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 features of traditional designs extrapolated from smaller HRSGs that operated predominantly continuously. Many combined-cycle units may be subjected to periods of regular overnight shut-down much earlier than expected and, unless anticipated during the initial design, there will be significant adverse impact on reliability and durability. Very premature problems already experienced 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 attention 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 procedures and suggests how prudent purchasers can enhance reliability and lower lifetime costs at a small premium in installed cost by specification of appropriate design features. Other weaknesses which cause problems for operation and maintenance are discussed.