Most large combined cycle power plants are equipped with steam attemperation equipment to control temperatures below design limits during normal operation and anticipated transient conditions. For power stations with reheat systems, design requirements place spray valves in the heat recovery steam generator (HRSG) high-pressure (HP) and reheat (RH) interstage piping and in turbine bypasses to cold RH and condenser. High-temperature piping exposed to spray water, commonly grade 91 or 22, have experienced the damaging effects of thermal quenching/shock resulting from direct contact between liquid water and hot metal surfaces, the consequent development of skin cracking (also sometimes called "craze" cracking), and after prolonged exposure, leaks and failures.
Three case studies will be presented in this article describing the engineering evaluation of service-induced cracks to support the scheduling of pipe replacements; the design of efficient nondestructive testing (NDT) inspection plans, in accordance with the American Society of Mechanical Engineers (ASME) B31.1 "Power Piping" code Chapter VII guidance; and issues with spray control during transient operation.
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