Estimating HP-IP Midspan Packing Leakage in Combined Cycles

When a turbine has combined high-pressure (HP) and intermediate-pressure (IP) sections, there is a steam flow path between the sections. In combined cycle steam turbines, this internal leakage flow rate needs to be determined for the steam turbine performance calculations. However, since the leakage is internal to the turbine, it cannot be measured directly. One method, which has been employed in determining the midpacking leakage flow rate, is the variation of initial and̸or reheat temperature method. It involves using the convergence of IP efficiency plots from multiple test runs to estimate the HP-IP leakage flow rate. Although this method has been employed successfully in large steam applications, it has generally not produced consistent results for combined cycle steam turbines. The lack of convergence for combined cycles may be due to the fact that some of the assumptions made in applying the method to large steam applications are not valid for combined cycle applications. Some of the assumptions, which need to be reviewed and modified for combined cycle application, are as follows: (a) constant IP efficiency for all test runs, (b) constant throttle flow during all test runs, (c) constant section pressure ratios for all test runs, and (d) no influence of external cooling or admission flows. This paper reviews the modifications to the traditional initial and/or reheat temperature variation method to make the midpacking leakage calculations more consistent for combined cycle applications. Some data have shown that incorporating these additional changes improves the convergence of midpacking leakage determination.