What is Pelton Turbine?
The Pelton wheel or Pelton turbine is an impulse type water turbine invented by American inventor Lester Allan Pelton in the 1870s. The Pelton wheel extracts energy from the momentum of the moving water, as opposed to the water’s own weight like the traditional overshot water wheel.
There were many earlier variations of impulse turbines, but they were less efficient than Pelton’s design. The water leaving these wheels was usually still at high speed and carried away much of the dynamic energy brought to the wheels.
Pelton’s paddle geometry was designed so that when the rim was running at half the speed of the water jet, the water exited the wheel at a very low speed. Therefore, his design extracted almost all of the water’s momentum energy – resulting in a very efficient turbine.
Design of Pelton Turbine
The Pelton turbine is built quite simply. A large circular disc is mounted on a type of rotating shaft known as a rotor. Mounted on this circular disc are cup-shaped blades known as buckets that are evenly spaced around the entire wheel.
Generally, the buckets are arranged in pairs around the edge. Nozzles are then placed on the wheel and serve the purpose of introducing water into the turbine.
Nozzles direct powerful, rapid streams of water against a series of spoon-shaped buckets, also known as impulse blades, that are mounted around the outer rim of a drive wheel. When the jet of water hits the blades, the direction of the water velocity is changed to follow the contours of the blades.
The impulse energy of the water jet exerts a torque on the bucket-and-wheel system and turns the wheel. The water jet makes a “U-turn” and emerges on the outside of the bucket, decelerating to a low speed. The impulse of the water jet is transmitted to the wheel and thus to a turbine.
Thus “impulse energy” acts on the turbine. Maximum performance and efficiency are achieved when the speed of the water jet is twice the velocity of the rotating blades.
A very small percentage of the original kinetic energy of the water jet remains in the water, emptying the bucket at the same rate as it is being filled, allowing the high-pressure inlet flow to continue uninterrupted and without wasting energy.
Typically, two buckets are mounted side by side on the wheel, with the water jet split into two equal streams. This balances the side load forces on the wheel and contributes to a smooth and efficient transfer of momentum from the water jet to the turbine wheel.
Since water is almost incompressible, almost all of the available energy is drawn off in the first stage of the hydraulic turbine. “Therefore, unlike gas turbines that work with a compressible fluid, Pelton wheels only have one turbine stage.”
How Does a Pelton Turbine work?
The operation of a Pelton turbine is fairly simple. In this type of turbine, high-speed jets of water emerge from the nozzles that surround the turbine. These nozzles are arranged so the water jet will hit the buckets at splitters, the center of the bucket where the water jet is divided into two streams.
The working of Pelton turbine is as follows:
- The water is drawn from the high-pressure source through a long pipe called a penstock.
- The nozzle arrangement at the end of the pressure pipe helps the water to accelerate and it flows out as a high-speed jet at high velocity and discharges at atmospheric pressure.
- The jet will hit the splitter of the buckets which will distribute the jet into two halves of the bucket and the wheel starts revolving.
- The kinetic energy of the jet is reduced when it hits the bucket and also due to the spherical shape of buckets the directed jet will change its direction and takes a U-turn and falls into the tailrace.
- In general, the inlet angle of the jet is between 1o to 3o, after hitting the buckets the deflected jet angle is between 165o to 170o.
- The water collected in the tailrace should not submerge the Pelton wheel in any case.
- To generate more power, two Pelton wheels can be arranged to a single shaft or two water jets can be directed at a time to a single Pelton wheel.
Application of Pelton Turbine
Pelton turbine is used in the hydroelectric power plant where the water available at high head i.e. 150 m to 2000 m or even more. In a hydroelectric power plant, it is used to drive the generator attached to it and the generator generates the mechanical energy of the turbine into electrical energy.
Advantages of Pelton Turbine
- It is easy to maintain.
- There is no cavitation problem in the Pelton turbine.
- Pelton turbine has a simple construction.
- It can work on the high head and low discharge.
- It is very easy to assemble.
- The overall efficiency of this turbine is very high.
Disadvantages of Pelton Turbine
- Pelton Turbine requires a very high head for operation.
- The efficiency decreases quickly with time.
- Its turbine size is generally large and hence requires a large space.
- As it only works in the high head, it is difficult to control vibrations in the operating head.