57.1.5 Gas Turbine Operation
Like other internal combustion engines, the gas turbine requires an outside source of starting power.
This is provided by an electrical motor or diesel engine connected through a gear box to the shaft
of the gas turbine (the high-pressure shaft in a multishaft configuration). Other devices can be used,
including the generator of large electric utility gas turbines, by using a variable frequency power
supply. Power is normally required to rotate the rotor past the gas turbine's ignition speed of 10-15%
on to 40-80% of rated speed where the gas turbine is self-sustaining, meaning the turbine produces
sufficient work to power the compressor and overcome bearing friction, drag, and so on. Below self-
sustaining speed, the component efficiencies of the compressor and turbine are too low to reach or
exceed this equilibrium.
When the operator initiates the starting sequence of a gas turbine, the control system acts by
starting auxiliaries such as those that provide lubrication and the monitoring of sensors provided to
ensure a successful start. The control system then calls for application of torque to the shaft by the
starting means. In many industrial and utility applications, the rotor must be rotated for a period of
time to purge the flow path of unburned fuel that may have collected there. This is a safety precaution.
Thereafter, the light-off speed is achieved and ignition takes place and is confirmed by sensors.
Ignition is provided by either a sparkplug type device or by an LP gas torch built into the combustor.
Fuel flow is then increased to increase the rotor speed. In large gas turbines, a warmup period of
one minute or so is required at approximately 20% speed. The starting means remains engaged, since
the gas turbine has not reached its self-sustaining speed. This reduces the thermal gradients experi-
enced by some of the turbine components and extends their low cycle fatigue life.
The fuel flow is again increased to bring the rotor to self-sustaining speed. For aircraft engines,
this is approximately the idle speed. For power generation applications, the rotor continues to be
accelerated to full speed. In the case of these alternator-driving gas turbines, this is set by the speed
at which the alternator is synchronized with the power grid to which it is to be connected.
Aircraft engines' speed and thrust are interrelated. The fuel flow is increased and decreased to
generate the required thrust. The rotor speed is principally a function of this fuel flow, but also
depends on any variable compressor or exhaust nozzle geometry changes programmed into the control
algorithms. Thrust is set by the pilot to match the current requirements of the aircraft, through takeoff,
climb, cruise, maneuvering, landing, and braking.
At full speed, the power-generation gas turbine and its generator (alternator) must be synchronized