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:
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:
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.
By-pass Governing:
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 |
Admission losses | Low | High |
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 |
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