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 (CF _{alt})**

**Power Correction factor for Inlet Loss**

where:

CF

_{inletloss}= correction factor

PDi

_{nlet}= 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:

CF

_{exhaustloss}= correction factor

PD

_{exhaust}= 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:

CF

_{Tamb}= correction factor

T

_{amb}= 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*CF**

_{alt}*CF_{inletloss}*CF_{exhaustloss}*CF_{Tamb}where:

GT Site Power = Re-rated GT Power at actual site, kW

GT ISO Power = Vendor Specified Power, kW