What is ISO Rating of Gas Turbines?

Page content

The same Gas Turbine performs differently in the high altitudes of LaPaz, Bolivia and the tropical island of Singapore. It performs differently in winter and in summer in the Saudi Arabian deserts. This has to do nothing with the Gas Turbine itself, but is due to the ambient atmospheric conditions.

For example : The manufacturers brochure promises a machine capable of 100 MW output at an efficiency of 35 %. But the actual performance is different. This is not due to the fault of the machine, but because of the different ambient conditions.

To eliminate such misunderstandings, the gas turbine output and performance is specified at Standard conditions called the ISO ratings. These are specified as per ISO standards 3977-2 ( Gas Turbines - Procurement - Part 2: Standard Reference Conditions and Ratings).

The Stanadard conditions are explained below.

Ambient temperature, Relative Humidity, and Elevation.

The three standard conditions specified in the ratings are Ambient Temperature - 15 deg C, Relative Humidity - 60 % and Ambient Pressure at Sea Level.

These conditions affect the air density. How does air density affect the Gas Turbine output and performance ?

The compressor section in a fixed volume of air for each rotation of the blades. The mass of the air depends on the air density. So at sea levels one rotation sucks in more kg of air than at place at high altitude.

The work done by the gas turbine i.e.: the heat energy to mechanical energy conversion depends on the mass of the hot gases, the specific heat and the Temperature difference. The mass of the hot gases depend on the mass of the air that is taken in. This means at sea level the Gas turbine gives more output than at high altitudes.

Inlet and Exhaust Losses.

The standard conditions specify that Inlet and Exhaust losses as Zero.

The intake system and the exhaust system offer resistance to the flow of air and the exhaust gases. The energy to overcome these resistances comes from the Gas Turbine. This reduces the nett output of the Gas Turbine. The configuration and layout of the intake and exhaust systems varies from plant to plant and accordingly the losses. These losses reduce the actual output of the Gas Turbine from the rated value.

Base Load Operation at 100 % rated power.

The standard considers that the Gas Turbine operates at 100 % rated load. The efficiency quoted at the standard ratings are for this 100 % Load. Efficiency of the gas turbine at part load operations is different from that at 100 % load. If you buy a machine rated at 100 MW and operate it at 75 MW you will not get the rated efficiency.

The performance of a plant operating as a base load plant is different than a peaking load plant. This is mainly due to the frequent starts and stops that lead to deterioration of the performance.


All the manufacturers provide correction factors for deviations from the various standard ratings discussed above. If the actual conditions ar known the output and efficiency at the standard ratings is corrected to theactual conditions. Consider these corrections when specifying, purchasing and operating Gas Turbine power plants.

Understanding of the ISO ratings and the actual plant conditions is essential when specifying and buying a Gas Turbine. Also this can be used to compare the performance of Gas Turbines.

This post is part of the series: Gas Turbine Power Plants.

Gas Turbines have in the recent times become one of the most efficient and reliable energy conversion devices. Used in Combined Cycle Power plants they give the highest efficiency for converting Fossil energy to electric power. Used in Simple cycle mode they have the shortest gestation time and the

  1. How does a Gas Turbine Power Plant Work? – The Main Equipment.
  2. How does a Gas Turbine Power Plant Work? The Auxiliary Systems.
  3. What is ISO rating of Gas Turbines ?