"It was a pretty traumatic event for everyone," began Joe Pittman, recalling the early days of safety valve partial stroke testing (PST). Back in the 1990s, when we wanted to ensure that normally open safety valves would close when called upon, a mechanical "jammer" was used to limit the valve's movement to about 20% closed, explained Pittman, who today serves as team leader, safety instrumented systems (SIS) and automation integrity, Chevron Energy and Technology Company. The operator would initiate a valve closing, the valve would bang up against the jammer, and that's how we knew the valve wasn't stuck in place, Pittman said.
Chevron has since implemented better ways of doing PST, driven by the desire to ensure the integrity of their safety integrity level (SIL)-rated safety functions and collect performance data for SIS valves, Pittman recently said in a presentation to users of Invensys Operations Management's Triconex safety systems in Galveston, Texas. Generally, Chevron is striving to use PST data in a more predictive way, allowing it to better define which valves to pull during a maintenance turnaround. Regulatory requirements also are a factor, Pittman said, citing requirements to test every safety valve in offshore application every 30 days. "That's a driver in itself," Pittman said.
Chevron's current set-up for PST relies on a HART-capable 4-20mA digital valve controller connected to the safety system logic solver. The PST itself is initiated through the digital HART signal, which also passes back data such as valve position, test results, faults and failure information back to a supervisory asset management system. "The logic solver has no PST control logic, and in fact doesn't even know that the PST is taking place," Pittman explained.
The Keys to Operator Acceptance
Pittman pointed to operator confidence and operator control as critical contributors to successfully implementing a PST program. PST routines should be set to operate in manual, semi-automated and automated modes. Start in manual, Pittman recommended, meaning that operators receive reminders when valves are due for a PST but must initiate the test manually. In semi-automated mode, operators are notified that an upcoming test will proceed automatically unless the operator intervenes. And in fully automatic mode, PSTs proceed in the background without operator intervention or notification but can readily be put on hold. Pittman has found that operators nearly always want manual control to start, but once they have confidence in the procedure they slowly migrate to automated mode—especially if it saves them 50 or more alerts in the course of a day.
"Operators need to understand what's happening and that they have control," Pittman said. Build confidence in the PST device by relating it to the hundreds of well-behaved positioners currently installed on existing control valves. "They're reliable and not bad actors," Pittman said. Keep it simple, using hardware and software designed for implementation of PST (don't build your own). Further, Pittman recommended that a PST "owner" be designated and trained to understand the PST data. Develop work practice defining what is done if faults or failures are identified. And test often, Pittman said. "When pre-scheduled and conducted automatically, frequent testing does not increase manpower."
Down the road, Chevron is looking to get even more predictive understanding of valve performance from its PST efforts, Pittman said. Are there opportunities to automatically stroke the valve further than normal based on lower than usual process flow requirements? Or could acoustics be used to tell whether an apparently closed valve actually stopped the flow? Ultimately, too, successfully applying PST requires human judgment to understand a valve's performance histogram and correctly identify any issues. The automatic flagging of which valves need attention—immediately or at the next scheduled turnaround—just might be another step forward in Chevron's continuing PST journey.