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80 Gas Turbine Handbook: Principles and Practices
down, thereby lowering the compressor speed as well as airflow and
pressure. The downward spiral continues until the desired shaft
horsepower is reached.
While it is possible to increase and decrease shaft horsepower
by making minute changes in fuel flow, this method is time consuming.
On the one hand, if the fuel flow is increased too rapidly than
excessive combustor heat is generated and either the turbine inlet
temperature will be exceeded or the increase in speed will drive
the compressor into surge, or both. On the other hand if the fuel
flow is decreased too rapidly then the reduction in fuel flow could
be faster than the rate at which the compressor will reduce airflow
and pressure and result in a flame-out or compressor surge (as speed
decreases the compressor operating point moves closer to the surge
line). High turbine inlet temperature will shorten the life of the turbine
blades and nozzles, and compressor surge could severely damage
the compressor blades and stators (and possibly the rest of the gas
turbine). Flame-out as a result of power decease takes its toll in the
long run because of the thermal stresses created with each shutdown
and re-start.
The control must also guard against surge during rapid power
changes, start-up, and periods of operation when compressor inlet
temperature is low or drops rapidly. Note: the gas turbine is more
susceptible to surge at low compressor inlet temperatures. Normally
changes in ambient temperature are slow compared to the response
time of the gas turbine control system. However, the temperature
range from 28°F (–2.0°C) to 42°F (6.0°C) accompanied with high humidity
is of major concern. Operation in this range has resulted in ice
formation in the plenum upstream of the compressor. To address this
problem, anti-icing schemes have been employed to increase the sensible
heat by introducing hot air into the inlet. Anti-icing, therefore,
becomes another control function that must address both the problem
of ice formation, as well as the effect temperature changes have on
the compressor relative to surge.
The acceleration schedule facilitates loading the unit as quickly
as possible. Without the acceleration schedule it would be very time
consuming to increase the load from the Idle-No-Load position to the
Full-Load position. As prescribed by the acceleration schedule, the
fuel valve is opened and loading is initiated. As load approaches its
target set point the speed governor starts to override the acceleration