Fixed Area Nozzle With Free Power Turbine: What Makes the Match?

The matching of free power turbine with gas producer/generator turbine in a twin shaft industrial gas turbine configuration is a complex fluid interaction with the fact that maximum output must be achieved at fully choked nozzle segments while fixing the compressor and gas producer turbine pressure ratios. The paper presents characteristics of different turbo-machines with power turbine chiefly the matching of fixed area nozzle with low Wobbe Index Fuels. It also addresses the peculiar behavior of bleed valve, control limits, power as well as correlation of low Wobbe Index fuels. Mechanically, the gas producer/generator turbine and power turbine are independent in twin shaft gas turbines for oil and gas applications; i.e., the speed of free power turbine is controlled by driven equipment like pump or compressor similar to single shaft gas turbine generator set applications. The only contact remains between them is burnt gases generated by Gas producer Turbine. The full/optimum power will be obtained with correct aerodynamic matching of free power turbine at different match temperatures. The paper describes how turbine rotor inlet temperature ‘TRIT’ can be controlled by fixing discharge pressure of axial flow compressor, gas producer speed, flow through first power turbine nozzle etc. Important fluid phenomenon, the high TRIT in fully choked nozzle and bleed valve closing delay at high ambient match will be discussed in more detail. Last but not least, a correction factor is introduced by mathematical correlation of fixing the exit area of nozzles with complex calculations performed for low Wobbe index fuels keeping in view the match temperature.