Gas Turbine Handbook : Principles and Practices

Gas Turbine Control 73
There are as many variations in controls as there are control
manufacturers, gas turbines, and gas turbine applications. Controls
can be divided into several groups: hydromechanical (pneumatic or
hydraulic), electrical (hard wired relay logic), and computer (programmable
logic controller or microprocessor) 1 . The hydromechanical type
controls consist of cams, servos, speed (fly-ball) governors, sleeve and
pilot valves, metering valves, temperature sensing bellows, etc. (Figure
5-4).
Electrical type controls consist of electrical amplifiers, relays,
switches, solenoids, timers, tachometers, converters, thermocouples,
etc. Computer controls incorporate many of the electrical functions
such as amplifiers, relays, switches, and timers within the central
processing unit (CPU). These functions are easily programmed in
the CPU and they can be just as easily reprogrammed. This flexibility
in modifying all or part of a program is especially useful to the
user/operator in the field. Analog signals such as temperature, pressure,
vibration, and speed are converted to digital signals before they
are processed by the CPU. Also the output signals to the fuel valve,
variable geometry actuator, bleed valve, anti-icing valve, etc. must be
converted from digital to analog.
Figure 5-1. Simplified gas turbine-generator control system.