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Gas Turbine Control 83
Figure 5-7 Control map.
Depicted is a typical control map for a single spool gas turbine. The parameters
used for this map are fuel flow (Wf) divided by burner pressure
(Pb), which represents the fuel-air ratio, and rotor speed (N). The upper
portion of the map represents maximum allowable operating conditions to
avoid compressor surge while attaining the maximum rate of acceleration.
The extreme right line represents the exhaust gas temperature or turbine
inlet temperature limit, which is a function of turbine disc creep temperature.
On aircraft engines, this is trimmed in as a function of engine pressure
ratio and exhaust gas temperature for standard ambient conditions.
The “governor” lines indicate the infinite number of conditions between
idle and maximum temperature limit at which the gas turbine can operate.
On jet engines this is a function of thrust or power lever angle. On
industrial engines it is a function of load. The lower split-lines represent
the gas turbine operating line for steady state conditions. The lowest line
is the deceleration limit function . This function permits rapid deceleration
to a lower power or idle without flame-out or loss of combustion. The
type of governor depicted is a speed-droop governor and is in common use
on aircraft engines and some mechanical drive gas turbines. An isochronous,
or zero speed-droop, governor is used almost exclusively on power
generating gas turbines.