Gas
turbines are important in the chemical process industry. They are often used as
mechanical drivers for large centrifugal and reciprocating compressors which
transport very large volumes of natural gas at high pressures over long
distances.
Gas turbine vendors specify gas turbine power at
standard conditions which are as per the "International Standards
Organization" abbreviated as ISO. This is called the ISO rating of the gas
turbine and is according to the ISO document ISO 3977-2:1997 Gas Turbines --
Procurement -- Part 2: Standard reference conditions and ratings.
Following are these conditions:
Ambient Temperature = 15 deg C
Altitude = 0 m (sea level)
Ambient Pressure = 101.325 kPa (abs)
Relative Humidity = 60%
The standard conditions also specify inlet and
exhaust losses as zero.
What about actual site conditions where the gas
turbine needs to be installed? Gas turbines need to be re-rated for actual site
ambient conditions and also considering inlet and exhaust losses. Power and
Heat Rate of the GT need to be re-rated on account of actual site conditions
and inlet / exhaust losses. These require the application of correction factors
to the ISO power and ISO heat rate of the GT. Today's blog entry provides the
procedure for re-rating (de-rating / up-rating) the ISO power for the actual
site and considering inlet / exhaust losses.
Some graphs and formulas are provided below to
obtain the correction factors:
Power Correction factor for Altitude at Site
(CFalt)
Power Correction factor for Inlet Loss
where:
CFinletloss = correction factor
PDinlet = Inlet Pressure drop,
kPa
Note: The inlet
pressure drop is in the range of 1 to 1.5 kPa (GT vendor can also provide this
data)
Power Correction factor for Exhaust Loss
where:
CFexhaustloss = correction
factor
PDexhaust = Exhaust Pressure
drop, kPa
Note: The
exhaust pressure drop is in the range of 0.5 to 1 kPa (GT vendor can also
provide this data)
Power
Correction factor for Ambient Temperature at Site
where:
CFTamb = correction factor
Tamb = Ambient temperature at
site, ⁰C
Note: The
correction factor for ambient temperature should be calculated for both minimum
average ambient temperature and maximum average ambient temperature of the GT
installation site and applied accordingly for re-rating the gas turbine.
After calculating the correction factors as
above the site re-rated gas turbine power output can be calculated as follows:
GT Site
Power = GT ISO Power*CFalt*CFinletloss*CFexhaustloss*CFTamb
where:
GT Site Power = Re-rated GT Power at actual
site, kW
GT ISO Power = Vendor Specified Power, kW