Pelton Wheel: Setting, Diagrams and Problem and Solutions!
Notes on Pelton Wheel:
The Pelton wheel (named after an American engineer L.A. Pelton is a parallel flow impulse turbine. The pelton wheel is today the most popular impulse turbine provided in situations where the head of water is 250 m or more. Since the head of water is large, the quantity of water required to work the turbine is less. This turbine has an overall efficiency of about 80%. The turbine consists of a number of buckets (also called cups) rigidly connected to the periphery of a wheel.
Water is conveyed from a reservoir to the turbine through penstocks. The penstock is connected to a pipe or branch pipes fitted with nozzles. The powerful jet issuing from the nozzle impinges tangentially on the buckets. As the jet moves round the bucket a dynamic force is imparted to the wheel and turns it. Each bucket is a double hemispherical cup with a sharp dividing central edge over which the jet impinges.
After striking this sharp edge, the jet divides into two parts each moving round a hemispherical cup. This arrangement eliminates any axial thrust on the wheel. The turbine is installed above the tail race level to avoid splashing of the buckets with the tail water. In most of the turbines, the deflection angle is about 165°. The jet of water, after imparting the major portion of its energy to the turbine is discharged into the tailrace.
ADVERTISEMENTS:
The discharge to the turbine is regulated by a needle valve provided in the nozzle or by a throttle valve provided in the supply pipe. The jet issues out of one or more nozzles. In the simplest arrangement a single jet supplies the discharge to the wheel. Two or more jets may also be provided to the wheel. A single jet is provided where its specifies speed N is less than 30. For the case where the specific speed is from 30 to 50 multijet pelton wheel is suitable.
Breaking Jet:
ADVERTISEMENTS:
This is a jet, which when needed can be directed towards the back of the buckets so as to stop the wheel. Suppose it is desired to stop the wheel for any special reason, it cannot be done merely by stopping the supply of water by thrusting the spear in the forward direction, in the needle valve, as the wheel would still continue to turn rapidly due to its inertia. In such a situation the breaking jet can act on the back of the buckets retard the wheel and bring it to rest quickly.
The bucket material must be strong enough to withstand the large force to which it is subjected to. For low heads of water cast iron is found suitable. Stainless steel is recommended for higher head.
The pelton wheel is provided with a casing not interfering with free access of atmosphere to the wheel. The object of the casing is to serve as a protective cover and to prevent splashing of water and to easily direct the flow to the tailrace.
Setting of the Turbine (Pelton Wheel):
ADVERTISEMENTS:
These turbines are generally set with the turbine shaft horizontal. Fig. 22.5 (a) shows a single overhung pelton wheel unit with the generator mounted between two bearings, and the wheel mounted outside. Turbines meant to develop high power have two turbine runners shown in Fig. 22.5 (b) and 22.5 (c). The two runners may be arranged side by side as shown is Fig. 22.5 (b) or may be arranged on the projecting ends of the shaft as shown in Fig. 22.5 (c). In the latter arrangement the turbine is called a double overhung pelton.
Inlet and Outlet Vector Diagrams of Pelton Wheel:
Bucket Dimensions of Pelton Wheel:
Let,
d = Diameter of the jet
ADVERTISEMENTS:
Depth of bucket = 1.2 d
Width of bucket =5 d
Number of Buckets:
In order to minimize frictional resistances the number of buckets should be the minimum. At the same time, there shall be enough number of buckets so that as a bucket facing the jet leaves, the following jet should be in the position to face the jet. In other words, there shall be enough number of buckets so that the jet is always intercepted by a bucket.
Let R = Radius of the mean bucket circle.
γ = Angle subtended by adjacent buckets at the centre of wheel.
The buckets are so arranged that as one bucket is just about to move out of the jet, another bucket has just moved in. Let a, b and c represent consecutive buckets. We know for maximum efficiency condition the jet speed is double the bucket speed. Thus in the interval of time in which the bucket moves from b to c the jet section will move from a to c. Thus for the condition of continuous interception of the jet by the buckets, the positions of the buckets should be as shown in Fig. 22.9.
