Steam Turbine Governing and its mechanism

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Governing System

Governor is a heart component of the turbine which controls it during steady state and transient conditions. The governor does this function by controlling the steam flow through the turbine by adjusting the control value. The governor is a control component with advanced protections for the turbine which ensure safe operation of the turbine. Governing system of the turbine does the following functions:

  • Controls the turbine speed during start-up or in no load condition to permit the unit to be synchronized with the grid.

  • Controls the turbine load when running in parallel with the grid/generating sets.

  • All protective functions to ensure the safe operation of the unit.

Methods of governing

Depending the final control component of governing system these are divided as nozzle governing and throttle governing.

Nozzle Governing:

Nozzle Governing mechnism of steam turbine

Nozzle Governing mechnism of steam turbine

This type of governing is suitable for simple impulse turbine and most efficiently in part load conditions. In this method the steam flow is regulated by opening or closing some set of nozzles of the turbine. These nozzles are actuated by independent control mechanisms. Depending upon the load demand some no of nozzles will be put in service and some put in out of service. This feature enables the use of nozzle governing for part load operation efficiently.

Nozzle governing of steam turbine with inner architecture

Nozzle governing of steam turbine with inner architecture

For example if the turbine consists of 10 nozzles for full power(100 %). If the load demand has reduced to 60 % then out of 10 nozzles, 4 nozzles can be closed to provide 60% load with out delay. This gives the advantage on throttle governing where the power reduction will happen gradually.

In nozzle governing method, there will not be any importance  to the steam inlet pressure and it is done by the boiler and bypass pressure control system. So the use of nozzle governing is limited to small turbines.

Throttle governing

In this case of governing mechanism, the mass flow rate is controlled by throttling the steam by means of control value. The control value position will be adjusted to allow the required steam for turbine operation. Here the controlling parameter can be turbine speed and load. With latest technologies steam generator pressure control is also incorporated in governing control system. This is more important in nuclear power plants where constant steam pressure is essential for efficient operation.

Throttling sleeve governor of steam turbine

Throttling sleeve governor of steam turbine

For example if the plant is operating at full power and it needs to be operated at 60 % according to the load demand. Then the controller checks the demand with the existing load gives a adjustment signal to control valve. The control valve will slowly throttle to reduce the steam flow for acquiring the load demand.

Due to throttling of control values there will be high energy loss across the control values. Hence the design of value should be adequate to withstand the pressure of the steam.

Comparison between Throttling and Nozzle governing mechanisms 

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