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Wednesday, 26 June 2019

PELTON WHEEL TURBINE AND ITS COMPONENTS

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machine, various types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine in our recent posts. 

Now we will focus here to understand the basics of Pelton wheel or Pelton hydraulic turbine and we will also find out here the various parts of Pelton wheel or pelton hydraulic turbine with the help of this post. 

So let us start here with the basics of Pelton wheel or Pelton hydraulic turbine

Pelton wheel or Pelton hydraulic turbines

Pelton wheel or Pelton hydraulic turbine is basically a tangential flow impulse turbine. In pelton wheel, water will strike the bucket along the tangent of the runner. 

Energy available at the inlet of the turbine will be kinetic energy. Pressure available at the inlet and outlet of the turbine will be atmospheric pressure. Pelton wheel or Pelton hydraulic turbuine is used for high heads. 

Following figure, displayed here, indicates the layout of hydroelectric power plant where pelton wheel hydraulic turbine is used to produce the electric energy. 

Water from the reservoir will flow through the penstock at the outlet of which there will be one nozzle. Nozzle, provided at the outlet of penstock, will increase the kinetic energy of the water flowing through the penstock. 

Water will come in the form of jet at the outlet of nozzle and this water jet will strike the bucket of the runner. 

Let us see here the main parts of the Pelton turbine

  1. Nozzle and flow regulating arrangement
  2. Runner and buckets
  3. Casing
  4. Breaking jet

Nozzle and flow regulating arrangement

The amount of water which will strike the buckets of the runner will be controlled by spear arrangement in the nozzle as displayed here in following figure. 

Spear is basically a conical needle which will be operated by hand wheel or automatically in the axial direction depending upon the size of the unit. 

In order to reduce the amount of water striking the buckets of the runner, spear will be moved forward into the nozzle. Similarly, in order to increase the amount of water striking the buckets of the runner, spear will be moved backward into the nozzle. 

Runner and buckets

Following figure, displayed here, indicates the runner of a Pelton wheel. It will have one circular disc and a number of buckets will be fixed evenly on the surface of periphery of this circular disc. 

The shape of the bucket will be a double hemispherical cup or bowl. Each bucket will be divided into two symmetrical parts by a dividing wall which is known as splitter. 

The jet of water will strike on the splitter and hence water jet will be divided in two parts by splitter and water jet will come out at the outer edge of the bucket. Buckets will be shaped in such a way that water jet will be deflected through 160 or 170 degree. 

Material selection of buckets will be based on the head at the inlet of the turbine. Buckets are usually made of cast iron, stainless steel or cast steel. 

Casing

Following figure indicates the casing of a Pelton wheel hydraulic turbine. 

There is no hydraulic function which will be performed by casing of pelton wheel turbine. Casing is basically used in order to prevent the splashing of the water and to discharge water to tail race. 

Casing also provides the safeguard against any accident. Material of casting of casing of pelton wheel turbine will be cast iron or fabricated steel plates. 

Breaking jet

As we have already discussed that in order to reduce the amount of water striking the buckets of the runner, spear will be moved forward into the nozzle. 

When the nozzle is completely closed by moving the spear in forward direction, the amount of water striking the buckets of the runner will be zero. 

But as we are quite aware that runner will keep moving for a long time due to its inertia and therefore a breaking jet will be provided to stop the runner in short interval of time. 

A nozzle is provided to direct the water jet on the back of vanes and this jet of water will be termed as breaking jet. 

Do you have any suggestions? Please write in comment box.  Further we will find out, in our next post, Radial flow reaction turbine

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google 

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