Comparison of Primary Flow Measurement Techniques Used During Combined Cycle Tests

One of the most significant contributors to the overall uncertainty of a performance test of a combined cycle steam turbine is the uncertainty of the primary flow measurement. ASME performance test codes provide many alternative methods for determining flow. In two actual combined cycle tests performed in 2005, the following three alternate methods were used to determine the HP steam flow into the combined cycle steam turbines: 1) Derivation from measured HP feedwater flow using calibrated PTC 6 throat tap nozzles, 2) Derivation from LP condensate using calibrated PTC 6 throat tap nozzles, and 3) Derivation from LP condensate using calibrated orifice metering sections. This paper describes the design, calibration, and installation of each flow meter involved, the methods used to calculate the HP steam flow, the estimated uncertainty of the HP steam flow derived using each method, and the actual test results using each method. A comparison of the methods showed that there are distinct advantages with one of the methods and that very low uncertainties in HP steam flow can be achieved if sufficient attention is applied to the design, calibration, and installation of all flow meters involved.