Steam turbine governing is to ensure that the turbine speed constant irrespective of steam turbine load conditions. The steam turbine governor controls the steam entry to the turbine to maintain the steam turbine speed. The principle methods of steam turbine governing are as follows:
- Nozzle Governing
- Throttle Governing
- By-pass governing
Throttle Governing is the most widely used method in steam turbine governing for smaller turbines where the steam flow is less. As the name indicates. in throttle governing the governor throttles the inlet steam flow through the turbine according to the steam turbine steam. When there is a load reduction than the desired economic load, then the speed increases to control this the governor closes the governor valve. This results in decreasing the steam flow reduces the steam turbine speed.
The figure shows the schematic of throttle governing. The inlet stop valve (SV) admits the full steam. This steam is controlled by a double beat valve (CV) which is carried out by a servo motor controlled by a centrifugal governor. If the steam turbine gains speed, the valve (CV) closes to throttle the steam ans reduces the supply to the nozzle.
In throttle governing the relation between the steam consumption and load is given by the willan’s line.
The above chat indicates that the efficiency of the turbine decreases at low loads in throttle governing. So this type of governing is in-efficient for low loads.
Nozzle governing is an alternative efficient method of steam turbine governing. Simple schematic of nozzle governing of steam turbine are shown in below figure. In this method of governing, the nozzles are grouped together as 3 or 5 and the supply of steam through this nozzles are controlled by regulating valves. Under full full load conditions all the regulating valves of nozzles are fully open.
When the load on turbine changes or deviates from the design valve, the supply of steam through one or some group of nozzles are regulated or fully closed to maintain the speed of the turbine.
Nozzle governing control can be only applied to the first stage of a turbine. It is suitable for impulse turbines and larger units which have impulse state followed by an impulse reaction stage. In this there will be a steam pressure drop at first stage outlet to second stage because of the nozzles cut down.
The above two methods of steam turbine governing i.e. nozzle governing and throttle governing controls the steam at inlet of the first stage or high pressure stage of turbine. When the load is greater than desired load, the additional steam required could not pass through the first stage since additional nozzles are not available.
Bypass regulation governing, the steam is by-passed through a valve to the lower stage turbines which results in increasing the turbine power. Once the bypass valve opens it is under control of turbine governor to control the speed of the turbine. Because if it not controlled by the governor it may allow more steam to lower stage which results in increase in turbine speed.
The secondary and tertiary supplies of steam increases the output of turbine power but the efficiency of the turbine is still less. In reaction turbines, because of the pressure drop required in the turbine balding, nozzles control governing is not possible and throttle governing plus bypass governing is used.
Comparison Table of Throttle and Nozzle Control Governing:
|Parameter||Throttle Governing||Nozzle governing|
|Throttling Losses||Very High||No throttling losses (partially|
|Heat drop available||Lesser||Larger|
|suitability||Small turbines||Medium and larger turbines|
|Use||Used in impulse and reaction turbines both||Used in impulse turbine and also in reaction turbine|