Normal boiler and Heat Recovery Steam Generator Operation have required a complete purge of the boiler by fresh air before startup firing to remove potentially explosive fuels and other substances from the boiler/HRSG. For power plants in daily two-shift cycling (or more frequent starts) this not only adds time to the startup sequence but can impose significant thermal stresses on hot components exposed to cooler purge air. Since the approval of a purge credit mode in NFPA 85 (2011) where the flow of air through the HRSG during shutdown can be credited as a purge, plants can operating without a startup purge with enhanced fuel systems and operating procedures. This paper will look at the impact of purge (and purge credit) on the thermal stress and fatigue lifetimes of HRSG operating with and without purge credit. Data and simulation results will demonstrate the range of fatigue life changes from implementation of purge credits as well as other operational impacts as seen in operating data.

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