Combined Cycle Steam Turbine Inlet Flow Passing Capability: Impact on Plant Operation, Turbine Design, and Test Result Accuracy

The publication of ASME Performance Test Code 6.2-2004 provided the industry with a Code document dedicated to calculating the performance of a steam turbine in a combined cycle power plant. Power output at specified steam flows and conditions was chosen as the Code’s primary performance parameter. That choice was based on the operating and cycle characteristics of a combined cycle plant operating, where the steam turbine is part of the bottoming cycle operating in a sliding pressure mode that follows ambient conditions and the gas turbine operating profile. This steam turbine generator output, corrected to reference heat consumption, is called Output Performance and is a measurement of steam turbine efficiency. Accompanying this new Code was a new correction methodology that focused on correcting the steam turbine generator output to the reference heat consumption of the cycle. In the development of the overall correction methodology, the corrections associated with high-pressure (HP) steam inlet conditions were given careful attention. The committee developing the Code and methodology concluded that three correction formulations were required to accurately and fairly correct back to the reference heat input of the high-pressure turbine inlet, and to account for changes in the as-built flow capacity versus the design flow capacity. The new correction formulations chosen were: • HP Steam Flow; • HP Steam Temperature; • HP Turbine Flow Capacity. Applying these three corrections on a sliding pressure steam turbine ensures that the output performance is corrected to the true reference high pressure steam heat input to the cycle. If any of these three corrections is excluded the calculated output performance will not be a true representation of the steam turbine efficiency.