The_Pelton_Wheel_Experiment.docx

7/24/2019 The_Pelton_Wheel_Experiment.docx

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TOPIC PA

ABSTRACT 3

INTRODUCTION 3

APPARATUS 6

PROCEDURE 7

THEORETICAL KNOWLEDGE 7

PRESAUTIONS 12

RESULTS 13

CALCULATIONS 14

DISCUSION

15

CONCLUSION

16

REFERENCES

17

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PERFORMANCE TEST OF A PELTON WHEEL TURBINE

Aim

To study the variance of the power output and overall efficiency against discharge with

the head retained as a constant at normal speed.

Abstract

The findings of an experiment carried out to study the properties and performance of a

pelton wheel are herein discussed with much emphasis placed on the output

measured. The resulting output was discussed against the theoretical output todetermine presence and causes of a deviation. The results were presented in graphical

method and the properties of the graph used to discuss the properties of the turbine

under study.

Flow was varied and head measured against each variance to indicate the power in the

system. Other parameters necessary for the study were also measured and recorded

for the study. The pelton wheel under study was of a smaller scale though it acted as a

representative of a similar system in large scale.

The results were also used for the checking of scaling laws used for rturbines.

Introduction

A pelton wheel turbine is a tangential flow impulse hydraulic machine that is actively

used for the production of power from kinetic energy of flowing water. It is the only

form of impulse turbine in common industrial use. It is a robust and simple machine

that is ideal for the production of power from low volume water flows at a high head

with reasonable efficiency.

The pelton wheel used in this experiment, although a model, reproduces all thecharacteristics of full size machines and allows an experimental program to determine

the performance of a turbine and also to verify the theory of design.

Impulse turbines operate through a mechanism that first converts head through a

nozzle into high velocity, which strikes the buckets at single position as they pass

by.jet flows past the buckets is quite essential at constant pressure thus runner

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passages are never fully filled. These turbines are suited for relatively low power and

high head derivations. The pelton wheel turbine is comprised of three basic

components that include the stationary inlet nozzle, the runner and the casing. The

multiple buckets form the runner. They are mounted on a rotating wheel. They are

shaped in a manner that divides the flow in half and turn in a velocity vector that isnearly 180degrees.

The nozzle is positioned in a similar plane as the wheel and is arranged so that the jet

of water impinges tangentially on to the buckets. The nozzle is controlled by movement

of the spear regulator along the axis of the nozzle which alters the annular space

between the spear and the housing. A static pressure tapping is provided to enable the

measurement of the water pressure in the inlet.

The nozzle is controlled by movement of the spear regulator along the axis of the

nozzle which alters the annular space between the spear and the housing, the spear

being shaped so as to induce the fluid to coalesce into a circular jet of varying

diameter according to the position of the spear.

A friction dynamometer consists of a 60mm diameter brake wheel fitted with a fabric

brake band which is tensioned by a weight hanger and masses with the fixed end

being secures via a spring balance to the support frame. A tachometer may be used to

measure the speed of the turbine.

4

Fig. Th !"#$g%&'(i"# ") (h #"**+ '#, -%!(/ i# ' P+("#

0h+ (%&-i#


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Fig. G#&'+ '&&'#g#( ") (h +("# 0h+ (%&-i#


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Apparatus used

For the purpose of the study, the following system of apparatus were used

V- 1,2,3

List of apparatus as labeled

in the diagram above

:Sluice valve X :BalanceN :Nozzle G :Hook Gauge

NV :Needle valvePG-

2:Pressure gauge

PB :Plony brake T :Main tank

W :Waterway TW :Triangular weir

A thermometer was also used for the determination of the water temperature.

6

FIG. A&&'#g#( ") A'&'(%/ %/, i# (h P+("# Wh+ T%&-i# T/(


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The tachometer was used optically in the determination of the speed of the turbine so

as to retain the speed at 900rpm.

Procedure

The sluice valve,V-2, was opened to supply water to the turbine, and the needle

valve of the nozzle,N, was opened manually by the handle,MV, to allow the water

flow. As the turbine rotated cooling water was supplied into the plony brake.

Importance was taken such that the temperature did not exceed 60 C for the most

efficient operation.

Initially the needle valve was fully opened, and the sluice was adjusted to bring the

pressure head on the turbine to 27m.

The pressure head was maintained at 27m throughout the experiment period,

and was monitored by the pressure gauge-PG -2. To maintain the turbine speed at

900rpm, the adjusting screw of the plony brake,Z, was tightened and when the arm of

the plony brake got. At that speed, the spring balance,X, reading (Kg) was recorded as

the load on the plony brake.

The experiment was performed several times (15 times) by shutting the needle valve in

bits. It was noted that for each revolution the needle advanced 1.25mm.

As a precautionary measure the needle valve,NV, was not shut completely

before shutting off the sluice valve,V-2, because the pump water pressure might

break some of the vinyl tubes between the sluice valve and the needle valve.

Theoretical Knowledge pertaining to the experiment

The efficiency of the turbine is defined as the ratio between the power developed by the

turbine to the available water power. Figure below shows the layout of a hydro-electric

power plant in which the turbine is pelton wheel. Water from the reservoir flows

through the penstock at the outlet of which is fitted a nozzle. The nozzle increases the

kinetic energy of the water jets. These water jets strike the bucket of the runner

making it rotate.

The two main parts of the pelton turbine are:

i. the nozzle and the flow regulating arrangement

ii. the runner with the buckets

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Fig. Indication of actual state of operation of a pelton wheel turbine

The amount of water striking the buckets is controlled by providing a spear in the

nozzle as shown in Figure below. The spear is a conical needle which is operated either

by a band wheel or automatically in an axial direction depending on the size of the

unit. When the spear is pushed forward into the nozzle, the amount of water striking

the runner is reduced, where as if the spear is pushed back the amount of water is

increased.

Fig. Velocity Analysis


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Figure below shows the pelton turbine. It consists of a circular disc (the runner) on the

periphery of which a number of buckets evenly spaced are fixed. The shape of the

buckets is a double hemispherical cup or bowl. Each bucket is divided into two

symmetrical parts by a dividing wall which is known as a splitter. The jet of water

strikes the splitter which then divides

the jet into two equal parts and the jet comes out at the outer edge of the bucket. The

buckets are shaped in such a way the jet gets deflected through 160 or 170.

Definition of terms

1.Total Head:The difference between the head race level and the tail race level

when no water is flowing is known as Total Head (Hg).

2.Net Head:It is also called the effective head and is the available head at the

inlet of the turbine. When water is flowing from head race to turbine, there ishead loss due to friction between the water and the penstocks. There could also

be minor head losses such as loss due to bends, pipe fittings and entrance loss

of penstock etc. If hfis the total head loss, then net head on the turbine is given

byH=Hgh f Pelton turbine is best suited to operating under very high heads

compared with other types of turbines.


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3.Overall Efficiency:The overall efficiency of a pelton turbine is the ratio of the

useful power output to the power input. Mathematically,

Power availableOverall efficiency ( ov)= the shaft

Power suppied at theinlet

Power supplied at the inlet of the turbine or the water horse power is given by the

expression gHQ

750 .

Where = density of water (kg/m3),

g = acceleration due to gravity (9.81m2/sec),

Q = discharge,

H = net head (m).

The power losses that occur within a turbine are attributed to volumetric, mechanical

and hydraulic losses. Volumetric losses ## some of the volume of the water is

discharged to the # without striking the runner buckets. Thus the ratio of the volume

of the water # striking the runner to the volume of the water supplied to the turbine is

defined as the volumetric efficiency.

Mathematically,

volumeof water supplied

Volumetricefficiency (v )=volume of water striking the bucket

theturbine

The shaft horse power (SHP) output is less than power input due to power consumed

in overcoming mechanical friction at bearings and stuffing boxes. The ratio of the

power available at the shaft of the turbine to the power developed by the runner iscalled the mechanical efficiency (m) of the turbine.

Mathematically,

m=Power at the shaft of theturbine

Power developed by the runner

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The water head actually utilized by a turbine is less than that available because of

frictional losses as water flows across the buckets. The water power at the inlet of the

turbine due to hydraulic losses as the vanes are not smooth and water jet is not

completely turned back. The ratio of the power developed by the runner to the

available power at the inlet is known as the hydraulic efficiency (h) of the peltonturbine.

Mathematically,

h=Power developed by the runner

Power available at the inlet

Normal overallefficiency (ov)=v m h

Performance characteristic curve of pelton turbines

These are curves with the help of which the exact performance behavior of the

turbines under different working conditions can be ascertained. The curves are plotted

from the results of the tests performed on turbines under different working conditions.

The quantities that can be varied during a test on a turbine are: speed, head,

discharge, power, overall efficiency and gate opening.

If the speed and water head of a pelton turbine are maintained at constant values,

then the curves obtained by plotting the discharge (Q) against both the power outputs

and the overall efficiencies are called the operating characteristic curves of the pelton

turbine.

Preparation of the Experiment

The asbestos of the plony brake (PB) (details as shown in Figure 4) should be oiled

before the experiment is started. This ensures easier reading of the load on the springbalance.

The sluice valves, V-1, 2, 3 are put in closed positions. Before the pump is started

ensure that it is filled up with water i.e. primary and once started it should not be

allowed to run for long before opening any of the valves V-1, 2, 3. This is to prevent it

from getting overheated.

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Figure: Details of the plony brake

A triangular weir is used to determine the discharge through the circuit. The water

head through the weir is measured with a hook gauge; first the zero water head is

measured. This is done as follows:

Keeping the water flowing over the weir, observe reflection of the end of the weir V on

the water from the upper stream side. Open the cork valve (V-8) positioned under the

waterway, to lower the surface water level and then read the water head with the hook

gauge when the end of the weir V coincides with end of the shade V reflected on the

surface of the water. This reading is recorded as the zero water head. Then close cork

valve (V-8) to prepare for the other readings.

The other water heads are read when the point of the hook gauge coincides with the

reflection itself in the water through a glass window. In every case allow the water to

settle before recording the reading i.e. waits for about 5 minutes after the flow

adjustment before you take the next reading.

Precautions taken

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1.It was ensured that the centrifugal pump that supplies water in this system

is primed first before the mortar is started.

2.The gate openings were set carefully and throughout each gate opening, the

spear wheel and the delivery valve were not changed.

Results

Fundamental Data

Properties of turbine

Revolution speed (N) 900 rpm

Pressure head on turbine 27 m

Length of the handle of the plony brake 0.130 m

Properties of V-notch

Half angle of V-notch () 45

Coefficient of discharge (CdV) 0.576

Coefficient (KV) 1.360

Crest level (hook gauge) 0.21805 m

Operation Data

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Stag

e

V-notch Properties of water

Theoretic

al power

input

(Pth)

HP

Spring

balance

reading

(w)

kg

Actual

power

(P

a )

HP

Ove

Effi

n

(

%

Reading

m

Head

(HV)

m

Discharge

(Q)

103

m3/

Temperatu

re

C

Density

()

kg/m3

1 0.17020 0.04785 0.681 18.0 998.595 2.401 14 2.287 95.

2 0.16280 0.05525 0.976 19.5 998.305 3.439 20 3.267 95.

3 0.15645 0.06160 1.281 19.5 998.305 4.514 27 4.411 97.

4 0.15255 0.06550 1.493 20.0 998.203 5.262 32 5.228 99.

5 0.14865 0.06940 1.726 20.0 998.203 6.081 36 5.881 96.

6 0.14700 0.07105 1.830 20.0 998.203 6.449 38 6.208 96.

7 0.14525 0.07280 1.945 20.0 998.203 6.853 40 6.535 95.

8 0.14400 0.07405 2.029 20.0 998.203 7.151 40 6.535 91.

9 0.14265 0.07540 2.123 20.0 998.203 7.481 41 6.698 89.

10 0.14180 0.07625 2.183 20.0 998.203 7.694 41 6.698 87.

Calculations

a)The theoretical power input (Pth) of the turbine given by the expression:

Pth= gHQ

7560HP

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Where = density of water (depends on the water temperature and atmospheric

pressure),

Q = discharge,

H = net water head on the turbine (given H=27m).

Example:

Pth=998.5959.81270.681

7560HP

Pth= 2.401W

b)The actual power output (Pa) of the turbine is obtained from the expression:

Pa=2! "Nw7560

HP

Where x = length of the handle of the plony brake (given as 0.130m),

N = revolution per minute of the turbine (supposed to be 900rpm),

w = load exerted by the plony brake (kg) read on the spring balance.

Example

Pa=2!0.1390014

7560HP

Pa= 2.287 HP

c)The overall efficiency of the pelton turbine (ov) is given by the formula:

ov= Pa

Pth100

Calculate the overall efficiencies of the pelton turbine at each discharge

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Example:

ov=2.287

2.401100

=95.248

Discussion

From the above calculations the values of actual power output are slightly lower

than the values of theoretical power output of the turbine and thus from this a

relationship between the discharge, actual output and efficiency can be shown using a

graph as indicated below.

1

2

3

4

5

6

7

4

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1

f(x) = - 15261743.52x^2 + 39796.77x + 73.25

f(x) = - 1409219912.52x^3 + 5140098.29x^2 - 2357.87x + 1.96

Powe O!" (#P) Po$%&o'$ (Powe O!" (#P))

e*e&% (,) Po$%&o'$ (e*e&% (,))

/e ('3)

Powe o!" (#P) E*e&% (,)

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From this relationship, it is possible to prove that the higher the power output of a

turbine, the higher the efficiency. These are functions of the discharge.

It is also correct to indicaate that efficiency of the system increases with increase in

the specific speed of the pelton wheel. This has been derived from the relationship of

the values collected, tabulated and graphed as herein.

Conclusion

This experiment was carried out with an acceptable level of accuracy. It was

generally a success as the results obtained were useful for the analysis of the

properties of the machine.

From the experimental results, it became possible for the real picture of the

operational basis of the machine to be displayed in such a way that the characteristics

of the turbine were visible in the graphical analysis used.

The experiment was not fully accurate due to several errors that resulted from

several misdoings. The greatest being that it became really difficult to acquire readings

from the spring balance since the setup was vibrating as result of the operation of the

machine. As such, this explains the slight deviation of the results obtained in the

experiment that were later reflected in the graphs drawn to represent the work.

Other errors may have resulted from unseen leakages in the system and

observational and computational errors. The experiment was, however, carried out

with a great level of keenness to reduce the occurrence of such errors.

References

1. Rajput, R. K. (2005). Elements of mechanical engineering. New elhi, !n"ia# $a%mi

&u'lications

2. gar, ., Rasi, *. (200+). n the use of a la'orator-scale &elton wheel water tur'ine

in renewa'le energ- e"ucation. Renewa'le Energ-, //(), 1511522.

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/. hang, . (200). low interactions in &elton tur'ines an" the h-"raulic efficienc- of the

tur'ine s-stem. &rocee"ings of the !nstitution of *echanical Engineers, &art # 3ournal of

&ower an" Energ-, 221(/), /4//55.

4. rn"t, R. E. (11). 6-"raulic tur'ines. Energ-, 2, 2

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