Introduction to Gas Turbines

General, Functional Description

 A package power plant, as furnished for most installations, is comprised of the single-shaft, simple cycle, heavy duty gas turbine unit driving a generator. Fuel and air are used by the gas turbine unit to produce the shaft horsepower necessary to drive certain accessories and ultimately the driven load generator.

The turbine unit is composed of a starting device, support systems, an axial-flow compressor, combustion system components, a three-stage turbine. Both compressor and turbine are directly connected with an in-line, single-shaft rotor supported by two pressure lubricated bearings. The inlet end of the rotor shaft is coupled to an accessory gear having integral shafts that drive the fuel pump, lubrication pump, and other system components.

About Gas Turbine Units

When the turbine starting system is actuated and the clutch is engaged, ambient air is drawn through the inlet plenum assembly, filtered, then compressed in the 17th stage, axial flow Compressor. For pulsation protection during start-up, the 11th stage extraction valves are open and the variable inlet guide vanes are in the closed position. When the speed relay corresponding to 95 per cent speed actuates, the 11th stage extraction bleed valves close automatically and the variable inlet guide vane actuator energizes to open the inlet guide vanes (I.G.V.) to the normal turbine operating position.

Compressed air from the compressor flows into the annular space surrounding the fourteen combustion chambers, from which it flows into the spaces between the outer combustion casings and the combustion liners.

The fuel nozzles introduce the fuel into each of the fourteen combustion chambers where it mixes with the combustion air and is ignited by both (or one, which is sufficient) of the two spark plugs.

At the instant one or both of the two spark plugs equipped combustion chambers is ignited, the remaining combustion chambers are also ignited by crossfire tubes that connect the reaction zones of the combustion chambers. After the turbine rotor approximates operating speed, combustion chamber pressure causes the spark plugs to retract to remove their electrodes from the hot flame zone.

The hot gases from the combustion chambers expand into the fourteen separate transition pieces attached to the aft end of the combustion chamber liners and flow towards the three stage turbine section of the machine. Each stage consists of a row of fixed nozzles followed by a row of rotatable turbine buckets. In each nozzle row, the kinetic energy of the jet is increased, with an associated pressure drop, and in each following row of moving buckets, a portion of the kinetic energy of the jet is absorbed as useful work on the turbine rotor.

After passing through the 3rd stage buckets, the exhaust gases are directed into the exhaust hood and diffuser which contains a series of turning vanes to turn the gases from the axial direction to a radial direction, thereby minimizing exhaust hood losses. Then, the gases pass into the exhaust plenum.

The resultant shaft rotation is used to turn the generator rotor, and drive certain accessories.

Design Basis

Simple Cycle Gas Turbine Flow Diagram

The turbine unit is composed of a starting device, support systems, an axial-flow compressor, combustion system components, a three-stage turbine. Both compressor and turbine are directly connected with an in-line, single-shaft rotor supported by two pressure lubricated bearings. The inlet end of the rotor shaft is coupled to an accessory gear having integral shafts that drive the fuel pump, lubrication pump, and other system components.