CYCLIC SERVICE FEATURES FOR HEATRECOVERY STEAM GENERATORS

摘要

Much has been learned about features of Heat Recovery Steam Generator’s, HRSGs, thatrnmake them susceptible to premature failures from cycling. The trend today is to convert fromrnbaseload operation to a cyclic mode of operation. This has created concern for owner-operators who worry that the equipment, originally designed to last twenty five to thirty years, may only last a few years without major mechanical revisions or updating. This article outlines some ofrnthe areas of concern in procuring new HRSGs or updating existing units, what features should bernconsidered in the design if cycling is required, and why these feature can provide a robust unitrncapable of handling the demands of today’s market.rnFor components such as reheater and HP Superheater coils, tube to tube temperaturerndifferences can create high internal thermal stresses. The ability of coils to absorb theserntemperature differences without creating the potential for low cycle fatigue is a necessary designrnfeature. The details necessary to minimize or eliminate these internally generated stresses arerndiscussed.rnThrough thickness temperature gradients are considered important when determining start uprnand shut down ramp rates. Keeping the headers that are susceptible to creep as thin as possiblernhelps to minimize the stresses associated with start up and shut down. Several techniques forrncalculating component thicknesses are presented.rnFor HRSGs that are required to cycle daily, condensate management becomes a realrnchallenge. Condensate quenching will damage boilers tubes, pipes and other hot sections of thernboiler faster than almost any other mechanism known. When the boiler is kept pressurized andrnhot and then restarted, large amounts of condensate are generated in the high pressurernsuperheaters and reheaters. This often floods lower headers and piping. How this condensate isrnremoved is important for boiler design when cycling.rnFinally, several other recommendations that can increase equipment longevity such asrncondensate pots for desuperheaters, feedwater recirculation systems, auxiliary equipment to holdrnboiler pressure and temperature, and the problems with certain types of piping layouts are allrnbriefly discussed.