PELTON WHEEL TURBINE@GYAN SINGH

PELTON TURBINE

This turbine is named after Lester A. Pelton an American Engineer who developed it in

the year 1880. A pelton wheel is a tangential impulse turbine, and the available energy at the

entrance is completely kinetic energy. Further, it is preferred at a very high head

and low discharges with low specific speeds. The pressure available at the inlet and

outlet is atmospheric. Pelton wheels operate best with heads from 15–1,800 metres (50–5,910 ft), although

there is no theoretical limit.

COMPONENTS OF PELTON TURBINE

NOZZLE WITH GUIDE MECHANISM The function of the nozzle of a Pelton wheel is to convert the available

pressure energy into high velocity energy in the form of jet. The quantity of water required is proportional to the load on the turbine.

Therefore, to control the flow through the nozzle, some sort of a regulating or a governing mechanism is necessary. This is generally done by using a spear inside the nozzle. The movement of spear inside the nozzle changes the area of flow through it, thus varying the discharge.

The nozzle is usually made of either cast iron or cast steel. Sometimes, a small brake nozzle is also used in case of large turbines. When

the wheel is to stopped, besides cutting off the supply of water through the main nozzle, the brake nozzle also directs the water on to the back of buckets to bring the wheel quickly to rest.

RUNNER WITH BUCKETS The runner of a Pelton turbine consists of a number of double cupped buckets

fixed to the periphery of the wheel. Each bucket has a sharp edge at the centre called the splitter The jet strikes each bucket at this splitter and is divided into two sides, thus

avoiding any unbalanced thrust on the shaft. As the splitter takes the full impact of the jet, so it has to be quite strong.

CASING It is made of cast-iron or fabricated steel plates. The casing does not perform any hydraulic function. However, a casing is necessary to

avoid accidents, splashing of water, to lead the water to the tailrace and to support the hosing for the bearing and the nozzle.

DIMENSIONS OF BUCKET

The minimum possible width B of the bucket is approximately 2d, where d is the diameter of jet. This dimension is usually taken as 2.8 to 3.2d to avoid losses. Some prefer to take it as 3 to 4d.

The depth of the bucket (T) usually lies between 0.8 to 1.2d. The height (H) of the bucket is 2.4 to 2.8d.

NUMBER OF JETS

Number of jets may be increased, usually upto 4, but never more than 6 because higher number of jets makes the governing complicated.

WORKING OF PELTON WHEEL TURBINE

High speed water jets emerging from the nozzles (obtained by expanding high pressure water to the atmospheric pressure in the nozzle) strike a series of spoon-shaped buckets mounted around the edge of the pelton wheel. High pressure water can be obtained from any water body situated at some height or streams of water flowing down the hills.

As water flows into the bucket, the direction of the water velocity changes to follow the contour of the bucket. These jets flow along the inner curve of the bucket and leave it in the direction opposite to that of incoming jet. When the water-jet contacts the bucket, the water exerts pressure on the bucket and the water is decelerated as it does a "u-turn" and flows out the other side of the bucket at low velocity.

The change in momentum (direction as well as speed) of water jet produces an impulse on the blades of the wheel of Pelton Turbine. This "impulse" does work on the turbine and generates the torque and rotation in the shaft of Pelton Turbine.

To obtain the optimum output from the Pelton Turbine the impulse received by the blades should be maximum. For that, change in momentum of the water jet should be maximum possible. This is obtained when the water jet is deflected in the direction opposite to which it strikes the buckets and with the same speed relative to the buckets.

Often two buckets are mounted side-by-side, thus splitting the water jet in half (see photo). The high speed water jets emerging form the nozzles strike the buckets at splitters, placed at the middle of the buckets, from where jets are divided into two equal streams. This balances the side-load forces on the wheel, and helps to ensure smooth, efficient momentum transfer of the fluid jet to the turbine wheel.

HYDROELECTRIC SETUP OF PELTON WHEEL TURBINE By using the pelton turbine setup electricity is generated with the help of the

water reservoir. The water reservoir must be situated at the height from the pelton wheel. Pressure channel is arranged and the water flows through the pressure channel to the penstock head. From the penstock head or with the help of supply pipeline the water passes through the nozzles. In that nozzles water flows with high speed and comes out as a high jet and it strikes the blades hardly.

To obtain quality power output, constant speed is maintained in the turbine. To maintain constant speed of turbine water flow through the nozzles is changed. In the jet servo controlled spear valves are used to control the change of jet load to a change in the flow rate. Due to sudden reduction in the load the jets are repelled by using the deflector plate. Due to that the jets does not strikes the blades. Over fast-moving of the turbine must be prevented.

APPLICATIONS

Pelton wheels are the preferred turbine for hydro-power, when the available water source has relatively high hydraulic head at low flow rates.

. The smallest Pelton wheels are only a few inches across, and can be used to tap power from mountain streams having flows of a few gallons per minute.