Hydraulic Machines Lab Manual 4s111p

Experiment: 1 To determine the operating characteristics of a Pelton turbine at various speeds. Method By measurement of torque, brake power and efficiencies against rotor speed.

Equipment In order to complete the demonstration you need a number of pieces of equipment:    

The F 1-10 Hydraulics Bench which allows you to measure flow by timed volume collection. The F1-25 Pelton Turbine Apparatus. A stopwatch to allow you to determine the flow rate of water. A tachometer to measure rotor speed

Theory: Fluid machinery _An Introduction: Hydraulic energy is that which may be possessed by a fluid. It may be in the form of kinetic, pressure, potential, strain or thermal energy. Fluid machinery is used to convert hydraulic energy into mechanical energy (turbines) or mechanical energy into hydraulic energy (pumps, compressors). Turbines can be impulse or reaction based on type of fluid head they utilize. Pelton turbine (or Pelton wheel), an impulse turbine, is one of the well-known type of water turbines. The water turbines converts the energy possessed by the water to mechanical energy.

Pelton Wheel Turbine: The Pelton wheel is an impulse turbine in which vanes, sometimes called buckets, of elliptical shape are attached to the periphery of a rotating wheel. One or two nozzles project a jet of water tangentially to the vane pitch circle. The vanes are of double-outlet section so that the jet is split and leaves symmetrically on both sides of the vane. This type of turbine is used for high head and low flow rates. It is named after the American engineer Lester Pelton.

Components of Pelton Wheel Turbine: Impeller with bucket: Impeller (runner) of Pelton turbine consists of a circular disc on the periphery of which a number of buckets are fixed. Nozzle: The water coming from the reservoir through penstock is accelerated to a certain velocity by means of a nozzle. Spear Valve: The spear (spear valve) is a conical needle which is operated either by a hand wheel or automatically in an axial direction depending upon the size of the unit. The amount of water striking the buckets of the runner is controlled the spear in the nozzle. Casing: Casing is used to prevent the splashing of the water and to discharge water to tail race. It is made up of cast iron or steel plate.

Working:     

The Pelton wheel turbine equipment is designed to be positioned on the side channel of the hydraulics bench and the inlet pipe should be connected to the bench supply The flow is controlled by a fully retractable spear valve. Water discharges into the volumetric tank through an orifice in the base of the Pelton turbine base plate The Pelton wheel buckets are clearly visible due to the transparent turbine cover A simple band brake connected to two spring balances allows the load applied to the turbine to be varied by adjustment of the tensioning device The speed of the turbine shaft can be determined by a tachometer

In the Pelton wheel turbine, the total head available is first converted into the kinetic energy. This is usually accomplished in one or more nozzles. The jets issuing from the nozzles strike vanes attached to the periphery of a rotating wheel. Because of the rate of change of angular momentum and the motion of the vanes, work is done on the runner (impeller) by the fluid and, thus, energy is transferred. The process can by shown as:

water jet

brake wheel

rotor

Pelton wheel turbine is highly efficient turbine. Ideally, the fluid should impart all of its kinetic energy to Pelton buckets resulting in zero exit velocity. Now, the buckets will not be empty rendering the possibility of 100 percent efficiency. The fluid must have some exit velocity so that the fluid should leave the buckets.

Also, the fluid energy which is reduced on ing through the runner is entirely kinetic, it follows that the absolute velocity at outlet is smaller than the absolute velocity at inlet (jet velocity). Furthermore, the fluid pressure is atmospheric throughout and the relative velocity is constant except for a slight reduction due to friction.

Torque measurement: The water jet striking the runner generates a torque of and rotates the runner with certain (rev/m), then power obtained from the runner can be expressed as: Brake power=

𝟐𝛑𝐧𝐓 𝟔𝟎

The torque produced can be measured by applying braking force with the help of band belt as there is some relative motion between shaft and belt. The braking force is then measured through spring balance attached at belt ends. T= (w₁-w₂) r Where, r= radius of shaft w₁-w₂ = difference between braking force shown on spring balances

Efficiency of Pelton turbine: During conversion of energy (hydraulic energy to mechanic energy or vice versa) there occur some losses like:  

The bearings which enable the shaft to rotate can’t have frictionless motion. All kinetic energy of fluid can’t be converted into mechanical motion.

For a turbine, Fluid Input Power = (Mechanical loss) + (Hydraulic losses) + (Useful shaft power output) Where, Hydraulic Losses = (Impeller loss) + (Casing loss) + (Leakage loss) The useful power supplied by fluid can be expressed as:

𝑷𝒘 = 𝝆𝒈𝑯𝑸𝒗 Where, H= input head of fluid 𝑄𝑣 = Volume flow rate of fluid The efficiencies of Pelton turbine can be expressed as: 𝐻𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =

𝑃𝑜𝑤𝑒𝑟 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑏𝑦 𝑟𝑜𝑡𝑜𝑟 × 100 𝑈𝑠𝑒𝑓𝑢𝑙 𝑝𝑜𝑤𝑒𝑟 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑏𝑦 𝑓𝑙𝑢𝑖𝑑

𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =

𝑃𝑜𝑤𝑒𝑟 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑏𝑦 𝑟𝑜𝑡𝑜𝑟 × 100 𝑃𝑜𝑤𝑒𝑟 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑏𝑦 𝑟𝑜𝑡𝑜𝑟

𝐹𝑟𝑖𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑙𝑜𝑠𝑠𝑒𝑠 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝑇𝑜𝑡𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =

𝑃𝑜𝑤𝑒𝑟 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑏𝑦 𝑏𝑟𝑎𝑘𝑒 × 100 𝑃𝑜𝑤𝑒𝑟 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑏𝑦 𝑟𝑜𝑡𝑜𝑟

𝑃𝑜𝑤𝑒𝑟 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑏𝑦 𝑏𝑟𝑎𝑘𝑒 × 100 𝑈𝑠𝑒𝑓𝑢𝑙 𝑝𝑜𝑤𝑒𝑟 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑏𝑦 𝑓𝑙𝑢𝑖𝑑 ƞ𝒕 =

𝟐𝛑𝐧𝐓 𝟔𝟎𝝆𝒈𝑯𝑸𝒗

The operating characteristics of a turbine are shown by plotting torque T, brake power 𝑃𝑏 , and overall turbine efficiency ƞt against turbine rotational speed n for a series of volume flow rates Qv. “The maximum efficiency of Pelton turbine will occur when rotor speed becomes half of the water jet speed”

Procedure:       

Position the apparatus in the working channel of the bench and connect to the bench supply. Lift the band brake assembly until it is clear of the brake drum. Switch on the bench pump and open the bench control valve fully. Adjust the spear valve until the maximum rev/min are indicated on the tachometer. Lift the band brake assembly over the brake drum and adjust the band brake for a range of readings on the spring balances. Record the spring balance and tachometer readings for each band brake setting. Measure the flow rate using a timed volume collection, and record the reading from the inlet pressure gauge. Adjust the flow rate using the spear valve, and repeat the experiment. Continue to do this until you have sets of readings for a variety of different flow rates. For each flow rate, plot a graph of rotational speed n against Torque T, Brake (mechanical) Power Pb and Efficiency ƞt.

Observations and Calculations:

Graphs:

Variation of Brake power with rotor speed 12

Brake power (W)

10 8 6 4 2 0 1000

1050

1100

1150

1200

1250

1300

1350

1400

Rotor speed (rpm)

Variation of Torque with Rotor speed 0.07 0.06

Torque (Nm)

0.05 0.04 0.03 0.02 0.01 0 1000

1050

1100

1150

1200

1250

Rotor speed (rpm)

1300

1350

1400

OVERALL EFFICIENCY (%)

Variation of Overall efficiency with Rotor speed 50 45 40 35 30 25 20 15 10 5 0 1000

1050

1100

1150

1200

1250

ROTOR SPEED (RPM)

Specimen Calculations: Diameter of shaft= 0.06m Diameter of tachometer device= 0.033m Using relation,

n₁D₁=n₂D₂ Putting values, (0.06) n₁ = 2300(0.033) n₁= 1265 rpm 𝑄𝑣 =

𝑉 𝑡

=

0.005 25

𝒎𝟑 𝒔

𝑸𝒗 = 𝟎. 𝟎𝟎𝟎𝟐

T= (w₁-w₂) r = (2-1)*0.03 T= 0.03 Nm 𝑃𝑏 =

2πnT 60

𝑷𝒃 = 3.9721 W 𝑃𝑤 = 𝜌𝑔𝐻𝑄𝑣 = 1000*9.81*8*0.0002 𝑷𝒘 = 15.696 W 2πnT

ƞ𝑡 = 60𝜌𝑔𝐻𝑄

𝑣

1300

1350

1400

ƞ𝒕 = 25.3%

Comments:    

The torque produced varies inversely with the rotor speed as evident from the graph. The brake power increases as rotor speed increases because the fluid is imparting more kinetic energy to the rotor of turbine. The overall efficiency increases with increase in rotor speed because maximum energy is being converted into mechanical work. The efficiency of Pelton wheel turbine depends upon proper measurement of torque and rotational speed of shaft by tachometer. If either of these measurements have any error, then efficiency will decrease.