A probe developed by EPRI has demonstrated the use of light scattering to measure liquid mass fraction in the condensing steam flow of low-pressure turbine sections. Wetness fraction is a critical parameter in assessing enthalpy of condensing steam, and therefore valuable in determining steam cycle performance. EPRI is planning field-testing and verification of the system in 2001, including benchmark testing conducted in parallel with standard ASME PTC-6 steam cycle measurements.
EPRI’s wetness probe development was initiated in 1995 as part of its low-pressure turbine efficiency improvement program, according to Steve Hesler, EPRI program manager for fossil steam turbines and generators. Prototype field testing during 1997 and 1998 led to the development of improvements in the probe optical elements, and ease of probe system deployment. EPRI received a U.S. patent in 2000 and is now moving to the system verification stage. In 1997 and 1998, steam wetness measurements were performed at a selection of EPRI member sites, including TVA’s Allen and Gallatin plants. The improved system will be validated in a benchmark test conducted at TXU’s Martin Lake lignite plant in April. TVA is also sponsoring additional tests of the probe this year at Texas A&M’s Aerodynamic Research Laboratory, according to Jim Terrell, project specialist at TVA.
The earliest field applications for in-situ enthalpy probes were in the 1970s, and were used to measure droplet size, wetness fraction, and aerodynamic parameters as part of overall steampath design improvement efforts. Enthalpy measurements are valuable for diagnosing performance deficits, verifying LP turbine performance upgrades, measuring hood losses and identifying trends in turbine performance. Turbine manufacturers, which have the difficult job of designing blades for use in highly three-dimensional condensing steam flow, can also benefit from detailed measurements of aero-thermal conditions within the steampath.
EPRI has recently completed hardware assembly of the first improved probe system. “We’d like to build another system in order to carry out more testing,” says Jeff Stallings, manager of EPRI’s Heat Rate and Cost Optimization Value Package, which is funding the TXU tests. “We are seeking additional partners in order to further demonstrate the value of these measurements, and implement more widespread use of this emerging technology in generating plants.” Future benefits to power producers include increased opportunity to measure and trend LP turbine performance, at a lower cost than conventional methods. Plant operators planning to upgrade their steampath can benefit from measurements made prior to and following the upgrade installation to document the LP section performance improvement.
EPRI’s enthalpy probe advances steam cycle performance measurements. Photo courtesy of EPRI.
Insertion through pre-installed ports at the turbine interstage and exhaust locations deploys the low-pressure turbine wetness and aerodynamic probes. Turbine exhaust access ports can be installed during a boiler outage. Through EPRI sponsorship, more than ten units in the U.S. now have these ports available for LP performance testing.
“We want to extend the use of steam wetness measurement technology to higher temperature and pressure conditions,” says Hesler. “These developments would enable the technology to assist in nuclear steam cycle performance measurements. EPRI is working with our membership, as well as turbine manufacturers, to maximize the future benefits of this technology to plant operators. We are also looking for ways to use steam enthalpy measurements to strengthen EPRI’s ongoing activities in the area of steam turbine performance assessment and improvement.”
Those interested in more information should contact Steve Hesler at 704-547-6183 or Jeff Stallings at 650-855-2427.