Gas Turbine Handbook : Principles and Practices

82 Gas Turbine Handbook: Principles and Practices
Figure 5-6. Control map.
Depicted is a typical control map for a dual-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 power lever angle. On industrial engines it is a function of
load. The lower line represents 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 split-shaft, mechanical drive
gas turbines. An isochronous, or zero speed-droop, governor is used almost
exclusively on power generating gas turbines.