Reliability and durability from large heat recovery steam generators

Experience with heat recovery steam generators (HRSGs) designed for larger- heat-input and higher-steam conditions highlights limitations in some featu res of traditional designs extrapolated from smaller HRSGs that operated pr edominantly continuously. Many combined-cycle units may be subjected to per iods of regular overnight shut-down much earlier than expected and, unless anticipated during the initial design, there will be significant adverse im pact on reliability and durability. Very premature problems already experie nced on large HRSG designs in combined-cycle and cogeneration applications give an early warning that more widespread problems will arise as HRSGs are subjected to more thermal cycling. Problems that arise when inadequate att ention is given to transient conditions over the full range of operation of the combined-cycle gas turbine unit, including at part loads, during shut- downs and restarts from diverse pre-start conditions, are highlighted. The paper gives examples of how the damage to HRSGs during shut-down and starts can be mitigated by minor modifications and sympathetic operating procedur es and suggests how prudent purchasers can enhance reliability and lower li fetime costs at a small premium in installed cost by specification of appro priate design features. Other weaknesses which cause problems for operation and maintenance are discussed.