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Accessories 89
Air or gas must be available at approximately 100 psig and in
sufficient quantity to sustain starter operation until the gas generator
exceeds self-sustaining speeds. Where a continuous source of air
or gas is not available, banks of high and low pressure receivers and a
small positive displacement compressor can provide sufficient air for
a limited number of start attempts. As a rule of thumb, the starting
system should be capable of three successive start attempts before
the air supply system must be recharged.
In gas pipeline applications, the pneumatic starter can use pipeline
gas as the source of power. In these applications it is critical that
the starter seals are leakproof and the area is well ventilated.
Hydraulic Motors
Hydraulic pumps often provide the power (motive force) to drive
hydraulic motors or hydraulic impulse turbine ( Pelton Wheel) starters.
Hydraulic systems are often used with aeroderivative gas turbines
as they are easily adaptable to the existing hydraulic systems.
Hydraulic systems offer many advantages such as small size, light
weight, and high time between overhaul.
Diesel Motors
Due to their large mass moment of inertia, heavy frame (25,000
SHP and above) gas turbines require high torque, high time starting
systems. Since many of these units are single shaft machines, the
starting torque must be sufficient to overcome the mass of the gas
turbine and the driven load. Diesel motors are the starters of choice
for these large gas turbines. Since diesel motors cannot operate at
gas turbine speeds, a speed increaser gearbox is necessary to boost
diesel motor starter speed to gas turbine speed. Diesel starters are
almost always connected to the compressor shaft. Besides the speed
increaser gearbox, a clutch mechanism must be installed to insure
that the diesel motor starter can be disengaged from the gas turbine.
Advantages of the diesel motors are that they are highly reliable and
they can run on the same fuel as the gas turbine, eliminating the
need for separate fuel supplies.
Small Gas Turbines
Small gas turbines are used to provide the power to drive either
pneumatic or hydraulic starters. In the aircraft industry a combus-