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Gas Turbine Control 81
schedule output and the fuel valve starts to approach its final running
position. During this excursion the temperature limit controller and
the pressure limit controller monitor temperatures and pressures to
ensure that the preset levels are not exceeded. The temperature limit
controller for turbine inlet temperature monitors the average of several
thermocouples taking temperature measurements in the same
plane. It is possible for the temperatures throughout this measurement
plane to vary significantly while the average remains the same.
This situation is dealt with in the Protection Controller, discussed
later. Should the temperature (or pressure) reach its set point the
limit controller will override the governor controller and maintain
operation at a constant temperature (or pressure). These different
controllers are always operating but the output signal to the fuel valve
only reflects the controller signal requiring the minimum fuel flow.
Considering the control maps (Figures 5-6 and 5-7), the operating
point will move along a locus of points that define the operating
line for various load conditions. The number of operating lines and
the number of governor lines are infinite. However, when the operating
point intercepts a pressure, temperature, speed limit, or acceleration
schedule limit condition, the gas turbine cannot operate beyond
that point. Barring any other restrictions, the gas turbine would be
able to operate at any load within the operating envelope.
The Protection Controller continuously checks speed, temperature,
and vibration for levels that may be detrimental to the operation,
the unit, and personnel. Usually two levels are set for each parameter,
an alarm level and a shutdown level. When the alarm level
is reached the system will provide an audible warning to the operator
that there is a problem. If the transition from alarm to shutdown condition
takes place so rapidly that operation response is not possible,
the unit is automatically shutdown. Overspeed is one of the parameters
monitored by the protection controller that does not include an
alarm signal. Overspeed* of the compressor, compressor-turbine, or
power extraction turbine could result from excessive fuel flow or loss
of load (as a result of coupling failure).
*The turbine manufacturer has determined the yield and burst speed of each
disc. Based on this information he has defined a speed above which the integrity
of the disc is compromised. Whenever possible overspeed is set at a point
below the disc yield point. When this is not possible, following an overspeed
situation, the unit must be overhauled and the affected disc replaced.