Page 352 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Gas Turbine Best Practices Be st Practice 6.2

Best
Best Practice 6.2Practice 6.2Practice 6.2
Best
Accurately define site conditions to be sure that sufficient Insufficient driver power will restrict maximum possible pump or
driver power will be available at site. compressor flow rates and generated power. The associated revenue
Gas turbine site power is determined by elevation, temperature, losses can exceed over $100 MM over the life of the plant.
inlet conditions, outlet conditions, humidity and fuel conditions.
Confirm in the design phase that all site conditions are correct, and Benchmarks
confirm ambient temperature conditions for high sites, and an accurate This best practice has been used since 1990 to ensure that sufficient
fuel gas analysis. Consult other end users in the area to confirm that the gas turbine power is available at site conditions. A combination of
anticipated conditions are correct. rigorous checks of anticipated site conditions, consultation with other
Perform a life cycle cost analysis to determine possible lost revenue end users in the geographic area and a life cycle cost analysis to justify
costs that would arise from using an undersized driver, in order to a larger driver selection, if warranted, has been performed to result in
justify a larger power driver. maximum driver reliability and product revenue.
Lessons Learned
Failure to consider actual site conditions and fuel gas
composition has resulted in power deficient gas turbines
that reduce product revenue for the life of the plant.

B.P. 6.2. Supporting Material both inlet and exhaust conditions are ‘open’ to the atmosphere
and vary with atmospheric conditions.
The steam turbine is an external combustion engine since the
Gas Turbine Performance hot vapor is produced external to the engine. The steam turbine
cycle is closed, in that both inlet and exhaust conditions are
A gas turbine is a dynamic internal combustion engine. When we controlled by the steam generation system (boiler), therefore
compare the performance of a gas turbine to that of a steam steam turbine conditions are constant and do not vary.
turbine, it becomes immediately evident that steam turbine Figure 6.2.2 presents performance parameters for steam
performance is much easier to calculate, since both the vapor turbines. Since inlet and exhaust conditions are controlled and
and the vapor conditions are fixed. For a gas turbine, the vapor the steam turbine is an external combustion engine, steam rate
condition depends on the type of fuel used and the atmospheric and external efficiency can be used to express performances.
conditions. This is because the inlet to the gas turbine engine is Since the gas turbine Brayton cycle is open, vapor conditions
from the atmosphere, and any change in temperature, humidity are variable and performance must be expressed as:
or pressure will affect the mass flow into, and consequently the
power produced by the gas turbine. The gas turbine cycle - Heat rate
(Brayton) is open. - Thermal efficiency
As a result, steam turbine performance can be expressed - Fuel rate
rather easily in terms of steam rate (pounds of steam per These facts are shown in Figure 6.2.3.
horsepower or kilowatt hour) and external efficiency. Since the
gas turbine vapor conditions are variable however, its perfor-
mance must be expressed in terms of heat rate, BTUs per Gas turbine ISO conditions
horsepower or kilowatt hour, thermal efficiency and fuel rate.
All of the above also must be expressed in standard terms. Since gas turbine performance varies as a function of fuel and
A set of standardized conditions has been established by ISO inlet conditions, a set of standard conditions has been estab-
(The International Standards Organization) to rate all gas tur- lished by the International Standards Organization to define gas
bines. We will discuss the various ISO standard requirements, turbine performance. These facts are presented in Figure 6.2.4.
and how a site rating is obtained by using vendor ISO derating Gas turbine vendors publish their performance data in terms
data for each turbine design. A performance example for an of ISO power rating and ISO heat rate. Typical vendor data is
actual gas turbine will be presented, and the effect of varying shown in Figure 6.2.5.
inlet conditions (temperature, pressure and humidity) on per-
formance will also be explored. Finally, the exhaust gas com-
position will be discussed, and the emission products examined. Site rating correction factors
In addition, various alternatives for meeting local emission re-
quirements will be presented and discussed. Gas turbine site performance is directly affected by inlet air
Figure 6.2.1 presents a comparison between gas turbine and density and air environmental conditions as shown in
steam turbine performance. Figures 6.2.6, 6.2.7 and 6.2.8 respectively.
A gas turbine is an internal combustion engine in that the hot Since produced power and heat rate vary as a function of inlet
vapor is produced internal to the engine. The cycle is open, since temperature, pressure and inlet duct and exhaust duct pressure

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