Pelton turbine is named after L. A. Pelton an American engineer; it is a high head, tangential flow and low specific speed turbine. This turbine is most suitable for high head. In the case of low head, flow is to be increased and for increased flow a bigger jet diameter is required. The bigger jet diameter requires bigger runner diameter which results into bulky turbine and low peripheral velocity. Thus, the efficiency of Pelton turbine decreases with low head of the water. A schematic diagram of Pelton turbine is shown in Figure 9.6.
Figure 9.6 General Layout of Pelton Turbine
9.8.1 Main Components of Pelton Turbine
Penstock: Penstock is a steel or concrete conduit through which water flows from reservoir to nozzle. Its size is large due to conduction of flow of water from high head (200 m) to the nozzle.
Nozzel and Guiding Mechanism: The pressure head at the inlet of the turbine is converted into kinetic energy using nozzle. The velocity of the water jet at the tip of the nozzel depends on the net head (H);
The high velocity jet of water strikes on the bucket and deviates at 165° and results into impulse on the buckets. Buckets mounted on shaft rotate at high velocity and produce shaft power.
Guide mechanism is used to control the flow of water from the nozzle and to control the speed of the turbine as shown in Figure 9.6. The main function of the spear is to change the flow area of the nozzle moving in forward and backward directions. The flow area is decreased by the movement of spear in forward direction and increased by the movement of spear in backward direction. But, water in penstock causes hemmering due to sudden increase and decrease of pressure resulting from sudden change in the flow area. Therefore, to prevent the high pressure generation in the penstock, a deflector is used in front of nozzle, which deflects the flow of water to decrease the shaft speed.
Bucket: The splitter, a sharp edge at the centre of the bucket, divides it into two hemispherical parts as shown in Figure 9.7. The splitter helps the jet to be divided into two parts without producing shock on the bucket and moving the same sideways in opposite directions. The rear of the bucket is so designed that water should not interfere during passage of the bucket preceeding in order of rotation. The jet should be deflected backward at an angle of 160° to 165°; the materials used for the bucket may be cast iron, bronze or stainless steel.
Figure 9.7 (a) Buckets of Pelton Wheel and (b) Cross-sectional View of Bucket
Casing: Casing has no hydraulic importance; it is only used to prevent the splashing of water and to discharge the water to tail race; the other purpose is to provide safeguard to the wheel.
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