Pelton wheel

Pelton wheel Points : Pelton wheel, Nozzle, Runner and buckets, Casing, and, Breaking jet The Pelton wheel is an impulsive turbine used for high heads of water. It has the following main components:
1. Nozzle,
2. Runner and buckets,
3. Casing, and
4. Breaking jet.

1. Nozzle: It is a circular guide mechanism, which guides the water to flow at a designed direction, and also to regulate the flow of water. This water, in the form a jet, strikes the buckets. A conical needle or spear operates inside the nozzle in an axial direction. The main purpose, of this spear, is to control or regulate the quantity of water flowing through the nozzle.

A little consideration will show, that when the spear is pushed forward into the nozzle, it reduces the area of jet. As a result of this, the quantity of water flowing through the jet is also reduced. Similarly, if the spear is pushed back out of the nozzle, it allows a greater quantity of water to flow through the jet. The movement of the spear is regulated by hand or by automatic governing arrangement depending upon the, requirement. Sometimes, it is very essential to close the nozzle suddenly. This is done with the help of spear, which may cause the pipe to burst due to sudden increase of pressure. In order to avoid such a mishap, an additional nozzle (known as by pass nozzle) is provided through which the water can pass, without striking the buckets. Sometimes, a plate (known as deflector) is provided to the nozzle, which is used to deflect the water jet, and preventing it from striking the buckets. The nozzle is kept very close to the buckets, in order to minimum the losses due to windage. 2. Runner and buckets: The runner of a Pelton wheel essentially consists of a circular disc fixed to a horizontal shaft. On the periphery of the runner, a number of buckets are fixed uniformly. A bucket resembles to a hemispherical cup or bowl with a dividing wall (known as splitter) in its centre in the radial direction of the runner. The surface of the buckets is made very smooth. For low heads, the buckets are made of cast iron. But. for high heads, the buckets are made of bronze, stainless steel or other alloys. When the water is chemically impure, the buckets are made of special alloys. The buckets are generally bolted to the runner disc. But, sometimes, the buckets and disc are cast as a single unit. Sometimes, all the buckets wear equally in a given time. But in actual practice, all the buckets do not wear equally. A few buckets get worn out and damaged early and need replacement. This can be done only if the buckets are bolted to the disc. 3. Casing: Strictly speaking, the casing of a Pelton wheel does not perform any hydraulic function. But it is necessary to safeguard the runner against accident, and also to prevent the splashing of water and lead the water to the tail race. The casing is, generally, made of cast or fabricated parts. 4. Breaking jet: Whenever the turbine has to be brought to rest, the nozzle is completely closed. It has been observed, that the runner goes an revolving for a considerable time, due to inertia, before it comes on rest. In order to bring the runner to rest in a short time, a small nozzle is provided in such a way, that it will direct a jet of water on the back of the buckets. It acts as a brake for reducing the speed of the runner.