Pelton wheel is an impulsive turbine used mainly for high head hydro electric schemes.
Pelton wheel has the following main components.
- Runner and buckets,
- Breaking jet
Nozzle of a pelton wheel is a circular guide mechanism which guides the water to flow at the desired directions.
It also regulate the flow of water. A conical or spear needle operates inside the nozzle in axial direction. The main purpose of the nozzle is to regulate the flow of water through nozzle.
When the needle is pushed forward into the nozzle, it reduces area if jet. As a result the quantity of water through the jet is also reduced. Similarly if the spear is pushed back out of nozzle, it increases the area of nozzle and discharge increases.The movement of spear is regulated by hand or by automatic governing arrangement.
Sometimes it is very essential to close the nozzle suddenly. It is necessary when there is a sudden decrease of load on turbine. To close the nozzle, if we use spear then it will cause the pipe to burst due to high pressure generation. In order to avoid such a mishap, an additional nozzle is provided through which the water can pass without striking the buckets. This needle is known as bypass needle.
Sometimes, a plate known as deflector plate is provided to the nozzle, which is used to deflect water jet.
- Runner and buckets
Runner of a pelton wheel consists of circulating disc fixed with horizontal shaft.
On the periphery of the runner, a number of buckets are fixed at a uniform distance. Bucket is a hemispherical cup or bowl with a divider in the middle. This divider is known as splitter. It split the water jet in two equal parts.
The surface of the bucket is made very smooth. For low heads, the buckets are made of cast iron. For high heads, buckets are made of stainless steel, bronze or other alloys. For impure water, buckets are made of special alloys.
The buckets are generally bolted to the runner disc. Sometimes the buckets and disc are cast as a single unit. Sometimes few buckets are damaged and need replacement. They can only be replaced if they are bolted with the runner disc.
Steel coverings provided over the runner of pelton wheel is known as casing. It does not play any hydraulic function.
But is necessary to provide the runner against accident. It has the following functions.
- It prevents accidents
- Minimize the wind losses
- stops splashing of water
- Facilitates to collect water
- Transmission of water to the tail race.
The casing is generally made of cast or fabricated parts.
- Braking jet
Whenever the turbine has to be brought to rest the nozzle is completely closed. But the runner of pelton wheel goes on revolving due to inertia. To bring the runner to rest in short time, a small nozzle is provided in such a way that it will direct the jet of water on the back of buckets. It acts as a brake for reducing the speed of the runner.