Combined heat and power (CHP) is an application that utilizes the exhaust heat generated from a gas turbine and converts it into a useful energy source for heating & cooling, or additional electric generation in combined cycle configurations. Compared to simple-cycle plants with no heat recovery, CHP plants emit fewer greenhouse gasses and other emissions, while generating significantly more useful energy per unit of fuel consumed. Clean plants are easier to permit, build and operate. Because of these advantages, projections show CHP capacity is expected to double and account for 24% of global electricity production by 2030. An aeroderivative power plant has distinct advantages to meet CHP needs. These include high thermal efficiency, low cost, easy installation, proven reliability, compact design for urban areas, simple operation and maintenance, fuel flexibility, and full power generation in a very short time period.

There has been extensive discussion and analyses on modifying purge requirements on cycling units for faster dispatch. The National Fire Protection Association (NFPA) has required an air purge of downstream systems prior to startup to preclude potentially flammable or explosive conditions. The auto ignition temperature of natural gas fuel is around 800°F. Experience has shown that if the exhaust duct contains sufficient concentrations of captured gas fuel, and is not purged, it can ignite immediately during light off causing extensive damage to downstream equipment.

The NFPA Boiler and Combustion Systems Hazards Code Committee have developed new procedures to safely provide for a fast-start capability. The change in the code was issued in the 2011 Edition of NFPA 85 and titled the Combustion Turbine Purge Credit. For a cycling plant and hot start conditions, implementation of purge credit can reduce normal start-to-load by 15–30 minutes. 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 heat recovery steam generator (HRSG).

This paper details the technical analysis and implementation of the NFPA purge credit recommendations on GE Power and Water aeroderivative gas turbines. This includes the hardware changes, triple block and double vent valve system (or drain for liquid fuels), and software changes that include monitoring and alarms managed by the control system.

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