Why choose an aeroderivative gas turbine over other units?

September 12 2013 - William E. Forsthoffer

Aeroderivative gas turbines (initially designed for flight and using anti-friction bearings) have been thought to have lower MTBFs than industrial types. End user fleet experience has shown anti-friction bearing life to exceed 100 months. One should always consider aeroderivative gas generator/industrial power turbine (hybrid type) gas turbine units.

The use of an aeroderivative gas generator while using an industrial type power turbine (with hydrodynamic bearings) has the following advantages:

  • Shorter start-up and shut down time
  • Equal or greater bearing MTBF than industrial type gas turbines
  • Module replacement of the gas generator (as low as 36 hours guaranteed) without the necessity for gas generator to power turbine alignment

The gas turbine is the most misunderstood rotating equipment item. Because of its many support systems and various configurations, the gas turbine is often approached with mystery and confusion. In order to thoroughly explain the gas turbine from a functional standpoint, comparing it to a steam turbine and an automotive engine in terms of its combustion cycle is necessary.

Comparison to a steam turbine

The major difference between a typical condensing steam turbine and an industrial gas turbine is that a steam turbine is an external combustion engine, whereas a gas turbine is an internal combustion engine. That is, the motive fluid for a steam turbine is generated external (in the boiler) to the engine. In the case of a gas turbine, the motive fluid is generated internal to the engine (air compressor and combustor).

The steam turbine cycle is known as the ‘Rankine cycle’. The hot vapor is generated in the boiler, which is external to the steam turbine (expander). The gas turbine cycle is known as the ‘Brayton cycle’. Here, air is brought into the engine by the axial compressor, and combined with fuel and an ignition source in the combustor to produce a hot vapor, which then is expanded through the HP (high pressure) turbine. The combination of the compressor, combustor and HP turbine is commonly known as the gas generator. This is because its function is to generate or produce a hot vapor from the combination of an air and fuel mixture.

Essentially, a gas generator can be considered to have the same function as a boiler – both produce a hot vapor. After the hot vapor is generated, it is then expanded additionally in the power turbine. The power turbine, therefore, serves exactly the same function as the steam turbine. That is, both components are hot gas expanders.

Comparison to an automotive engine

An automotive engine is a positive displacement internal combustion engine, having an intake, compression, combustion and exhaust stroke. A gas turbine engine is a dynamic internal combustion engine. The process in this case is continuous – and not intermittent as is the case for the automotive engine. Both engines have compression, combustion and exhaust sections. The two are similar in that both require starters, ignition sources, inlet air filters, inlet fuel systems, cooling systems and monitoring systems.

Industrial gas turbines are used for mechanical drive applications based on the pre-conceived idea that they have a higher reliability than aeroderivative/industrial power turbine units. There has finally been a realization by end users that greater MTTR times, longer start-up and cool-down times for industrial types outweigh any reliability advantages for the industrial – if one actually exists.

The use of hybrid (aeroderivative/industrial) gas turbines, for mechanical dive and generator applications above 15 MW site power rating, has been recommended in project design phases since 1990.