Construction and Working of Pelton Wheel

An arrangement of Pelton turbine is shown in Figure A.

The water from reservoir flows through the penstock to the nozzle which converts the pressure energy (high head of water) into kinetic energy. The resultant high velocity jet from nozzle strikes the buckets or vanes fitted at outer periphery of runner.

1. Nozzle and spear assembly
2. Casing
3. Deflector
4. Runner and buckets
5. Braking jet

1. The needle spear is provided in the nozzle to regulate the water flow through the nozzle.
2. Also, it provides the smooth flow of water with negligible loss of energy A spear is a conical needle which can be moved in axial direction by operating the wheel either manually or automatically.
3. When the spear is moved in forward direction into the nozzle, it reduced the nozzle exit area, hence, the quantity of water flow striking the buckets is reduced.
4. If the spear is moved backwards, it increases the flow rate of water.
5. The nozzle converts the potential energy of water into kinetic energy before jet strikes the buckets.
6. Pressure at exit of nozzle is reduced to atmospheric pressure.

1. The turbine rotor called runner is a circular disc fixed with buckets.
2. It is provided with cylindrical boss and keyed to the supporting shaft in small thrust bearings.
3. The runner carries cup-shaped buckets more that 15 in number which are mounted at equidistance around its periphery.
4. The buckets are either cast integrally with the circular disc or these are bolted individually to the runner, it helps in easy replacement of buckets when worn out.
5. Buckets are made of cast iron cast steel, special steels or stainless steel with inner surface polished to reduce friction losses of water jet.
6. Type of metal used for bucket depends on the head at turbine inlet.
7. The shape of the buckets is of double hemispherical cup or bowl. Each bowl of the bucket is separated by a wall called splitter or a ridge.
8. The shape and dimensions of a bucket are shown in Figure B.

The commonly adopted dimensions of bucket are :

d = diameter of jet

L = Length of height of bowl inside the rim = 2 d to 3 d

B = Width of bucket between the rims of bowl = 3d to 4d

T = Depth of bowl = 0.27 B to 0.32 B

M = Notch width = 1.1 d to 1.2 d

Splitter angle, w = 10° to 20°

• The water strikes the bucket at the splitter which splits the water into two equal streams of the hemispherical bowl.
• The maximum force will be obtained when the jet is deflected through 180° into exact hemispherical bowl.
• However, in practice the jet is deflected through 160° to 170° [splitter angle y = 10° to 20°] It avoids striking the exit jet with the back of the succeeding bucket, thus exerting a retarding force on it.
• It would reduce the power output and the overall efficiency of turbine. This also avoids the splashing of water with a splitter.
• Pelton wheel is provided with two hemispherical cups since the splitter splits the jet into two equal streams, the axial component of each stream velocity is equal and opposite due to which the axial thrust on the shaft is negligible.
• Therefore, Pelton wheel needs very small thrust bearings. An undercut is provided and surface of spoons is raised so that water can be deflected back through the angle of 160° to 170° with the vertical without disturbing the incoming bucket.

1. A casing does not have any hydraulic function to perform therefore it is not actually needed in case of impulse turbines because the runner runs under atmospheric pressure.
2. However, a casing is provided to prevent the splashing of water and lead the water to tail race, and to safeguard the persons against accidents.
3. It is made of cast iron in two halves.

1. Whenever the turbine is brought to rest, the nozzle is completely closed by pushing forward the spear.
2. However, the runner continues to rotate due to its inertia for a considerable period of time till it comes to rest.
3. In order to bring the runner to stop in a shortest time, a small nozzle is provided which issues the water jet and falls on the back of buckets.
4. It acts as a hydraulic brake for reducing the speed of runner.

1. A deflector is provided which is hinged to the casing to deflect the jet of water away from striking the buckets in case the load on turbine suddenly reduces.
2. It prevents the runner of turbine attaining unsafe speeds called runaway speed.
3. A governing mechanism is also provided to control the speed of turbine according to variation in load which shall be discussed later.