By - Pass Governing Of Steam Turbines

This method is adopted in modern high - pressure impulse steam turbines which contain a large number of stages of small mean diameter in high pressure stages. Such turbines are usually designed for a definite load known as economic load at which the efficiency is maximum. This load is taken as about 80% of the maximum continuous rating. According to the principle of by-pass governing , some extra quantity of steam is by - passed to the far down stages of the turbine when the load exceeds the economic load.

Nozzle control governing is not preferable because of small enthalpy drop in the first stage of a high- pressure turbine. Further , incase of higher loads the extra steam required cannot be admitted through additional nozzles in the first stage due to many practical reasons. Those difficulties are overcome by using by-pass governing.

Steam through a throttle valve enters the nozzle box or steam chest. The throttle valve is controlled by a speed regulator or governor. In general , up to economic load, the governing of steam turbine speed is done by throttling. For loads greater than the economical load , a bypass line is provided in such a way that steam passes directly from the first stage nozzle box into a latter stage. The by-pass of steam is automatically controlled by the lift of the valve. This valve is controlled by the speed governor for different loads within this range.

By-pass governing is most suitable for reaction turbine and a single by-pass valve.

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THROTTLE GOVERNING OF STEAM TURBINES

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