Combined-cycle plants will continue to be cycled on and off frequently as long as natural gas costs more than coal. The need to start a plant's gas turbine quickly—to meet short-term electricity demand—sometimes overshadows the potential effects of cycling metal temperatures of heat-recovery steam generator (HRSG) tubes and those of superheater and reheater headers. In particular, the combined effect of this thermal cycling on water and steam chemistry can be significant.
A metallurgical penalty must be paid each time an HRSG goes through a thermal cycle. Many of the physical damage mechanisms have been addressed in POWER (October 2002; March, April, and September 2004; and March 2006) since the boom in combined-cycle plant construction began over a decade ago (Figure 1).
1. Fast mover. Heat-recovery steam generators like this one are now cycled often, sometimes daily, to meet swings in demand. Water chemistry has to keep up with changes in HRSG operating conditions. Courtesy: David Daniels
Excessive HRSG cycling can result in physical damage to the unit and increase the frequency of corrosion and tube failures. But there is another less-appreciated, but nonetheless important, chemistry-related variable in the failure equation. Disruptions in HRSG water chemistry are caused by various actions. Some are expected and others are artifacts of unit cycling. This article discusses both types.
Prevention first
Corrosion products can form quickly if water, air, and metal are allowed to occupy the same space in an HRSG. Proper lay-up is critical to good start-up chemistry, particularly on a cycling unit. Whenever possible, operators should maintain steam pressure on drums and the vacuum on the condenser for as long as possible. Many HRSGs use stack dampers to keep the heat in and bottle up the unit.
HRSGs that are cycled daily typically maintain steam pressure overnight or even over a weekend, depending on the ambient temperature and the condition of the stack dampers. If the unit has an auxiliary boiler, steam seals on the steam turbine can be maintained, to preserve condenser vacuum. This improves the chemistry during start-ups. If an HRSG is to remain off-line long enough that all pressure will be lost, drums and steam piping should be filled with water and blanketed with nitrogen.
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