CYCLIC SERVICE FEATURES FOR HEAT RECOVERY STEAM GENERATORS

摘要

Much has been learned about features of Heat Recovery Steam Generator's, HRSGs, that make them susceptible to premature failures from cycling. The trend today is to convert from baseload 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 of the areas of concern in procuring new HRSGs or updating existing units, what features should be considered in the design if cycling is required, and why these feature can provide a robust unit capable of handling the demands of today's market. For components such as reheater and HP Superheater coils, tube to tube temperature differences can create high internal thermal stresses. The ability of coils to absorb these temperature differences without creating the potential for low cycle fatigue is a necessary design feature. The details necessary to minimize or eliminate these internally generated stresses are discussed. Through thickness temperature gradients are considered important when determining start up and shut down ramp rates. Keeping the headers that are susceptible to creep as thin as possible helps to minimize the stresses associated with start up and shut down. Several techniques for calculating component thicknesses are presented. For HRSGs that are required to cycle daily, condensate management becomes a real challenge. Condensate quenching will damage boilers tubes, pipes and other hot sections of the boiler faster than almost any other mechanism known. When the boiler is kept pressurized and hot and then restarted, large amounts of condensate are generated in the high pressure superheaters and reheaters. This often floods lower headers and piping. How this condensate is removed is important for boiler design when cycling. Finally, several other recommendations that can increase equipment longevity such as condensate pots for desuperheaters, feedwater recirculation systems, auxiliary equipment to hold boiler pressure and temperature, and the problems with certain types of piping layouts are all briefly discussed.