Power Plant Engineering

(b) Turbine Overspeed. The turbine overspeed control is similar to the prime speed control as
shown in Fig. 9.22. In this case, the reference speed is the maximum allowable speed instead of rated
speed. The speed sensor could be a centrifugal governor, a tachometer generator or magnetic pick up. If
the speed increases a certain fixed value, the speed acts upon the relay system to shut down the fuel
valve.
(c) Low Lube Oil Pressure. It is essential to protect the turbine from low lube oil pressure to
ensure proper lubrication and cooling of the turbine bearings. This is accomplished by the use of a
pressure switch in the lube oil supply line. The switch operates an alarm signal or shut down fuel valve
if the oil pressure drops a safe valve.

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(d) High Lube Oil Temperature. High lube oil temperature in the lubrication system is a dan-
gerous signal as it is an indication of low lube oil supply or failure in bearings, gears, etc. This protection
is generally accomplished through the use of a temperature-sensing device immersed in the lube oil. The
sensing device may be a thermal switch, which triggers an alarm or shut down the turbine.
(e) Excessive Vibration. A slight increase in vibration is a cause of warning. Protection against
vibration is accomplished by stalling one or moves vibration pick-ups. The output signal is fed to a
monitoring device, which may shut down the turbine if the vibration increases a certain value.
(2) Modulating Controls
The purpose of modulating control is to sense an impending malfunction or a condition, which
could adversely affect turbine life and make some modification to the operating condition of the turbine
in order to alleviate the undesired conditions. An example of this control may be maximum turbine inlet
temperature and maximum speed. The modulating control is more complex and more costly than the
shutdown control. But it offers and advantage in allowing a turbine or turbine driven plant to continue
operating when normally a shut down occurs. There are some conditions such as high vibrations and
low lube oil pressure which cannot be taken care by corrective measures as in this case shut down of the
turbine is essential.
Modulating controls for maximum turbine inlet temperature and maximum speed are slight modi-
fications for over temperature and overspend controls.