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Peter Jackson, President of Tetra Engineering Group was invited to present Tetra's experience on Purge Credit implementations at the 501F User Conference this February (2019).Combined cycle power plants use fuels and other materials that can cause fires or explosions in the combustion turbine, ducting, or heat recovery steam generator. Purging that equipment with ambient air to displace residual combustible gases before starting is a normal safety practice. But when plants are cycled, the disadvantages of purging often outweigh the advantages.
During the startup of the triple-pressure HRSGs, plant staff noticed steam coming from the stack and a discrepancy between feedwater and steam flows. On shutting down the unit a T23 superheater tube was found with a complete circumferential failure. The failed tube was removed and the bore was plugged with some difficulty due to poor access and the uncommon material.
As combined cycle gas turbine plants are called upon to play a larger and more flexible role in the generation mix, it’s important to schedule a comprehensive assessment of major components at key intervals to ensure reliable operation.
Waterside Deposits in evaporator tubes have been an issue in steam generators as long as boilers have been used. Substantial experience in deposit formation and management has been gained in conventional goal and oil/gas boilers over time. The role of boiling modes in the steam generator tubes is very critical to areas of deposit formation.
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.
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