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THROTTLE GOVERNING OF STEAM TURBINES

Steam Pressure at inlet to a steam turbine is reduced by throttling process to maintain the speed of the  turbine constant at part load and hence this method of governing is called "Throttle Governing". 




Construction :

Throttle governing system consists of  a centrifugal governor, a lever , an oil pump , a pilot piston , control valve , a relay piston and a throttle valve. The throttle valve is moved by a relay piston. The relay piston is actuated by pilot piston control valve. There are two piston valves covering ports in the pilot piston control valve without any overlap. These piston valves are operated by lubricating oil supplied by a gear pump at 2 to 4 bar. The oil returns to the drain from this chamber. 

Working :

When the turbine works on full rated load, the throttle valve will remain open. When the load is decreased, the energy output of the turbine becomes in excess and the turbine shaft speed increases. Hence, governor sleeve will lift. The upward movement of the sleeve will lower the control valve rod, opening the lower part to oil pressure and upper port to oil return. 

The relay piston will move the piston and throttle valve towards right which will partially close the area of the nozzle. As a result the steam flow rate into the turbine decreases which in turn brings the speed of turbine to lower range. 

Throttle Governing is mechanically simple but thermodynamically it is inefficient and therefore, this method is used in small machines. 

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