Working of Pelton Wheel
A Pelton turbine was created by Lester Pelton in 1880. The Pelton wheel or Pelton turbine is an impulse-type hydroelectric turbine. This turbine is frequently used in hydropower plants.
Generally, this turbine is used for sites with heads greater than 300 meters.
In the Pelton turbine, the water moves quickly and the turbine extracts energy from the kinetic energy of water and slows down the velocity of water.
The water from a reservoir flows through the penstock to the nozzle. And it converts the pressure energy to kinetic energy.
The resultant high-velocity jet from the nozzle strikes the vanes fitted on an outer periphery of a runner.
The arrangement of the Pelton wheel is shown in the figure below.
Components of Pelton Wheel
The main components of the Pelton wheel are;
- Nozzle and spear assembly
- Runner and buckets
- Braking jet
Nozzle and Spear Assembly
To regulate the water flow through the nozzle, a need spear is provided in the nozzle. Also, it provides a smooth flow of water with negligible loss of energy.
A spear is a conical needle and it can move in an axial direction by operating a wheel. This wheel can be manually or automatic.
It reduces the quantity of water by moving forward direction (reducing the area to flow water). And it increases the quantity of water by moving backward direction (increasing the area to flow water).
Before the jet strikes the buckets, the nozzle converts the potential energy of water into kinetic energy. At the exit of the nozzle, the pressure is reduced to atmospheric pressure.
Runner and Buckets
The turbine rotor is known as a runner. It is a circular disc fixed with buckets. The runner carries more than 15 cup-shaped buckets that are mounted at equidistance around its periphery.
The buckets are connected integrally with circular discs or these are bolted individually to the runner. This type of arrangement helps to replace easily when it is worn out.
Buckets are manufactured from cast iron, cast steel, special steel, or stainless steel. The inner surface is policed to reduce the friction losses of a water jet.
The material used for the bucket depends on the water head. Mostly it is in the shape of a double hemispherical cup or bowl.
Each bowl of the bucket is separated by a wall called a splitter or a ridge. The water strikes the bucket at a splitter that splits water into two equal streams of the hemispherical bowl.
When the jet is deflected through 180˚ into an exact hemispherical bowl, the maximum force is obtained.
In practice, the jet is deflected through 160˚-170˚ to avoid sticking the exit jet with the back of the succeeding bucket.
It will reduce the power output and the overall efficiency of the turbine. But it 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.
Hence, the Pelton wheel requires a very small thrust bearing.
A casing does not have any function to perform. It is not needed in the case of the impulse turbine because the runner runs under atmospheric pressure.
However, the casing is provided to prevent splashing of water and leading the water to tail race and safeguard the persons against accidents. It is made of cast iron.
The nozzle is completely closed by pushing forward the spear whenever the turbine is brought to reset. But the runner rotates continuously due to its inertia for a considerable time till it comes to rest.
To stop the runner in a short time, a small nozzle is provided that issues the water jet and falls back of buckets.
For reducing the speed of the water, it acts as a hydraulic brake.
A deflector is hinged to the casing to deflect the jet of water away from striking the buckets in case of the load on the turbine suddenly reduces.
It prevents the runner of a turbine to rotate above the unsafe speed (runaway speed).
To control the speed of a turbine according to variation in load, a governing mechanism is provided.
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