Gas turbines are limited, as are all rotating assemblies, by either speed or power. For an electric motor, the power limit is manifested by maximum amperage, (more precisely, the maximum permissible winding temperature). The situation with gas turbines is similar. The ultimate amount of power (i.e. work, horsepower), that can be developed by the turbine blades, is limited by the turbine exhaust temperature.
A typical maximum turbine exhaust temperature is 1,100°F. This limit is imposed by the metallurgy of the turbine’s blades. Continuous operation above the turbines design exhaust temperature will lead to accelerated deterioration of the blades and a consequent reduction in engine horsepower.
When neither end of the centrifugal compressor is running at its peak speed, and the turbine exhaust temperature is below it’s design limit, there are two other possibilities which may be limiting horsepower output:
• Fuel gas firing is limited by a faulty over-ride on the temperature controller. That is, the exhaust temperature is artifically surpressed by an instrument malfunction.
• The fuel gas flow control valve is wide open; or it is partially plugged by natural gas hydrates.
When the turbine exhaust temperature is at it’s limit, and horsepower output is deficient, other possible causes are:
• Excessive wear to turbine blades.
• Air/fuel ratio problems.
• Carbon deposits on turbine blades. Periodic detergent washing of the combustion air compressor will help reduce this effect.
• Lack of proper flow from the combustion compressor.