Short Notes: Pelton-Wheel | Short Notes for Mechanical Engineering PDF Download

 Page 1


Pelton-Wheel
There are three basic hydraulic machines: Pelton, Francis and Kaplan.
The fluid density is constant but the complication is that cavitation may 
occur in the machine. Cavitation is effectively the boiling of the liquid as 
it is exposed to extrem ely low pressures in certain parts of the turbine. 
The Pelton wheel or Pelton turbine is a tangential flow impulse turbine. 
The water strikes the bucket along the tangent of the runner. The energy 
available at the inlet of the turbine is only kinetic energy. The pressure 
at the inlet and outlet of the turbine is atmospheric. The turbine is used 
for high heads and is named after L.A Pelton, an American Engineer.
The Pelton wheel is composed of a nozzle which converts the whole 
available head to kinetic energy and a rotor made up of a series of 
double hemispherical buckets fastened on the periphery of the rotor. The 
rotor is not enclosed, and the water leaving the buckets goes 
immediately to the tailrace.
The Pelton wheel falls in a large class of these machines known as the 
axial-flow type where the nozzle angle relative to the runner is zero. 
However, the bucket cannot have a 180° camber angle, since the water 
must have a finite radial velocity component away from the wheel in 
order to avoid interference.
The main parts of the pelton turbine are: -
• Nozzle and flow regulating arrangement.
• Runner and buckets.
• Casing.
• Breaking Jet.
Page 2


Pelton-Wheel
There are three basic hydraulic machines: Pelton, Francis and Kaplan.
The fluid density is constant but the complication is that cavitation may 
occur in the machine. Cavitation is effectively the boiling of the liquid as 
it is exposed to extrem ely low pressures in certain parts of the turbine. 
The Pelton wheel or Pelton turbine is a tangential flow impulse turbine. 
The water strikes the bucket along the tangent of the runner. The energy 
available at the inlet of the turbine is only kinetic energy. The pressure 
at the inlet and outlet of the turbine is atmospheric. The turbine is used 
for high heads and is named after L.A Pelton, an American Engineer.
The Pelton wheel is composed of a nozzle which converts the whole 
available head to kinetic energy and a rotor made up of a series of 
double hemispherical buckets fastened on the periphery of the rotor. The 
rotor is not enclosed, and the water leaving the buckets goes 
immediately to the tailrace.
The Pelton wheel falls in a large class of these machines known as the 
axial-flow type where the nozzle angle relative to the runner is zero. 
However, the bucket cannot have a 180° camber angle, since the water 
must have a finite radial velocity component away from the wheel in 
order to avoid interference.
The main parts of the pelton turbine are: -
• Nozzle and flow regulating arrangement.
• Runner and buckets.
• Casing.
• Breaking Jet.
1. Nozzle and flow regulating arrangement: The amount of water 
striking the buckets of the runner is controlled by providing a spear 
in the nozzle. The spear is a conical needle which is operated either 
by a hand wheel or automatically in an axial direction depending 
upon the size of the unit. When the spear is pushed forward into the 
nozzle the amount of water striking the runner is reduced. On the 
other hand if the spear is pushed back, the amount of water striking 
the runner increases.
2. Runner with buckets: It consists of a circular disc on the periphery of 
which a number of buckets evenly spaced are fixed. The shape of 
the buckets is of a double hemispherical cup or bowl. Each bucket is 
divided into two hemispherical parts by a dividing wall which is 
known as splitter.
3. Casing: The function of the casing is to prevent the splashing of the 
water and to discharge water to tail race. It also acts as a safeguard 
against accidents. It is made of cast iron or fabricated steel plates. 
As pelton wheel is an impulse turbine, the casing of the pelton wheel 
does not perform any hydraulic function.
4. Breaking Jet: When the nozzle is completely closed by moving the 
spear in the forward direction the amount of water striking the 
runner reduces to zero. But the runner due to inertia goes on 
revolving for a long time. To stop the runner in a short tim e, a small 
nozzle is provided which directs the Jet of water on the back of the 
buckets. This Jet of water is called breaking Jet.
Working of Pelton wheel Turbine:
The water from the reservoir flows through the penstocks at the outlet of 
which a nozzle is fitted. The nozzle increases the kinetic energy of the 
water flowing through the penstock by converting pressure energy into 
kinetic energy. At the outlet of the nozzle, the water comes out in the 
form of a Jet and strikes on the splitter, which splits up the jet into two 
parts. These parts of the Jet, glides over the inner surfaces and comes 
out at the outer edge. The buckets are shaped in such a way that 
buckets rotates, runner of the turbine rotates and thus hydraulic energy 
of water gets converted into mechanical energy on the runner of turbine 
which is further converted into electrical energy in a 
generator/alternator.
The Pelton wheel shown in the following figure is a pure impulse (R = 0) 
turbine. It is used in very high head installations and develops efficiencies very 
close to the Francis and Kaplan reaction turbines.
Page 3


Pelton-Wheel
There are three basic hydraulic machines: Pelton, Francis and Kaplan.
The fluid density is constant but the complication is that cavitation may 
occur in the machine. Cavitation is effectively the boiling of the liquid as 
it is exposed to extrem ely low pressures in certain parts of the turbine. 
The Pelton wheel or Pelton turbine is a tangential flow impulse turbine. 
The water strikes the bucket along the tangent of the runner. The energy 
available at the inlet of the turbine is only kinetic energy. The pressure 
at the inlet and outlet of the turbine is atmospheric. The turbine is used 
for high heads and is named after L.A Pelton, an American Engineer.
The Pelton wheel is composed of a nozzle which converts the whole 
available head to kinetic energy and a rotor made up of a series of 
double hemispherical buckets fastened on the periphery of the rotor. The 
rotor is not enclosed, and the water leaving the buckets goes 
immediately to the tailrace.
The Pelton wheel falls in a large class of these machines known as the 
axial-flow type where the nozzle angle relative to the runner is zero. 
However, the bucket cannot have a 180° camber angle, since the water 
must have a finite radial velocity component away from the wheel in 
order to avoid interference.
The main parts of the pelton turbine are: -
• Nozzle and flow regulating arrangement.
• Runner and buckets.
• Casing.
• Breaking Jet.
1. Nozzle and flow regulating arrangement: The amount of water 
striking the buckets of the runner is controlled by providing a spear 
in the nozzle. The spear is a conical needle which is operated either 
by a hand wheel or automatically in an axial direction depending 
upon the size of the unit. When the spear is pushed forward into the 
nozzle the amount of water striking the runner is reduced. On the 
other hand if the spear is pushed back, the amount of water striking 
the runner increases.
2. Runner with buckets: It consists of a circular disc on the periphery of 
which a number of buckets evenly spaced are fixed. The shape of 
the buckets is of a double hemispherical cup or bowl. Each bucket is 
divided into two hemispherical parts by a dividing wall which is 
known as splitter.
3. Casing: The function of the casing is to prevent the splashing of the 
water and to discharge water to tail race. It also acts as a safeguard 
against accidents. It is made of cast iron or fabricated steel plates. 
As pelton wheel is an impulse turbine, the casing of the pelton wheel 
does not perform any hydraulic function.
4. Breaking Jet: When the nozzle is completely closed by moving the 
spear in the forward direction the amount of water striking the 
runner reduces to zero. But the runner due to inertia goes on 
revolving for a long time. To stop the runner in a short tim e, a small 
nozzle is provided which directs the Jet of water on the back of the 
buckets. This Jet of water is called breaking Jet.
Working of Pelton wheel Turbine:
The water from the reservoir flows through the penstocks at the outlet of 
which a nozzle is fitted. The nozzle increases the kinetic energy of the 
water flowing through the penstock by converting pressure energy into 
kinetic energy. At the outlet of the nozzle, the water comes out in the 
form of a Jet and strikes on the splitter, which splits up the jet into two 
parts. These parts of the Jet, glides over the inner surfaces and comes 
out at the outer edge. The buckets are shaped in such a way that 
buckets rotates, runner of the turbine rotates and thus hydraulic energy 
of water gets converted into mechanical energy on the runner of turbine 
which is further converted into electrical energy in a 
generator/alternator.
The Pelton wheel shown in the following figure is a pure impulse (R = 0) 
turbine. It is used in very high head installations and develops efficiencies very 
close to the Francis and Kaplan reaction turbines.
The utilization factor described in Equation 3 can be further simplified as 
follows: For these machines there is no change of rotor radius, so U- 1 =
U2. Further, since the energy transfer is entirely at atmospheric pressure, 
the absolute flow velocity remains unchanged; i.e., an impulse machine of 
the axial-flow type has
. Thus the energy transfer is wholly derived from a change in the 
velocity's direction. This resulting change in momentum (im pulse) causes 
a force on the turbine buckets. The denominator of Equation 3 
representing energy available then becomes sim p ly ^ 2.
The ideal utilization factor, e, is the ratio of Eutil to Eavail, i.e.,
- f u , v - u ^ g
g. * ‘ '
^ + ( u f - u
( u ^ - u A g
The Vu terms in the numerator of above equation can be simplified by 
referring to the above vector diagram.
VU [ = Vj = U + VI ;
= U — V, cos a.
where a2 is described in the above given figure.
Thus the utilization factor for a Pelton wheel turbine becomes
2U ((u + V ,) - (u - v f; C O S Q , ) ) 2UV, (l+ c o sa 2)
v r
V.-
2U | V — U i(l-i- c o s a ,)
= ------^ 4 -------------—= 2 il-c c o s a
Vf
ij
u u
:
,vf.
The theoretical maximum value of utilization factor should occur when
Page 4


Pelton-Wheel
There are three basic hydraulic machines: Pelton, Francis and Kaplan.
The fluid density is constant but the complication is that cavitation may 
occur in the machine. Cavitation is effectively the boiling of the liquid as 
it is exposed to extrem ely low pressures in certain parts of the turbine. 
The Pelton wheel or Pelton turbine is a tangential flow impulse turbine. 
The water strikes the bucket along the tangent of the runner. The energy 
available at the inlet of the turbine is only kinetic energy. The pressure 
at the inlet and outlet of the turbine is atmospheric. The turbine is used 
for high heads and is named after L.A Pelton, an American Engineer.
The Pelton wheel is composed of a nozzle which converts the whole 
available head to kinetic energy and a rotor made up of a series of 
double hemispherical buckets fastened on the periphery of the rotor. The 
rotor is not enclosed, and the water leaving the buckets goes 
immediately to the tailrace.
The Pelton wheel falls in a large class of these machines known as the 
axial-flow type where the nozzle angle relative to the runner is zero. 
However, the bucket cannot have a 180° camber angle, since the water 
must have a finite radial velocity component away from the wheel in 
order to avoid interference.
The main parts of the pelton turbine are: -
• Nozzle and flow regulating arrangement.
• Runner and buckets.
• Casing.
• Breaking Jet.
1. Nozzle and flow regulating arrangement: The amount of water 
striking the buckets of the runner is controlled by providing a spear 
in the nozzle. The spear is a conical needle which is operated either 
by a hand wheel or automatically in an axial direction depending 
upon the size of the unit. When the spear is pushed forward into the 
nozzle the amount of water striking the runner is reduced. On the 
other hand if the spear is pushed back, the amount of water striking 
the runner increases.
2. Runner with buckets: It consists of a circular disc on the periphery of 
which a number of buckets evenly spaced are fixed. The shape of 
the buckets is of a double hemispherical cup or bowl. Each bucket is 
divided into two hemispherical parts by a dividing wall which is 
known as splitter.
3. Casing: The function of the casing is to prevent the splashing of the 
water and to discharge water to tail race. It also acts as a safeguard 
against accidents. It is made of cast iron or fabricated steel plates. 
As pelton wheel is an impulse turbine, the casing of the pelton wheel 
does not perform any hydraulic function.
4. Breaking Jet: When the nozzle is completely closed by moving the 
spear in the forward direction the amount of water striking the 
runner reduces to zero. But the runner due to inertia goes on 
revolving for a long time. To stop the runner in a short tim e, a small 
nozzle is provided which directs the Jet of water on the back of the 
buckets. This Jet of water is called breaking Jet.
Working of Pelton wheel Turbine:
The water from the reservoir flows through the penstocks at the outlet of 
which a nozzle is fitted. The nozzle increases the kinetic energy of the 
water flowing through the penstock by converting pressure energy into 
kinetic energy. At the outlet of the nozzle, the water comes out in the 
form of a Jet and strikes on the splitter, which splits up the jet into two 
parts. These parts of the Jet, glides over the inner surfaces and comes 
out at the outer edge. The buckets are shaped in such a way that 
buckets rotates, runner of the turbine rotates and thus hydraulic energy 
of water gets converted into mechanical energy on the runner of turbine 
which is further converted into electrical energy in a 
generator/alternator.
The Pelton wheel shown in the following figure is a pure impulse (R = 0) 
turbine. It is used in very high head installations and develops efficiencies very 
close to the Francis and Kaplan reaction turbines.
The utilization factor described in Equation 3 can be further simplified as 
follows: For these machines there is no change of rotor radius, so U- 1 =
U2. Further, since the energy transfer is entirely at atmospheric pressure, 
the absolute flow velocity remains unchanged; i.e., an impulse machine of 
the axial-flow type has
. Thus the energy transfer is wholly derived from a change in the 
velocity's direction. This resulting change in momentum (im pulse) causes 
a force on the turbine buckets. The denominator of Equation 3 
representing energy available then becomes sim p ly ^ 2.
The ideal utilization factor, e, is the ratio of Eutil to Eavail, i.e.,
- f u , v - u ^ g
g. * ‘ '
^ + ( u f - u
( u ^ - u A g
The Vu terms in the numerator of above equation can be simplified by 
referring to the above vector diagram.
VU [ = Vj = U + VI ;
= U — V, cos a.
where a2 is described in the above given figure.
Thus the utilization factor for a Pelton wheel turbine becomes
2U ((u + V ,) - (u - v f; C O S Q , ) ) 2UV, (l+ c o sa 2)
v r
V.-
2U | V — U i(l-i- c o s a ,)
= ------^ 4 -------------—= 2 il-c c o s a
Vf
ij
u u
:
,vf.
The theoretical maximum value of utilization factor should occur when
A-
_U
Vj
d?
U
= 0 = 2|1 — cos a . 1 - 2 —
v , .
u
= 0 = 2 11 — c o s q , l 1 - 2 —
v ,
v,
IJ
U 1 
or — = —
Vt 2
U 1 
or — = —
Vi 2
i.e ., when the linear velocity of the bucket is half that of the absolute 
velocity of the impinging jet.