Heat Recovery Steam Generator Purge Credit Success

At the 2019 Ovation Users’ Group Conference, a U.S. power producer shared their story about shrinking down to a 17-minute purge on their combustion turbine (CT) startup. A heat recovery steam generator (HRSG) purge removes combustible levels of fuel mixtures prior to startup. A purge credit allows credit for a prior purge for the next startup with a normal shutdown occurs or modified fuel systems for both CT and duct burners are incorporated and functional.

NFPA 85: Boiler and Combustion Systems Hazards Code sets design, installation, operation, maintenance and training standards for safe equipment operation for fired and unfired steam generators.

A failed startup could mean a $300,000 penalty to the utility needing the electrical power. The retrofit incorporate Ovation system for burner management and instrumentation to monitor the fuel and purge lines. The system relies on air or inert gas and valve position verification on the fuel and purge air systems to receive the purge credit to make sure the combustion chamber is clear of fuel gas before startup. Plants with less that one-hour startups have a market advantage over other producers supplying electrical power to the grid.

The result was a reduction on hot start startups from 37 minutes down to 17 minutes saving 20 minutes on each startup. And thermal stress from cycling on the HSRG’s were significantly reduced by minimizing condensation caused from cooler purge air. Additional online time increases revenues, reduces penalties from not meeting utility power demand requirements. Overall project payback is estimated to be 2-3 years.

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