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
- Nozzle and flow regulating arrangement
- Runner and buckets
- Casing
- 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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