Gas Turbine Handbook : Principles and Practices

Gas Turbine Control 79
• Open the process compressor recycle valve (if applicable)
• Actuate the gas turbine starter
• Actuate the gas turbine starter time-out clock
• Energize the ignition system
• Actuate the ignition system time-out clock
• Open the fuel valve to preset point
• Stop the gas turbine starter when starter time-out clock expires
• De-energize the ignition system when the time-out clock expires.
When the start sequence is complete the gas turbine will have
reached self-sustaining speed. At this time control can be, and usually
is, handed over to the Routine Operation Controller. This controller
will maintain stable operation until it receives an input (from the
operator, or the process) to load the unit. Prior to initiating loading
the unit, control is turned back over to the Sequence Controller
to properly position inlet and discharge valves, electrical breakers,
etc. On electric generator drives this is the point that the automatic
synchronizer is activated to synchronize the unit to the electric grid.
When these steps are completed, control is again turned back over
to the Routine Operation Controller and the speed control governor,
acceleration scheduler, temperature limit controller, and pressure
limit controller all come into play. Controlling the gas turbine during
steady state operation is a major function of the control system. However,
while the control operates in this mode most of the time, it is the
other modes of operation that are most critical. These are Starting,
Stopping, Increasing Power and Decreasing Power. The direct result
of varying fuel flow is higher or lower combustion temperatures. As
fuel flow is increased, combustor heat and pressure increase and
heat energy to the turbine is increased. Part of this increased energy
is used by the compressor-turbine to increase speed which, in turn,
causes the compressor to increase airflow and pressure. The remaining
heat energy is used by the power extraction turbine to produce
more shaft horsepower. This cycle continues until the desired shaft
horsepower or preset parameter limit (temperature, speed, etc.) is
reached.
Similarly, to reduce shaft horsepower, the control starts by reducing
fuel flow. The lower fuel flow reduces combustion heat and
pressure and reduces the heat energy available to the compressorturbine.
With less available energy the compressor-turbine slows