The 24-in. characterized-seat ball valve was one of the minority of instrument âthingsâ in the plant that constituted a single point of failure, bringing all production to a halt immediately if, for example, it failed to open. Like many continuous process plants, an outage would mean a day or more of lost production, unusual wear and tear on equipment and catalyst, and increased risk for operators forced to engage in many non-routine procedures. That it was the anti-surge valve for the facilityâs un-spared turbine-driven air compressor was another reason to be mindful of its healthâdamaging this turbomachinery through an unmitigated compressor surge event could mean weeks instead of days of downtime.
It stood to reason that operators were skittish about its health when they noticed it didnât consistently respond to small position setpoint changes. One click, two clicks of the mouse: no movement. Each click was less than a half-percent change in setpoint; even if you climbed up to the top deck of the reactor where it was located, you would be challenged to visually detect such a small move. Maybe such a large valve shouldnât be expected to respond to such a small change, but if the valve was sticking, was it going to jump and overshoot the desired position, upsetting the process?
Operations entered a work request to pull this valve for overhaul in the next outage, months away. Itâs not small and being high in the structure, requires a large crane to bring it down and reinstall it. Wouldnât it be great, one imagines, if we could precisely determine this valveâs condition and confirm it needed an overhaul?
The operators could even detect their setpoint changes werenât getting the valve to move in response was evidence they had position feedback from a smart positioner. Controller I/O capable of HART or fieldbus communication can be employed to display the valve position âas seen byâ its positioner. Having been incorporated into MMI graphics for years, operators were accustomed to observing whether a valve was moving to its desired position, sticking or unstable. Valve position feedbackâif reliableâis interesting and useful information for routine operations as well as control systems troubleshooting.
Valve positioners were among the first instruments to become âsmart,â i.e., microprocessor-based and capable of self-diagnosis. Even before smart positioners were commonplace, experts could be engaged to hook up sensors to laptops and assess (offline) whether a given valve was healthy or not. This particular valveâs positioner had been smart for nearly two decades. You would think that a critical âthingâ such as this would get some measure of routine inspectionâ"Let âs see how itâs doing todayââwhether it was weekly or even monthly. Indeed, there are probably a number of critical valves that warrant some routine attention, control valves whose health and performance have an immediate impact on continuous operation. But whoâs excited to tinker with critical positioners that are functioning adequately?
If youâre fortunate enough to have a system that can communicate with smart devices, consider an investment in a late-model positioner, like the Fisher FieldVue DVC6200 and its kin, that have been on the market for a few years. One can license access to features like process diagnostics: while on process, the DVC can automatically monitor for a number of trigger eventsâa sustained deviation between setpoint and measured position would be one example, whereupon the positioner itself gathers data from its numerous internal sensorsâsupply pressure, actuator pressures, position sensors, drive signals, etc.âat 100 millisecond intervals. The HART version will store this data in a buffer for a minute or two after the trigger event, while the fieldbus version will store the data for both two minutes preceding and two minutes after. These records are preserved and can be examined offline, and sent to a specialist at your representative or the factory.
Even without a compatible host system, diagnostic data from critical valves can be retrieved using a handheld device like the Emerson AMS Trex handheld communicator, or a laptop/tablet running Fisher ValveLink, Metso Valve Manager, Masoneilan ValVue, Flowserve ValveSight, or compatible software from your favorite control valve supplier. If youâre looking to get started on utilizing âintelligentâ devices to avert unplanned outages and better prepare for scheduled shutdowns, deploying smart valve positioners on critical valves is a great place to start.
