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.
The primary benefit of the purge credit is fast system response. For an overnight shutdown resulting in hot start conditions, implementation of purge credit can shave 10 to 30 minutes off normal start-load times. Part of the time saving is the reduction of the purge time itself, and the rest is faster ramp rates due to a higher initial temperature and pressure in the HRSG. This design approach is now being used by several OEMs, including GE on its fast-start, flexible plant offerings. Background to Purging issues, Benefits and actual data from case studies were presented and discussed.