A gas turbine is a dynamic internal combustion engine. When we compare the performance of a gas turbine to that of a steam turbine, it becomes immediately evident that steam turbine performance is much easier to calculate, since both the vapor and the vapor conditions are fixed.
For a gas turbine, the vapor condition depends on the type of fuel used and the atmospheric conditions. This is because the inlet to the gas turbine engine is from the atmosphere, and any change in temperature, humidity or pressure will affect the mass flow into, and consequently the power produced by the gas turbine. The gas turbine cycle (Brayton) is open.
As a result, steam turbine performance can be expressed rather easily in terms of steam rate (pounds of steam per horsepower or kilowatt hour) and external efficiency. Since the gas turbine vapor conditions are variable, however, its performance must be expressed in terms of heat rate, BTUs per horsepower or kilowatt hour, thermal efficiency and fuel rate. All of the above must also be expressed in standard terms. A set of standardized conditions has been established by ISO (The International Standards Organization) to rate all gas turbines.
Gas turbine site performance is directly affected by inlet air density and air environmental conditions. The effects of inlet air density on produced power and heat rate are:
- A given engine design limits air volume flow capacity
- Produced power is a function of actual energy extracted per pound of vapor and mass flow of vapor
- For a given engine therefore, produced power varies directly with inlet air density
- Produced power does become limited by low volume (stall and surge) flow
Care must be taken when selecting gas turbines to ensure sufficient shaft power is available at high temperature conditions and fouled inlet conditions, and gas turbine applications tend to be ‘fully loaded’ since gas turbines (unlike steam turbines) are not custom designed.
Gas turbine vs. steam turbine performance
A gas turbine is an internal combustion engine in that the hot vapor is produced internal to the engine. The cycle is open, since 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 hot vapor is produced external to the engine. The steam turbine cycle is closed, in that both inlet and exhaust conditions are controlled by the steam generation system (boiler), therefore steam turbine conditions are constant and do not vary.
Since the gas turbine Brayton cycle is open, vapor conditions are variable and performance must be expressed as:
- Heat rate
- Thermal efficiency
- Fuel rate
Gas turbine site performance is directly affected by inlet air density and air environmental conditions. Since produced power and heat rate vary as a function of inlet temperature, pressure and inlet duct and exhaust duct pressure drop, vendors supply correction curves to convert ISO conditions to site conditions. A small increase in firing temperature has a significant effect on produced horsepower and on engine efficiency.
Gas turbine site power is determined by elevation, temperature, inlet conditions, outlet conditions, humidity, and fuel conditions. Confirm in the design phase that all site conditions are correct, and confirm ambient temperature conditions for high sites, and an accurate fuel gas analysis. Consult other end users in the area to confirm that the anticipated conditions are correct.
Perform a life cycle cost analysis to determine possible lost revenue costs that would arise from using an undersized driver, in order to justify a larger power driver. 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. Insufficient driver power will restrict maximum possible pump or compressor flow rates and generated power. The associated revenue losses can exceed over $ 100 MM over the life of the plant.
This best practice has been used since 1990 to ensure that sufficient gas turbine power is available at site conditions. A combination of rigorous checks of anticipated site conditions, consultation with other end users in the geographic area and a life cycle cost analysis to justify a larger driver selection, if warranted, has been performed to result in maximum driver reliability and product revenue.