Some combined-cycle plants have two feedwater-control valves: a small startup valve with the extra cavitation-protection needed during startup and low-load operations, and a larger main valve with the high flow-capacity needed during full-load ops. This design is intended to yield a longer service life of these valves. However, as the plant ages, this design also can yield a severe modulation, or “hunting,” in these valves.

At one 10-year-old combined-cycle plant, the two feedwater-control valves started hunting violently, eventually causing a loud boom.

The entire feedwater-piping system lurched, nearly out of its pipe rack. That startled everyone in the plant, and prompted the control room operator to hit his shutdown switch, and put in a trouble call to the maintenance shop. The maintenance technician grabbed his tool bag, went to the feedwater-control station, and began troubleshooting. He noted in his logbook that the incident had occurred shortly after a plant startup, just as the feedwater-control logic was transitioning from the startup valve to the main valve.…

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