Pelton wheel experiment

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an experimental approach on the study of the pelton wheel and how it operates

Transcript of Pelton wheel experiment

THE PELTON WHEEL TURBINE UNDER STUDY

TOPIC

PAGE

ABSTRACT3

INTRODUCTION3

APPARATUS6

PROCEDURE7

THEORETICAL KNOWLEDGE7

PRESAUTIONS12

RESULTS13

CALCULATIONS14

DISCUSION 15

CONCLUSION 16

REFERENCES 17

PERFORMANCE TEST OF A PELTON WHEEL TURBINEAimTo study the variance of the power output and overall efficiency against discharge with the head retained as a constant at normal speed.AbstractThe 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 to determine 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. IntroductionA 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 the characteristics 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 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 is nearly 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.

Fig. The configuration of the nozzle and buckets in a Pelton wheel turbine

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.

Fig. General arrangement of the pelton wheel turbine

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

FIG. Arrangement of Apparatus used in the Pelton Wheel Turbine Test

V- 1,2,3

List of apparatus as labeled in the diagram above

:Sluice valve

X

:Balance

N:NozzleG:Hook Gauge

NV:Needle valvePG-2:Pressure gauge

PB:Plony brakeT:Main tank

W:WaterwayTW:Triangular weir

A thermometer was also used for the determination of the water temperature. The tachometer was used optically in the determination of the speed of the turbine so as to retain the speed at 900rpm.ProcedureThe 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 experimentThe 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 arrangementii. the runner with the buckets

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

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 terms1. 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 is head 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 hf is the total head loss, then net head on the turbine is given by Pelton turbine is best suited to operating under very high heads compared with other types of turbines.

3. Overall Efficiency: The overall efficiency of a pelton turbine is the ratio of the useful power output to the power input. Mathematically,Power supplied at the inlet of the turbine or the water horse power is given by the expression .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,

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 is called the mechanical efficiency (m) of the turbine.Mathematically,

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