GOVERNING OF STEAM TURBINES

GOVERNING OF STEAM TURBINES

Requirement of governing system:-

• Controlling and regulating of a turbine according to variable demand.
• The regulation of turbine is done by maintaining control of speed, pressure and flow by the help of governors.
• Governing of turbine maintains the speed of turbine at constant level irrespective of load.

Governing of steam turbine is classified according to the mechanism 

(i) Throttle governing

(ii) Nozzle control governing

(iii) By-pass governing

(iv) Combined governing (i.e., Combination of above as throttle and nozzle control governing or ‘throttle and by-pass governing).

(i) Throttle Governing: 

Throttle governing of steam turbine bases upon the throttling of steam up to suitable pressure and regulate the mass flow of steam through the turbine so as to control the output of turbine. 

Here in throttle governing, aim is to alter the mass flow rate as for example by reducing it, the available energy gets reduced and hence lowering of output. 

In throttle governing the steam entering is regulated by opening and closing of valve. 

As the valve is closed, the throttling or constant enthalpy process occurs across the valve with an increase in entropy and corresponding decrease in availability of energy per unit mass flow of steam.

 Also due to throttling the state of steam entering turbine stage gets modified and the modified expansion line for each load is obtained. It may be noted that even when the governor valve is full open the pressure drop does occur and thus it can be said that throttling is evident at all loads on turbine.

Throttle governing offers following disadvantages due to throttle action at inlet:

(a) Throttling increases initial superheat at inlet and the greatest variations in steam velocity occur in the later stages. 

(b) The wetness of steam gets reduced in later stages due to throttling. Due to this reduced wetness there occurs reduction in stage efficiency at part load operation of turbine.

(ii) Nozzle Control Governing: 

In Nozzle control governing steam flowing through nozzles is regulated by control valves. 

Nozzle control governing is generally employed at first stage of turbine due to practical limitations.

 The nozzle areas in remaining stages remain constant.

In nozzle governing the nozzles of turbines are grouped in two, three or more groups upto six or eight groups. 

When nozzle governing is employed then the pressure and temperature of steam entering first stage nozzles are independent of load.

"As the valves are being regulated for actuating nozzle control governing so there occurs some throttling of steam at each valve."

(iii) By Pass Governing: 

In case of by-pass governing arrangement is made for by-passing surplus quantity of steam without allowing total steam quantity to contribute in turbine output when load reduces. 

Diagram shows that steam from main line enters the main valve which is controlled by speed governor. Steam from main valve enters the nozzle box or steam chest. By pass valve is also provided on the nozzle box. By pass valve is connected to a passage which delivers steam being by passed to later end of turbine. By pass valve is actuated when load varies, thus allowing only part of steam entering main valve to contribute in power output. By pass valve is controlled by speed governor for all loads within its’ range. In this kind of governing depending upon turbine and its’ application there may be more than one by pass valves. 

(iv) Combined governing: Some times when the governing requirements are not met by any one kind of arrangements of governing i.e. throttle, nozzle control and by pass governing, then the combination of two governing mechanisms may be employed. These popular combinations are ‘throttle and nozzle control combined governing’ and ‘throttle and by pass combined governing’.