Governing of Hydraulic Turbines
- “The governing of a turbine is defined as the operation by which the speed of the turbine is kept constant under all working conditions (irrespective of the load variations).”
- The governing of a turbine is necessary as, a turbine is directly coupled to an electric generator, which is required to run at a constant speed under all fluctuating loads conditions.
- It is done automatically by means of a governor, which regulates the rate of flow through the turbines according to the changing load conditions on the turbine.
- The governor used in hydraulic turbines should be very strong as it has to deal with the water coming at a very large force and huge quantity.
All type of turbines use oil pressure governor, which consists of the following parts:
- Oil sump
- Oil pump (Gear pump), which is driven by the power obtained from the turbine. It supplies oil at high pressure.
- The servo motor, also known as a relay cylinder, which consists of a cylinder in which piston reciprocates under the action of oil pressure. It is connected at both ends with the distributor valve through the pipelines.
- The distributor valve or control valve or relay valve, which slides whenever load changes and thereby allows the oil to go to either side of the servomotor.
- The centrifugal governor or actuator, which is connected to the turbine main shaft through a belt or gears.
Governing of Pelton Wheel – Governing of Impulse Turbine
- In Pelton wheel turbine, the quantity of water supplied by the nozzle can be regulated by anyone of the following methods:
a) Spear Regulation
b) Deflector Regulation
c) Double Regulation
All these methods are operated by oil pressure governor.
a) Spear Regulation
- It consists of a nozzle in which spear moves to and fro by the action of the servomotor piston and controls the quantity of water at changing demands.
- This movement is useful when the fluctuations in load are small. But when the load changes suddenly, a sudden change in the nozzle area causes a water hammer in the penstock. Therefore a simple regulation system is not used in modern turbines where fluctuations in the load are sudden.
b) Deflector Regulation
- It is a plate, pivoted outside the nozzle, and is connected to oil pressure governor through levers.
- When the load drops it deflects a part of the jet and thus controls the quantity of water striking the buckets.
- In this system a large amount of water goes into waste but there are no chances of water hammer in the penstock.
- This system is also not widely used due to wastage of large amount of water.
c) Double Regulation
- This method is widely used by all modern Pelton wheels because it has the advantages of both spear as well as deflector regulation system.
- If load on the turbine drops suddenly, then deflector comes into action immediately and obstructs a part of water reaching to the buckets. In the meantime, spear gradually comes forward to its new position and thus the risk of water hammer is avoided.
- As this type of governing controls the speed of turbine and pressure (i.e. water hammer) in the penstock by the combination of spear and jet deflector, this system is known as “Double Regulation”.
Working of Governing Of Impulse turbine ( Pelton Wheel ):
- When the load on the generator decreases, the speed of the generator increases. Hence the speed of the turbine also increases beyond the normal speed.
- The centrifugal governor which is connected to the turbine main shaft will be rotating at an increased speed and hence centrifugal force on the fly ball increases and it moves upward. The sleeve of the governor will also move upward.
- As the sleeve moves upward, a horizontal lever turns about the fulcrum, and the piston rod of the control valve moves downward. This closes the valve V1 and opens the valve V2 as shown in Fig.
- The oil pumped from the oil pump to the control valve under pressure will flow through the valve V2 to the servomotor and will exert force on the face A of the piston of the relay cylinder.
- Piston along with piston rod and spear will move towards the right. This will decrease the area of flow of water at the outlet of the nozzle and it will reduce the rate of flow to the turbine which consequently reduces the speed of the turbine.
- Meanwhile, the bell crank lever moves downward, the jet deflector will operate and divert the whole or part of the jet away from the buckets.
- As soon as speed becomes normal, the fly balls, sleeves, lever and piston rod come to its normal position.
Governing of Francis Turbine – Governing of Reaction Turbine
- The guide blades of the Francis turbine are pivoted and connected by levers and links to the regulating ring. The regulating ring is attached with two regulating rods connected to the regulating lever. Thus regulating lever, in turn, is connected with regulating shaft, which is operated by the piston of servomotor.
- When the load on the turbine decreases, speed tends to increase, which moves fly balls upwards and thus raises sleeves. The main lever on the other side of the fulcrum pushes down the control valve rod and opens port V1. Oil under pressure enters the servomotor from left and pushes the piston to moves towards right.
- When the piston of the servomotor moves towards the right, regulating the ring is rotated to decrease the passage between the guide vanes by changing guide vane angles. Thus the quantity of water reaching the runner blades reduces and speed decreases to the
- Sudden reduction in passage between the guide blades may cause a water hammer which can be prevented by providing a relief valve near the turbine which diverts the water directly to the tailrace. Thus it functions similar to that of jet deflector as in the Pelton wheel. Thus double regulation is also well performed in Francis turbine.
Read More :
- Pelton Wheel – Parts, Working, Diagram, Applications, Advantages
- Francis Turbine – Diagram, Working, Parts, Advantages, Applications
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