<>Pelton Turbine - Working & Design Aspects
Pelton turbines/wheels are suitable for power extraction, when the water energy is available at high head and low flow rate. In this video we will go through working principle and design aspects of Pelton turbine.
Following article gives detailed description of the video lecture.
Pelton Turbine – The Basic Working Principle
Working principle of Pelton turbine is simple. When a high speed water jet injected through a nozzle hits buckets of Pelton wheel; it induces an impulsive force. This force makes the turbine rotate. The rotating shaft runs a generator and produces electricity.
|Fig.1 Pelton wheel derives rotation from impulse force produced by the water jet|
In short, Pelton turbine transforms kinetic energy of water jet to rotational energy.
Governing in Pelton Wheel
Demand of power may fluctuate over time. A governing mechanism which controls position of the spear head meets this requirement. With lowering power demand the spear head at water inlet nozzle is moved in. So that water flow rate is reduced. If power demand increases spear head is moved out this will increase the flow rate. Following figure illustrates this mechanism. The first position of the spear head produces a low flow rate, while the second position produces a high flow rate.
|Fig.2 Water flow rate control in Pelton wheel by monitoring position of spear head|
So in Pelton turbine synchronization between power demand and power supply is met by controlling the water flow rate. The same technique is used in other types hydroelectric turbines. If the power supply is more than the demand, then the turbine will run over speed otherwise in under speed. But such a governing mechanism in turn will balance the power supply and demand and will make sure that the turbine rotates at a constant specified RPM. This speed should also conforms to the power supply frequency. So this mechanism acts as a speed governing mechanism of Pelton wheel.
Number of Buckets in Pelton Wheel
One of the most important parameter of Pelton turbine design is number of buckets on the disk. If number of buckets is inadequate, this will result in loss in water jet. That means when one bucket departs from the water jet next bucket may not get engaged with the jet. This will result in loss in water jet for a small time duration, thus sudden drop in turbine efficiency. Following figure illustrates what happens when the number of buckets are lowered. With lowering number of buckets at some point of operation, complete water jet might be lost (3rd figure). So there should be an appropriate number of buckets, which will make sure that no water is lost (1st figure).
|Fig.3 Effect of number of buckets on water-bucket interaction|
Pelton Bucket - Design & Features
Most vital component of Pelton wheel is its bucket. Buckets are casted as single solid piece, in order to avoid fatigue failure. You can note that force acting on the turbine bucket is not constant with time. If you follow one particular bucket, it will have high force for a small time duration (at the time of jet impingement) after that a larger idle period where no jet interaction takes place. So the force acting on the bucket is also not constant. It varies with the time but it is having a cyclic nature. If bucket were made using pieces by welding attachment such cyclic fore will easily lead to premature fatigue failure.
|Fig.4 Different views of Pelton bucket|
Water jet is split into 2 equal components with help of a splitter. The special shape of bucket makes the jet turn almost 180 degree. This produces an impulsive force on bucket. Force so produced can easily be derived from Newton’s 2nd law of motion. Blade outlet angle close to 180 degree is usually used in order to maximize impulsive force.
A cut is provided on bottom portion of buckets. This makes sure that water jet will not get interfered by other incoming buckets.
Pelton – An Impulse Turbine
Since the water jet is always open to atmosphere, inlet and exit pressure of water jet will be same and will be same as atmospheric pressure. However absolute velocity of fluid will have huge drop from inlet to exit of bucket. This kinetic energy drop is the maximum energy the bucket can absorb.
So it is clear that Pelton turbine gains mechanical energy purely due to change in kinetic energy of jet, not due to pressure energy change. Which means Pelton turbine is a pure impulse machine.
|Fig.5 Pressure and velocity variation across Pelton bucket|
Impulse force produced by water jet is high when jet is having high velocity. Water stored at high altitude can easily produce high jet velocity. This is the reason why Pelton turbine is most suitable for operation, when water is stored at high altitude.
You can easily understand why there is a nozzle fitted at water jet injection portion. Nozzle will increase velocity of jet further, thus will aid in effective production of impulse force.
Extracting Maximum Power from Water Jet
Pelton turbine design is always aimed at extracting maximum power from water jet, or maximizing efficiency. Power extracted by the bucket, P is product of jet impulse force and bucket velocity.
So power extraction is maximum when product of impulsive force and bucket velocity is maximum. Let's consider 2 different operating conditions.
Buckets are Held Stationary
Bucket Speed Same as Jest Speed
If Pelton wheel buckets are held stationary, there will be a huge impulse force produced. But power extraction will be zero since buckets are not moving.
If buckets are moving with same speed of jet, water jet won't be able to hit the bucket. This will lead to zero impulse force. Again power extraction will be zero.
|Fig.6 Relative magnitude of bucket and jet velocity is important in power extraction from fluid|
In short, power extraction is zero both at zero bucket speed and when bucket speed is same as jet speed. So with respect to jet to bucket speed ratio, power extraction will vary with as shown below.
|Fig.7 This graph shows how power extraction from fluid varies with respect to bucket to jet velocity ratio|
It is clear from the above graph that optimum power extraction happens in between. It can be shown using Euler's turbo machinery equation that maximum power extraction happens when bucket speed is half the jet velocity. So it is always desirable to operate Pelton wheel at this condition. Pelton turbines can give efficiency as high as 90 %, at optimum working conditions.