Pelton Wheel
 

 

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                                                    Pelton Wheel Lab sheet

 

 

The accessory 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 pressure gauge mounted on the support assembly allows the inlet pressure of the turbine to be monitored.

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 non-contactable type tachometer  for which a clamping arrangement is provided.

 

NOMENCLATURE

Column

Units

Nom.

Type

Heading

 

 

 

Radius of Brake

m

r

Given

Drum

 

 

 

Tachometer

Hz

n

Measured

Reading

 

 

 

 

Spring Balance

  N

w1

Measured


Spring Balance

 

N

w2

Measured

Volume

m3 

V

Measured

Collected 

 


Time to Collect

 


t


Measured

Head

H20

Hi

Measured



Flow Rate




Torque

 


m3/s




Nm

 

Qv

 

T



Calculated


Calculated


Brake Power


W


Pm


Calculated



Turbine

 




Et

Calculated

Efficiency

 

 

 

 


Radius of drum on which band brake
operates


The tachometer reading is measured in rpm. Convert to Hertz for calculations (divide by 60).


Reading from spring balance 1.

 
Reading from spring balance 2.  


Volume of water collected in a known time period. Converted to cubic metres for calculations.


Time taken to collect a known
volume of water.

Head loss across turbine. Head loss is measured in metres of water
 

                           

      
    T = (w2-w1)*r    


     
Pm = 2π nT
 

                 2  π  n T 

   EtρgH1Qv

 

EXPERIMENTAL PROCEDURE

Objective
To determine the operating characteristics of a Pelton turbine at various speeds.

Method
By measurement of power, torque and volume flow rates 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 non-contactable type tachometer to measure rotor speed.

Technical Data

The following dimensions from the equipment are used in the appropriate calculations.  If required these values may be checked as part of the experimental procedure and replaced with your own measurements.

Radius of brake drum                           r = 0.030m 

 

Theory

The Pelton turbine is the most visually obvious example of an impulse machine. A spear valve directs a jet of water at a series of buckets which are mounted on the periphery of a rotor.  As the water exiting the spear valve is at atmospheric pressure, the force exerted on the rotor is entirely due to changes in the direction of the flow of water.  The Pelton turbine is therefore associated with considerable changes of kinetic energy but little change in pressure energy.  The spear valve allows the jet diameter to be varied which allows the water flow rate to be varied with a constant jet velocity.  Large turbines may include more than one spear valve around the periphery of the rotor.

The operating characteristics of a turbine are often conveniently shown by plotting torque T, brake power Pb, and overall turbine efficiency Et against turbine rotational speed n for a series of volume flow rates Qv, as shown in chart provided. It is important to note that the efficiency reaches a maximum and then falls, whilst the torque falls constantly and linearly. The optimum conditions for operation occur when the required 'duty point' of head and flow coincides with a point of maximum efficiency.

The basic terms used to define, and therefore measure, turbine performance in relation to rotational speed include:

 

i)                       volume flow rate,

ii)                     head,

iii)         torque, power output and efficiencies.

 

Each of these is considered in turn. 

The flow rate of fluid through the turbine is the volume passing through the system per unit time.

Qv = V/t                  [m3/s]                                  ..... (1)

 

The term 'head' refers to the elevation of a free surface of water above or below a reference datum. In the case of a turbine we are interested in the head of the water entering the rotor, which of course has a direct effect on the characteristics of the unit. In this apparatus the head of water is generated by the pump on the hydraulics bench rather than an elevated reservoir.

Terms specifically applied to the analysis of turbines and generating systems are  briefly defined below.

 

1.                     Manometric suction head Hm 1 is the gauge reading (metres) measured at the inlet nozzle of the turbine, referenced to the rotor centreline datum.

2.                     Manometric discharge head Hm2 is the gauge reading (metres) measured at the discharge nozzle of the turbine, referenced to the rotor centreline.

3.                     Input head to the turbine (Hi) is the head used by the turbine in performing work. For the turbine Hi is given by:

Hi = [Hm1 - Hm2]                                                           ..(2)

For a control volume enclosing the turbine outlet and inlet, as Hm1 and Hm2 the measured pressures are equal to 

z1 + (P1/ρg)  and  z2 + (P2/ρg)      respectively

 

The gauge has been set up so that inlet pressure is measured in relation to atmospheric pressure (P2).  As the outlet of the turbine is at atmospheric pressure, it can be assumed that the reading given by the gauge is the head loss due to pressure difference across the turbine.

Hence

            Hi = P1 /ρ g       [m]

The hydraulic power supplied by the water, Ph, can be calculated as

Ph = ρ g Hi Qv           [Nm/s = Watts]                                            ..(3)

The mechanical power, Pm, produced by the turbine in creating a torque T on the brake at rotor speed n is given by

Pm = 2 π n T                 [Nm/s = Watts]                                            ..... (4)
The torque itself is given by the equation:

T = Fb r                         [Nm]                                                          (5)
where Fb is the brake force reading on the spring balances and r is the pulley radius.

However, the fluid friction 'losses' in the turbine itself, require a hydraulic efficiency Eh to be defined as:-

Further, the mechanical losses in the bearings etc. require a mechanical efficiency Em to be defined as

The Armfield turbine units do not include the direct measurement of mechanical power Pm, but instead measure brake force applied to the rotor via the band brake.  A further efficiency is therefore required, expressing the friction losses in the brake assembly Eb:-

  

The overall turbine efficiency Et is thus:- 

                                                         

 

Which is equal to :               

                                                                                                                                                                                         Thus:                Et = Eh Em Eb

 

Procedure - Equipment Set Up

Position the apparatus in the working channel of the bench and connect to the bench supply using the quick release connector.  Clamp the optical tachometer into the clip provided.  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.

 

Procedure - Taking a Set of Results

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 Pm and Efficiency Et.

 

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Last Edited :  22 January 2011 14:26:18