Gas Turbine Handbook : Principles and Practices

86 Gas Turbine Handbook: Principles and Practices
• engage starter
• purge inlet and exhaust ducts
• energize ignitors
• switch fuel on.
The primary function of the starting system is to accelerate the
gas generator from rest to a speed point just beyond the self-sustaining
speed of the gas generator (Figure 6-1). To accomplish this the
starter must develop enough torque to overcome the drag torque
of the gas generator’s compressor and turbine, any attached loads
including accessories loads, and bearing resistance. The single shaft
gas turbines with directly attached loads (such as electric generators)
represent the highest starting torque as the driven load must also
be accelerated from rest to a speed sufficiently above gas generator
self-sustaining speed. Two shaft gas turbines (consisting of the gas
generator and the driven load connected to the free power turbine)
represent the lowest starting torque requirements. In this case only
the gas generator is rotated.
Another function of the starting systems is to rotate the gas generator,
after shutdown, to hasten cooling. The purge and cool-down
functions have lead to utilization of two-speed starters. The low speed
is used for purge and cooling and the high speed is used to start the
unit. When sizing the starter, the designer should keep in mind that
the gas generator must move 3 to 5 times the volume of the exhaust
stack to insure purging any residual gas from that area. Also control
system programmers and operators should be aware that the purge
time, within the start cycle, is necessary for safe operation.
Gas generators are started by rotating the compressor. This is
accomplished in a number of ways:
• starter directly connected to the compressor shaft
• starter indirectly connected to the compressor shaft via the accessory
gearbox
• impingement air directed into the compressor or compressorturbine.
Devices used to start gas generators include electric ( alternating
current and direct current) motors, pneumatic motors, hydraulic motors,
diesel motors, and small gas turbines.