An accurate correction methodology is essential when analyzing test data and trending performance. One of the most critical parameters for steam turbines, which can result in large corrections, is condenser back-pressure or exhaust pressure.
Many large fossil and all nuclear steam turbines are configured with either two or three low pressure condensing exhaust hoods. All exhaust hoods may not operate with the same condenser pressure. Factors affecting condenser pressures between hoods could include the cooling water arrangement, uneven fouling, tube plugging, air removal effectiveness, etc. The biggest impact is likely due to the cooling water arrangement. In a parallel arrangement, the condenser cooling water splits between the shells with each shell receiving an equal amount of flow at the same inlet temperature. In a series arrangement, all cooling water enters and exits the first shell before entering the next shell. In this arrangement the cooling water temperature entering the second shell is higher than the temperature entering the first shell resulting in the condensers operating at different exhaust pressures.
One common practice is to apply a single exhaust pressure correction factor based on the average exhaust pressure of all condenser shells. In cases where the differences in condenser pressure are small, this practice can provide accurate corrected turbine performance. As the difference in condenser pressures increases, the potential for introducing error in the corrected performance results also increases.
This paper will discuss the mechanism of why multi-pressure operation can result in correction errors if not modelled correctly and will also quantify the potential impact of these errors on the corrected performance results. In addition, guidance will be given on how exhaust pressure correction curves should be created and applied to most accurately model the performance of the turbine cycle when multi-pressure operation exists.