Some combined-cycle crops have two feedwater-control valves: a small startup valve with the additional cavitation-protection wanted throughout startup and low-load operations, and a bigger important valve with the excessive flow-capacity wanted throughout full-load ops. This design is meant to yield an extended service life of those valves. However, because the plant ages, this design can also yield a extreme modulation, or “looking,” in these valves.
At one 10-year-old combined-cycle plant, the 2 feedwater-control valves began looking violently, finally inflicting a loud increase.
The whole feedwater-piping system lurched, practically out of its pipe rack. That startled everybody within the plant, and prompted the management room operator to hit his shutdown swap, and put in a hassle name to the upkeep store. The upkeep technician grabbed his device bag, went to the feedwater-control station, and started troubleshooting. He famous in his logbook that the incident had occurred shortly after a plant startup, simply because the feedwater-control logic was transitioning from the startup valve to the primary valve. His subsequent entry within the log stated, “But it seems just like the management logic is confused, as a result of it’s driving the primary valve full-shut—indicating it desires much less feedwater—whereas on the similar time it’s driving the startup valve full-open—indicating it desires extra feedwater!”
The upkeep technician requested for assist from the valves’ unique gear producer (OEM) consultant. The OEM rep stated it sounded to him like “valve-swapping,” and requested if the tech had been doing the periodic calibrations that these valves require. The tech stated “no,” there had been no discuss of periodic calibration, and requested how that needs to be completed.
|Feedwater-control logic sometimes incorporates a mixture of single- and three-element management. Courtesy: Justin Fullemann, marketing consultant|
The OEM rep defined that the transition from startup valve to important valve is the important parameter that must be re-calibrated on a periodic foundation, to make it possible for it’s nonetheless programmed to the “80/20” rule: The startup valve needs to be at 80% of its capability, exactly when the primary valve is at 20% of its capability. Any deviation from this rule may cause unstable drum-level management, the OEM rep emphasised. The upkeep tech although that might clarify the present dilemma, so he went again to the feedwater-control station and carried out the calibration examine. However, he was disenchanted to seek out that the calibration was spot-on right. He backtracked to his store and referred to as the OEM for a second time.
The OEM additionally was disenchanted that the issue wasn’t a scarcity of calibration, and he instructed that one other attainable trigger was improper sizing of the valves again within the plant’s design and building part. To examine this risk, the troubleshooters pored by their design and building recordsdata. They studied every valve’s coefficient of move (Cv)—the important design parameter for large-bore industrial valves—and in contrast the design worth within the OEM information to the as-built worth stamped on the put in valves. But this examine additionally discovered nothing amiss—the valves had been sized correctly.
That was the troubleshooters’ second swing-and-a-miss, so their disappointment grew into outright frustration. They noodled on the issue some extra, and determined that the following factor to examine was the situation of the valves’ trim (the inner elements). As talked about above, this plant was 10 years previous—which is downright aged in comparison with many of the combined-cycle fleet, so that they figured decade of wear-and tear on the valves might need broken the trim, regardless of its sturdy design.
To examine this risk, they opened and inspected the valves. And, simply as they suspected, there was, certainly, some proof of abrasion injury. Figuring they’d discovered the…