Page 64 - Offshore Electrical Engineering Manual

Gas Turbines 51

Although these unpredictable phenomena are avoided as much as possible by good
process design, it is worth considering some means of catering for them such as one
or more of the remedies described in the following.

1. A small separator or knockout pot may be located close to the engine intake
to remove condensate. If the supply pipe is long, it will be necessary to pro-
vide this anyway in order to remove condensate which has condensed in the
pipe.
2. The engine combustion system may be modified for dual fuel operation. When
set up properly, this system can automatically transfer from gas to diesel com-
bustion during fuel gas disturbances with negligible effect on engine power
output.
3. If there is a problem with fluctuations in the calorific value of the gas to the
extent that the electrical load cannot be met on a downward fluctuation, it
may be necessary to install a fast-acting automatic load shedding system, as
described in PART 2 Chapter 15.

TURBINE TEMPERATURE LIMITS
Gas turbines depend for their cooling on the vast quantity of air which passes
through them, in other words high air mass flow. A basic design limitation on the
operation of any gas turbine is the operating temperature of the power turbine
blades. Disastrous changes in their mechanical properties will occur should they
get too hot. By monitoring the exhaust gas temperature using a sophisticated con-
trol system, such as the GE ‘Speedtronic’ or the Ruston ‘Rustronic’ governor, it is
possible to bring the engines up to power automatically and to continuously control
the supply of fuel in such a way that the maximum exhaust temperature is never
exceeded.
As the cooling effect of the incoming air is proportional to its density and
temperature, the altitude and ambient air temperature have a very significant
effect on the available output power. Altitude, of course, is fixed, but if the engine
is running close to its rated power output then a small increase in ambient air
temperature may cause the engine control system to limit the fuel supply in order
to prevent the maximum exhaust temperature being exceeded. As it is unlikely
that the generator load would have been reduced at the same time, the effect of
this fuel supply reduction is for the generator set to begin slowing down until an
underfrequency trip occurs. Even in the North Sea, warm weather conditions can
sometimes reduce turbine power output capability below the required rating. The
problem is often exacerbated by the poor location of combustion air intakes and
exhausts.
The following must be considered:

1. Location of intakes away from any engine exhaust. Give these, including that of
the engine under consideration, as wide a berth as possible.