Power Plant Engineering

9.7.2 PROTECTIVE CONTROLS
The objective of the protective control is to ensure adequate protection for the turbine in prevent-
ing its operation under adverse conditions. Whenever, unsafe operating conditions are approached, the
prime control is overtaken by the protective control to protect the turbine or driven equipment.

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POWER PLANT ENGINEERING
Basically, the protective control is of two types:
1. Shutdown control and
2. Modulating control
Outer
inputs
Ped
Orifice
Main fuel
control
Fuel supply
By-pass
Fuel
valve
C
C C
T
Temp
sensor
Exhaust
Reference
Amplified
error signal
Alternate
temp control
Vent
Pneumatic
valve
Fig. 9.23. Turbine Over temperature Control.
1. Shut Down Control
The shut down type control detects a condition which can cause a serious malfunction and actu-
ate the shut-off valve to stop the turbine following are the various types of shut down controls.
(a) Turbine over temperature,
(b) Turbine over speed,
(c) Low lube oil pressure,
(d) High lube oil temperature, and
(e) excess vibration.
(a) Turbine Over temperature. Fig. 9.23 shows a turbine overtemperature control. Turbine
temperature may be well sensed at the turbine inlet but the sensing device put at the turbine inlet
goes wrong and therefore it is sensed at the turbine exhaust, which is also a indication of turbine
inlet temperature. The temperature sensor may be a thermocouple, bimetal or mercury vapor. As
soon as the turbine inlet/exhaust temperature increases a predetermined value, the relay system acts
upon the shut-off valve and shut down the turbine by stopping the supply of fuel completely to the
combustion chamber.