Study On A Method And Application Of Wetness Measurement On Line For Steam Turbine

In modern thermal power plants, the final several stages in condensate steam turbineand all the stages in nuclear steam turbine operate in wet steam region, the drain erosionon blade is caused by the wetness fraction, the operation safety of steam turbine isthreaten by the blade erosion. At the same time, the blade erosion increases the flowlosses in flow passage, which decreases the efficiency of steam turbine. Furthermore, themoisture loss decreases the efficiency of steam turbine too. so the wetness fraction havea great influence on the operation of steam turbine about safety and efficiency.The enthalpy could not be determined by pressure and temperature due to theexhaust in the wet steam region when we compute the thermal parameters on efficiency,on line monitoring the exhaust wetness fraction is necessary. If the exhaust wetnessfraction were monitored, the thermal parameters on efficiency would be credible.Because they show the real operation state on steam turbine, the operation instructionand maintain of equipment is based on the real condition.Therefore,it is significant for monitoring the wetness fraction on line in exhaust ofsteam turbine.Many methods of measuring wetness fraction is analyzed and compared in thisthesis, the heating method is chosen as the basis of monitoring the wetness fraction online in exhaust of steam turbine. At present, heating method have some disadvantage, Inorder to overcome the drawback of measuring method, a new method of double regionheating is presented for on line monitoring the wetness fraction in steam turbine exhaust.The evaporation tube length and critical heat have important influences on design andmeasuring for heating method wetness probe, the evaporation tube length of water dropis computed by numerical method at the conditions of turbulence flow, compressible,and coupled two-phase, then combined with critical heat, the range adjustment of heaterpower is determined, which provides the basis of design of double region heatingwetness probe and adjustment of measurement.Based on the double region heating method, a new style wetness measurement probeis designed. At present condition, the special wet steam with certain wetness fractioncould not be produced, the calibration boiler for wetness probe is designed and thecalibration experiment is done with that calibration boiler, To analyze the measurementerror of double region heating wetness probe, the measurement error of calibration boiler,and the calibration result, it shows that the measurement accuracy of thermal balancecalibration method is high enough, it is available for calibration, on the other hand itshows that the measurement of wetness fraction with double region method is feasible.With the core of double region heating method, the system of on line monitoringwetness fraction in steam turbine exhaust is developed. The industrial tests were done inUnit 9(200MW) of Datang Changshan thermal power plant, Unit 2(350MW) and Unit1(350MW) of Huaneng Dalian power plant. According to the results of industrial test,the method and system of online monitoring wetness fraction in steam turbine exhaust isimproved.The result of industrial test shows, with the core of double region heating method,the system of on line monitoring wetness fraction in steam turbine exhaust has no badinfluence on the routine operation of power unit, it is suitable for the requirements of online monitoring wetness fraction in steam turbine, it could become the routinemeasurement instrument. To contrast the results of industrial test of on line monitoringwetness fraction in steam turbine exhaust with that results of thermal efficiency test ofunit at the same operating condition, to analyze the wetness fraction of steam turbineexhaust, which is measured by a improved wetness probe, and the data of unit, we findthe fractional error between them is 11.9%. Together with other analyses, we draw theconclusion that the wetness fraction online measuring is credible.