Gas Turbine Handbook : Principles and Practices

202 Gas Turbine Handbook: Principles and Practices
FUEL CONTROL PROBLEMS
Starting
Common starting problems are “hot starts” and “hung starts.”
Hot starts are so called because they produce excessive exhaust gas
or turbine inlet temperatures. Sometimes hot starts are also associated
with compressor surge. Hot starts are caused by too rich a fuel
schedule. A “too rich” fuel schedule will rapidly increase the temperature,
but drive the gas generator-compressor into surge. Other causes
might be bleed valve malfunction or variable geometry malfunction or
an “out of calibration” condition. Hot starts may also be the result of
FOD in the compressor. The hung start is evidenced by a very slow
acceleration and a slow increase in exhaust gas or turbine inlet temperature.
Usually it results from insufficient starter motor torque or
too lean a fuel schedule.
Failure to start due to a fuel mixture that is too lean or too rich
is not an unusual problem. To determine that the fuel in the combustor
has ignited, manufacturers utilize turbine temperature measuring
sensors ( thermocouples). Some gas turbine manufacturers also install
ultraviolet (UV) flame detectors in the combustor. Depending on the
method of control, the thermocouples or UV detectors may directly activate
a fuel shutoff valve or the signal may be processed through the
main control system. If complete shutoff is not achieved, it is probable
that excess fuel will accumulate in the combustor and turbine. After
repeated start attempts this excess fuel could produce a “hot start”
situation.
A failed fuel shutoff valve spring was the cause of a turbine
failure I investigated several years ago. This failure went undetected
until the unit was shutdown. Without a tight shutoff, fuel gas leaked
into the gas turbine over a period of several days. When operations
attempted to bring the unit back on line, the first two attempts to
start failed (the fuel gas mixture was too rich). On the third start
attempt enough air had been mixed with the fuel to reduce the fuel
mixture to the upper end of the explosive limit. The explosive mixture
spread from the combustor through the turbine and into the exhaust
duct. When the mixture ignited it resulted in an exhaust duct explosion
with most of the damage occurring at the exhaust duct elbow
section.