LAY-UP AND START-UP PRACTICES
Lay-up and start-up of a combined cycle power plant are particularly important as the plants are often cycled regularly to meet peak demands. Start-up is not gradual, as in a conventional fossil-fired boiler, but very sudden because the combustion turbine exhaust gases produce very rapid heating of the tubes.
Combustion turbine purge cycles that occur just prior to ignition exacerbate the situation by pushing cold air through the HRSG followed by the engine ignition and sudden temperature rise. These factors can produce condensate in superheaters that can result in thermal low-cycle fatigue. This result can be minimized with proper design and operational procedures.
Start-up practice should ensure that oxygen is less than or equal to 10 ppb in the feedwater and any other water that will be fed to the HRSG before the deaerator is operational. This stipulation applies to water in the HRSG drums and the condenser hotwell and may even extend to the condensate storage tanks in some configurations. The purity of steam supplied by auxiliary boilers to a HRSG should be the same as that of steam produced by the HRSG.
Additional information on proper lay-up practices can be found in the ASME publication "Consensus for the Lay-up of Boilers, Turbines, Turbine Condensers, and Auxiliary Equipment" (CRTD-66).