Pelton turbine is an impulse turbine named after an American engineer Lester A.Pelton.
Pelton turbines are suitable for power extraction when water energy is available at a high head and low flow rate.
In this type of turbine, the potential and the pressure energy of water is converted to kinetic energy. A nozzle is used which increases the velocity of water and hence increases the kinetic energy.
Pelton wheel is a tangential flow turbine means the water jet will stikes the blade of the turbine tangentially.
It is a high head turbine means this turbine is used only in the condition where water is available at a high head.
The turbine whose head is very high has very low specific heat. So the turbine is also called low specific heat turbine. It is also a low discharge turbine.
Properties Of Pelton Turbine:
1) It is a tangential flow turbine.
2) It is a high head turbine.
3) It is a low specific heat turbine.
4) It is a low discharge turbine.
5) The pressure in the inlet of the turbine is equal to the pressure in the outlet of the turbine. Hence, there is no problem of cavitation in this turbine.
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Parts of Pelton Turbine:
1) Nozzle’s flow regulating arrangement:
It is used to increase the kinetic energy of water.
Jet Of Water:
The penstock is a channel or pipe for conveying the water to the Pelton turbine.
iii) Spear Arrangement:
Spear arrangement is to regulate the flow of water. As the turbine is coupled with the generator. Sometimes when the load of generator is less , the speed of the turbine has to be decreased and hence the flow of water is needed to be decreased. In this condition, the spear arrangement is used to regulate and decrease the flow of water from the nozzle.
A wheel is present to regulate the spear. When the spear of the nozzle is pushed forward towards the nozzle the flow rate through the nozzle decreases and when the spear of the nozzle is pulled away from the nozzle the flow rate of the nozzle decreases.
2) Runner And Bucket:
The runner is present in the center and the buckets are mounted all around the bucket. These buckets can also be called vanes or impulse blades.
The runner consists of a circular disc. Equal sized and evenly spaced buckets are mounted on this circular disc. If these buckets are not equally spaced then there will be a balancing problem in the Pelton turbine.
In the Pelton turbine, every bucket can be of two shapes:
a) Double hemispherical cup.
b) Double Ellipsoidal cup.
Vane or bucket in the Pelton turbine is designed specifically such that the water strikes the center part of the bucket and will split up into two streams and the water will come out from the two dips of the bucket.
The material used in making buckets of Pelton turbine is cast iron, cast steel bronze, or stainless steel.
The material used in making buckets is selected on the basis of how high the water head is. If the water head is very much high, very hard material is used to make the buckets.
The line between the two cups of the bucket or vanes is called a splitter. This splitter divides the buckets into two parts and these two parts are symmetrical.
Also, a cut is provided on the bottom portion of buckets, this makes sure that the water jet will not get interfered with by other incoming buckets.
One of the most important parameters of Pelton turbine design is the number of buckets on a disc. If the number of buckets is inadequate, this will result in a loss of water jet. With a lowering number of buckets, at some point of operation, a complete water jet might be lost.
This will decrease the efficiency of the Pelton Turbine drastically. So, there should be an appropriate number of buckets that will make sure no water is lost.
The enclosure in which the entire assembly is fitted is called the turbine casing.
The function of Casing:
1) It prevents splashing of water.
2) It collects the water and discharges it to the tailrace.
3) It provides a safeguard to the turbine.
Unlike other turbine’s casing, the casing of the Pelton turbine does not have any hydraulic functions.
4) Brake Nozzle:
The braking jet which comes put from the braking nozzle is used to stop the Pelton turbine in less time.
When the water supply to the Pelton turbine is stopped, the turbine still rotates and does not stop due to inertial as the components of the turbine are made of heavy materials.
If a situation comes when the complete stoppage of the turbine is required, this brake nozzle supply the braking jet, and this braking jet hit the buckets on the opposite side and hence due to force in the opposite direction the turbine stops.
In this Pelton turbine, there are two openings that help to maintain the pressure inside the turbine which is equal to the atmospheric pressure.
Important Angles to consider in Pelton Turbine:
Total splitter angle:
Total splitter angle is the angle between two tangents drawn from the curve of two cups present near the splitter. It is denoted by 2βs.
The splitter angle is half of the total splitter angle and is denoted by βs.
When the water strikes the bucket, water will exert force on the bucket and as the water splits up the force is divided into x-direction and y-direction. But the y-direction forces of the two cups will cancel each other as they will be equal in magnitude and opposite in direction, but the force in the x-direction of both cups will combine and help the turbine to rotate.
The deflection angle is the angle of deflection of water after striking the becket.
If the deflection angle is 180 degrees, then the force exerted on the buckets is maximum. So, 180 degrees is the ideal deflection angle.
But we not used the ideal deflection angle because when the ideal deflection angle is used the water from the bucket reflects with high force and starts hitting the opposite side of the preceding bucket and hence the force in the preceding bucket reduces as this force is in opposite direction.
Normally a deflection angle between 165° to 170° is used for optimum efficiency.
In practice, the inlet angle used is between 1° to 3°. But theoretically, all calculations are done by taking inlet angle 0°.
Working Of Pelton Turbine:
The water is stored at a high head and passes through the penstock to reach the nozzle of the Pelton turbine.
After reaching the Pelton turbine, the kinetic energy of flowing water increases as it passes the nozzle.
After that the water with high kinetic energy strikes the bucket, this water jet exerts a force in the bucket and this bucket is known as impulse force.
The flow of water from the nozzle is controlled using a spear fixed near the end of the nozzle.
The wheel and the turbine arrangement is used to regulate the flow of water through the nozzle. When the spear is moving forward towards the nozzle, the water flow rate through the nozzle decreases, and when the spear is moved away from the nozzle, the water flow rate through the nozzle increases. When the water from the nozzle reaches the bucket of the turbine, the water split into two ways into the cups of the bucket, and the momentum of the water is transferred to the bucket of the turbine. When the water exerts impulse force to the turbine, the turbine starts rotating due to this impulse force. Splitting of water into the cups helps the turbine to balance as the component of the force exerted by water is canceled after splitting as these components of force are equal in magnitude and opposite in direction.
For maximum efficiency and power, the turbine is designed in such a way that the velocity of the water jet is twice the velocity of the water.
After that, the main function is of a generator which converts
Applications of Pelton Turbine:
1 Pelton Turbines are used where the water is available at the high head from 150m to 2000m in a hydroelectric power plant.
2 They are also used as set up in the labs of Educational Institutions.
Advantages of Pelton Turbine:
1 It is easy to maintain.
2 There is no cavitation problem in Pelton turbine.
3 Pelton turbine has a simple construction.
4 It can work on the high head and low discharge.
5 It is very easy to assemble.
6 Overall efficiency of this turbine is very high.
Disadvantages of Pelton Turbine:
1 Pelton Turbine requires a very high head for operation.
2 The efficiency decreases quickly with time.
3 Its turbine size is generally large and hence requires a large space.
4 As it only works in the high head, it is difficult to control vibrations in the operating head.