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74 Gas Turbine Handbook: Principles and Practices
Figure 5-2. Temperature-power-speed interrelationships.
From the turn of the century through the late 1970s, control
systems operated only in real time with no ability to store or retrieve
data. Hydromechanical controls had to be calibrated frequently
(weekly in some applications) and were subject to contamination and
deterioration due to wear. A requirement for multiple outputs (i.e.,
fuel flow control and compressor bleed-air flow-control) required completely
independent control loops. Coordinating the output of multiple
loops, through cascade control, was a difficult task and often resulted
in a compromise between accuracy and response time. In addition,
many of the tasks had to be performed manually. For example, station
valves, prelube pumps, and cooling water pumps were manually
placed into the running position prior to starting the gas generator.
Also protection devices were limited. The margin between temperature
control set points and safe operating turbine temperatures was
necessarily large because the hydromechanical controls could not react
quick enough to limit high turbine temperatures, or to shutdown
the gas generator, before damage would occur.