TAILIEUCHUNG - GAS TURBINES P2

Like other internal combustion engines, the gas turbine requires an outside source of starting power. This is provided by an electrical motor or diesel engine connected through a gear box to the shaft of the gas turbine (the high-pressure shaft in a multishaft configuration). Other devices can be used, including the generator of large electric utility gas turbines, by using a variable frequency power supply. | 1740 GAS TURBINES Gas Turbine Operation Like other internal combustion engines the gas turbine requires an outside source of starting power. This is provided by an electrical motor or diesel engine connected through a gear box to the shaft of the gas turbine the high-pressure shaft in a multishaft configuration . Other devices can be used including the generator of large electric utility gas turbines by using a variable frequency power supply. Power is normally required to rotate the rotor past the gas turbine s ignition speed of 10-15 on to 40-80 of rated speed where the gas turbine is self-sustaining meaning the turbine produces sufficient work to power the compressor and overcome bearing friction drag and so on. Below selfsustaining speed the component efficiencies of the compressor and turbine are too low to reach or exceed this equilibrium. When the operator initiates the starting sequence of a gas turbine the control system acts by starting auxiliaries such as those that provide lubrication and the monitoring of sensors provided to ensure a successful start. The control system then calls for application of torque to the shaft by the starting means. In many industrial and utility applications the rotor must be rotated for a period of time to purge the flow path of unbumed fuel that may have collected there. This is a safety precaution. Thereafter the light-off speed is achieved and ignition takes place and is confirmed by sensors. Ignition is provided by either a sparkplug type device or by an LP gas torch built into the combustor. Fuel flow is then increased to increase the rotor speed. In large gas turbines a warmup period of one minute or so is required at approximately 20 speed. The starting means remains engaged since the gas turbine has not reached its self-sustaining speed. This reduces the thermal gradients experienced by some of the turbine components and extends their low cycle fatigue life. The fuel flow is again increased to bring the rotor

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