Page 343 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 6 .1 Gas Turbine Best Practices

Best
Best
Best Practice 6.1Practice 6.1Practice 6.1
Always consider aero-derivative gas generator/industrial have a higher reliability than aero-derivative/industrial
power turbine (hybrid type) gas turbine units. power turbine units. There has finally been a realization
Aero-derivative gas turbines (initially designed for flight and using by end users that greater MTTR times, longer start-up
anti-friction bearings) have been thought to have lower MTBFs than and cool-down times for industrial types outweigh any
industrial types. End user fleet experience has shown anti-friction reliability advantages for the industrial e if one actually
bearing life to exceed 100 months.
exists.
The use of an aero-derivative gas generator while using an industrial
type power turbine (with hydrodynamic bearings) has the following Benchmarks
advantages:
The use of hybrid (aero-derivative/industrial) gas turbines, for me-
Shorter start-up and shut down time
chanical dive and generator applications above 15 MW site power
Equal or greater bearing MTBF than industrial type gas turbines
rating, has been recommended in project design phases since 1990.
Module replacement of the gas generator (as low as 36 hours
guaranteed) without the necessity for gas generator to power
turbine alignment
Lessons Learned
Industrial gas turbines are used for mechanical drive
applications based, on the pre-conceived idea that they

B.P. 6.1. Supporting Material that a steam turbine is an external combustion engine, whereas
a gas turbine is an internal combustion engine. That is, the
In this section, we will discuss functions and types of gas tur- motive fluid for a steam turbine is generated external (in the
bines. In my personal experience, the gas turbine is the most boiler) to the engine. In the case of a gas turbine, the motive
misunderstood rotating equipment item. Because of its many fluid is generated internal to the engine (air compressor and
support systems and various configurations, the gas turbine is combustor).
often approached with mystery and confusion. In order to Figure 6.1.2 compares the gas and steam turbine cycles; the
thoroughly explain the gas turbine from a functional standpoint, latter is known as the ‘Rankine cycle’. As shown, the hot vapor is
we will build on prior knowledge. We will also compare the gas generated in the boiler, which is external to the steam turbine
turbine to an automotive engine in terms of its combustion (expander).
cycle. Having done this, we will then use a building block ap- The gas turbine cycle is known as the ‘Brayton cycle’. Here, air
proach to explain the total configuration of a gas turbine and is brought into the engine by the axial compressor, and combined
conclude with a brief history of its evolution. with fuel and an ignition source in the combustor to produce a hot
Gas turbine classifications will then be presented, vapor, which then is expanded through the HP (high pressure)
specifically: turbine. The combination of the compressor, combustor and HP
turbine is commonly known as the gas generator. This is because
- Design type its function is to generate or produce a hot vapor from the
- Number of shafts combination of an air and fuel mixture. Essentially, a gas
- Drive and number of shafts generator can be considered to have the same function as
- Cycle aboiler e both produce a hot vapor. One can think of the gas
- Drive and location generator as a ‘rotating boiler’. After the hot vapor is generated, it
We will discuss the major design differences between aero- then is expanded additionally in the power turbine. The power
derivative and hybrid (aero-derivative gas generator/industrial turbine, therefore, serves exactly the same function as the steam
power) turbines. Single and multiple shaft gas turbines will be turbine. That is, both components are hot gas expanders.
discussed and reviewed. The three major application cycles for
gas turbines: simple, regenerative, and combined will be
presented and discussed. Comparison to an automotive engine
Finally, we will present applications of different gas turbine
types, and provide information concerning where the different Figure 6.1.3 shows the similarities between an automotive
types are used. engine and a gas turbine. If one considers a gas turbine as only
a dynamic internal combustion engine, understanding it be-
comes significantly easier. As shown in the figure, an automotive
Comparison to a steam turbine engine is a positive displacement internal combustion engine,
having an intake, compression, combustion and exhaust stroke.
Figure 6.1.1 shows a typical condensing steam turbine and an A gas turbine engine is a dynamic internal combustion engine.
industrial gas turbine. The major difference between the two is The process in this case is continuous e and not intermittent as

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