Last Stage Performance Considerations in Low-Pressure Turbines of Power Plants: A Case Study

The last stage blades (LSBs) of low-pressure (LP) turbine power plants have been historically specified and designed on the basis of optimization studies by matching the turbine to the condenser/cooling system for a specified unit rating. LSB sizes for U.S. nuclear applications currently range from 38 in. to 52 in. for unit ratings of 600 MWe to 1200 MWe. LP turbine arrangements usually consist of two or three double-flow sections in parallel. Last stage end loadings (last stage mass flow divided by the last stage annulus area) vary from approximately $8000lb∕hsqftto14,000lb∕hsqft$⁠, with corresponding unit loadings (electrical output in megawatts divided by last stage annulus area) of 1.1 $MWe∕sqftto2.1MWe∕sqft$⁠. Several power plants have been upgrading/replacing their LP turbines. Considerations include efficiency, reliability, power uprates, operating license renewals (nuclear), aging, inspection, and maintenance. In some cases, LP turbine rotors are being replaced with new rotors, blading, and steam path. Others are replacing LP turbines with new and advanced designs incorporating improved technology, better materials, optimized steam paths, more efficient blading, longer LSB sizes, redesigned exhaust hoods, etc. Unlike the other stages in the LP turbine, the last stage performance is affected by both the upstream (load) and downstream (condenser) conditions. While the LP turbines are being upgraded or replaced, no major modifications or upgrades are being made to the condensers. To address vibration effects due to increased flows and velocities from power uprates, the condenser tubes may be staked. Circulating water pumps may or may not be upgraded depending upon the particular application. Consequently, while improvements in LP turbines lead to more efficient utilization of the available energy and higher output, the last stage performance may be out of synchronization with the existing condenser/cooling system. Undersized or oversized LSB sizes in relation to the unit rating and end loading may result in less than optimum performance depending upon the design and operating range of the condenser/cooling system. This paper examines the various factors that affect last stage performance of LP turbines. Using a case study, it discusses the relationships between the last stage, the unit rating, the end loading, and the operating range of the condenser/cooling system. It examines different last stage exhaust loss curves and provides recommendations for selection of LSB sizes for optimum performance.