controller becomes active and regulates to its own set point. The emergency controller level transmitter output is reversed; transmitter failure to zero is interpreted as a high level. The level set points are fixed and no manual control facilities are provided. These loops are in automatic mode at all times. 11.2

LP HTR DRAINS TANK LEVEL

The LP drains tank level is controlled by regulating the flow from the two LP drains pumps to the condensate system. Two parallel valves are provided, these are operated sequentially in split-control configuration from a single controller. The controller is reverse acting. Refer to Sheet 279. No manual operating facilities are provided, the loop is in automatic mode at all times. A transient "close" signal is added to the valve position demand when a second pump is started; this is to avoid draining the tank.

42 11.3

LP HTR DRAINS PUMP RECIRCULATION

The pump recirculation system ensures that there is sufficient flow to avoid overheating of the pumps under low load conditions. Refer to Sheets 273,673,676. The total flow from the two pumps is used by F(x)-70 to compute the required valve opening. If two pumps are running the flow signal is halved. On decreasing flow, the recirculation valves are ramped open once the flow is lower than the minimum threshold. No manual regulating facility is provided. position if manual is selected. 11.4

The valves run to the fully open

HP HTR DRAINS PUMP RECIRCULATION

The HP drains pumps lift the drains from HP6 to the deaerator when insufficient pressure differential exists. The recirculation system ensures that there is sufficient flow through the pumps at all times so as to prevent overheating. Refer to Sheets 385, 785. If the flow falls below the set value, the recirculation valve controller opens the valve and regulates the flow at the required minimum. The recirculated flow is returned to the HP6 heater drains cooling section. The flow transmitter measures the total flow; if two pumps are in service, the flow signal is halved. The resulting signal approximates single pump flow. A guidance message advises the operator to shut down the pumps when the pressure differential is sufficient. When both pumps are off, the recirculation valve is closed by external logic. No manual control facility is provided. If manual is selected, the valve goes to the fully open position.

43

12

MISCELLANEOUS

12.1

SEAL STEAM PRESSURE

Starting from a low load condition, the turbine glands sealing steam pressure is obtained from main steam. As load increases, the cold reheat pressure increases. When sufficient pressure is available, the steam source transfers to cold reheat and the main steam control valve will close. At higher loads, there is sufficient leakage through the glands for the HP glands to be self sealing. At his point, the cold reheat valve also closes and the pressure is regulated by two spillover valves. These dump sufficient leakage steam to drop the pressure to the required value. The steam is directed first to LP Heater 1A extraction steam and then, if the pressure continues to rise, to the condenser. Refer to Sheet 217. The cold reheat and main steam valves operate as pair from a single controller in split control configuration; cold reheat valve leading. The spillover control valves also work as a split control pair from a single controller [reverse acting]; the LP2 valve leads. No manual control or auto/manual selection facilities are provided for the four valves. The system is always in auto mode. 12.2

SEAL STEAM TEMPERATURE

The temperature of the seal steam to the LP turbine seals is regulated by a desuperheater. Refer to Sheets 213,613. The temperature controller and its associated valve controls the flow of condensate to the desuperheater. The seal steam temperature to HP-IP seals is not regulated. A low steam temperature switch applies a close override to the spraywater valve. 12.3

CLOSED CYCLE CWTEMPERATURE

The closed cycle cooling water temperature is controlled by regulating the proportion of the total flow ing through the C.C.C.W. heat exchanger. Refer to Sheets 245,645. The heat exchanger IS fitted with outlet and by- control valves. The temperature controller output regulates the two valves in opposite directions. Increasing temperature opens the outlet valve and closes the by valve. This action forces a greater proportion of the C.C.C.W. through the heat exchanger and lowers the temperature. A single subwindow iA provided for operator interface to both valves; [the MV shows outlet valve position].

44 12.4

AUX STEAM PRESSURE

The auxiliary steam is supplied from the tertiary superheater inlet header. Alternative supply is available from cross-ties to Units 2 and 4. Refer to Sheets 331,731. The pressure is controlled by two parallel pressure regulating valves. The pressure controller output operates the valves sequentially in split-control configuration. A single subwindow is provided for operator interface to both valves.

45

13

SIMPLE INDEPENDENT LOOPS

The Simple Independent Loops are defined as stand-alone single modulating loops having standard sensor configuration, single output and regulating device, standard operator subwindow with set point, auto/manual selection and manual control facility. These loops do not have special features such as function generators and feedforwards. Interpretation of functional operation is clear from relevant functional diagrams and PID; additional explanation is not required. Reverse acting loops are designated by a sign reversal block on the error signal. Outputs which operate fail-open valves are inverted [(1-X) block]. Simple Independent Loops

SHEET No.

151 155 159 163 167 179 201 205 209 233 237 241 328 346-364 376 388 394

DESCRIPTION Atomizing Steam Pressure Ignitor Gas Pressure Generator Hydrogen Temperature Turbine Lube Oil Temperature Ignitor Oil Supply Pressure Warm Up Oil Pressure Continuous Blowdown Tank Level Main Steam Warm Up Temperature Mise drains Receiver Level Ignitor Air Windbox/Furnace DP Closed Circuit CW Differential Press EHC Cooling Water Collecting Tank Level BFP Steam Supply Pressure BFP Seal Water Temperature Sootblower Steam Pressure M-BFP Seal Water Drain Tank Level CW Pump Motor Bearing Water Press