GAS TURBINE DRIVEN GENERATORS 39this affects the maintenance staff and total noise level permitted by international or national standards.
The effects of a silencer are similar to a filter since the silencing elements cause a pressure drop.
With offshore platforms it is not always practical to locate the main generators in a good
place regarding the position and routing of the inlet and exhaust ducting. Long runs of ducting are
sometimes unavoidable. It is then necessary to allow a derating factor for the pressure drop that
will occur. The manufacturer should be consulted for advice on this aspect. For a typical offshore
or onshore situation with a reasonable degree of silencing a rule-of-thumb derating factor would be
98%. In a poor location assume 95%.
2.4 Starting Methods for Gas Turbines
Gas turbines are usually started by a DC motor or an air motor. Either system is available for
most turbines up to about 20 MW. Occasionally AC motors are used. Beyond 20 MW, when heavy
industrial machines tend to be used, it becomes more practical to use air motors or even diesel engine
starters. DC motors require a powerful battery system. The DC motor and battery systems tend to
be more reliable and less space consuming, which is important for offshore systems. Air motors
require air receivers and compressors. The compressors require AC motors or diesel engines. Air
start and diesel start systems are more popular for onshore plants especially remote plants, e.g. in
the desert. This is partly due to the fact that batteries tend to suffer from poor maintenance in hot,
dry locations. Air systems require regular maintenance and must be kept fully charged in readiness
for a quick start. Air system receivers can become very large if more than three successive starting
attempts are required. More starts can probably be obtained by a battery system that occupies the
same physical space.
Occasionally process gas can be used instead of air to drive the air/gas starter motor. This
eliminates the need for receivers and compressors. However, there should always be a reliable source
of gas available. The exhaust gas from the starter motor should be safely discharged e.g. into a
ventilating pipeline.
2.5 Speed Governing of Gas Turbines
2.5.1 Open-loop speed-torque characteristic
The ungoverned or open-loop speed-torque characteristic of a gas turbine has a very steep negative
slope and is unsuitable for regulating the power output of the generator. The open-loop characteristic
is explicitly determined by the thermodynamic design of the gas turbine, together with the mechani-
cal inertial and frictional characteristics of the rotating masses. Without closed-loop feedback control
action the initial decline in speed in response to an increase in shaft torque would be mainly deter-
mined by the shaft inertia. LetT,ωandP be the torque, speed and shaft power respectively in
per-unit terms. The expression relating these variables is,
P=Tω ( 2. 45 )The open-loop speed-torque function may be expressed as,
ω=f(T) ( 2. 46 )