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The findings of the valve inspections were reported to the OEM. The plant was naturally concerned about the blue blush material and the number of valve parts that could not be separated or were stuck together, and a great amount of force had to be used to separate the parts. The OEM came up with a recommendation to the valve blue blush problem. The existing valve stems and bushings are made up of 12Cr alloy steel, which is more susceptible to scale oxidation. The plant decided to replace the 12Cr alloy steel stems to an incoloy material. The material does not scale easily and is more oxidation resistant. Stellite alloy does not build up scale as much with time compared to the existing 12 Cr alloy currently being used. Using stellite alloy, the stem is less likely to bow out at high temperature and has a high vibration strength. The material also has a high creep rupture resistance. The bushing materials are being recommended to be made of stellite alloy, which also are oxidation resistant and do not scale easily due to oxidation. The stellite bush has a high abrasion resistance. Prevention
Currently, the TS Power Plant valves are on a two-year schedule for inspection and maintenance; upgrading to stellite bushings and incoloy stems will push the inspection cycle to every 4 years.
Although using better valve alloys is beneficial, it has to be coupled with better operation practices. During plant start up after an outage, the exfoliated products from the boiler can be made to go through a bypass system to the condenser so that the solid particle materials going to the valves or turbine will be minimized. Of course, this means a longer startup period, but in the long run it will be worth it. Effective, bypass steam lines are very expensive to add to turbine systems and are not commonplace in U.S. plants, but are more widely used in Europe. Steam valves should be exercised every day, especially in situations where the steam turbine is operated with valves wide open in the boiler sliding pressure mode. Boiler water and steam chemistry should be strict and follow recommended standards, such as those under EPRI guidelines.
The work presented in this paper equally applies to large power plants (> 600 MW) with separate multiple main steam stop valves and two or more control valves, depending on the number of steam admission pipes to the HP turbine section. Valves should completely close during a turbine shutdown, whether it is an emergency trip or controlled shutdown. When valve parts no longer meet the expected clearances and cannot satisfy recommended specifications with time, they must be replaced. Overspeed prevention systems can be said to be effective only if the outcome is a complete stop of steam flow to the turbine as a whole after the valves have shut down completely.
If it allows any quantity of steam flow to the turbine, even small valve sticking can still cause some serious problems.
Because of increased main steam temperatures (> 1,005°F) which cause some boiler superheater and reheater tubes to exfoliate during shut downs; and magnetite material is carried to the steam valves and turbine during start up; maintenance or inspection of turbine valves has become very important. By careful inspection of valves; sticking caused by blue blush or oxide scale build can be avoided. Valve inspection of main stop, control, steam extraction and non-return should follow a strict inspection schedule of every two years or best practices. If necessary, valves should be upgraded to use better alloys, such as incolloy for stems and stellite for bushings to replace the 12Cr alloy steel parts. New plants should be supplied with new alloy material that is oxidation resistant to avoid build-up of scale. Valve exercise should be performed once a day, as well as visual inspection. This is especially important in plants that use the boiler sliding pressure mode with valves wide open. This would reduce the chances of valves sticking and ensure they close properly. The turbine overspeed trip test should be performed annually to ensure it works. All important valves around the turbine, including main stop, throttle, control and extraction non-return valves, should be visually inspected and tested for functionality. Finally, it cannot be over-emphasized that turbine over-speed events can be avoided if main stop valves, control valves, reheat stop valves, intercept valves and extraction valves close properly after a turbine trip. This will avoid a failure in turbine shaft, blades and generator retaining rings.
The author would like to thank Newmont Nevada Energy Investments for permission to present this work and the engineers at Toshiba International Company for their technical support to TS Power Plant on steam turbine valve inspection and improvements.
Editor’s note: This paper, PWR2011-55150, was printed with permission from ASME and was edited from its original format. To purchase this paper in its original format or find more information, visit the ASME Digital Store at www.asme.org.