The station, Kansas’ only nuclear power plant, installed Emerson’s Fisher Fieldvue DVC6000 digital valve controllers to extend the life of critical control valves that needed stabilizing. The Wolf Creek plant produces electricity for more than 800,000 homes. Fisher is located in Marshalltown, Iowa.
Plant operators had noticed an accumulation of graphite dust on the packing flange of a feedwater regulator valve, indicating a problem. Packing friction acts to stabilize the valve and keep its plug from vibrating or “chattering,” which can accelerate packing failure. Loss of friction on these critical service valves could lead to a plant trip or unscheduled shutdown at any time.
The maintenance crew discussed ways to keep the valve operating until the plant’s next scheduled outage for refueling, almost 18 months down the road. Their stop-gap measure was to inject leak sealant into the packing while the valve remained in service. The sealant would extend the life of the packing and enable the valve to continue operating. The injection process would require careful monitoring, however, because the sealant might also affect packing friction. Too little friction and the valve would continue to leak and chatter. Too much friction and the valve could lock up.
According to Justin Keim, supervisor of engineering, the online repair process would not have been possible without the Fieldvue instruments. The positioner’s “accurate measurement and monitoring capabilities enabled us to maintain system stability while packing friction dropped from 500 pounds to zero during the sealant injection,” he says, noting that the instrument’s fast response kept up with the changing friction load, in spite of the valve’s heavy chatter.
The Fieldvue instruments were used in combination with AMS ValveLink software to provide online real-time data about the valves’ position and condition, including variables such as valve packing friction. This performance data is easily documented and trended over time, enabling plant operators to diagnose problems, and to plan and prioritize valve maintenance.
Keim said the team would not have attempted the project using the old pneumatic positioner that the digital valve controller replaced.
“Online valve diagnostics via Fieldvue instruments enabled our personnel to document drive signal, valve position and packing friction levels throughout the injection process,” Keim says. “The valve remained in service the entire time, and we avoided a potentially costly plant shutdown.”
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