While the majority of natural gas field and transmission compressors are reciprocating machines, a sizable minority are centrifugal compressors driven by gas turbines. Only on rare occasions can electric, steam or deisel oil drives compete with natural gas as compressor fuel in pipeline service.
A gas turbine works on the same principle as a jet engine. Air is compressed (typically to 110 PSIG), and discharged into a combustion zone. Fuel gas is also injected into the combustion zone. The pressurized, burning gas expands as it passes across the blades of a turbine. The turbine serves two functions:
• One or more wheels of the turbine drives the combustion air compressor.
• One or more wheels of the turbine drives the gas compressor.
The major part of the horsepower developed in a gas turbine is consumed by the combustion air compressor. The gas compressor absorbs about one third of the gas turbines power output. Work done by the combustion air compressors is recycled back to the turbine blades via the pressurized combustion air.
An important feature of the gas turbine driven compressor is that the two ends of the machine are not mechanically coupled. This is called a split shaft design; which means that the combustion air compressor and the gas compressor operate at different speeds. This permits the air compressor end to run at a speed consistant with developing full horsepower, while the gas compressor end may be running at a lower speed due to factors such as high discharge pressure.