It is not uncommon for some users to perform partial stroke testing (PST) using cumbersome mechanical or pneumatic devices, or avoid PST altogether due to fears of spurious trip. This workshop addresses the false trip fears around PST and explores the use of smart digital valve controllers to improve safety, reduce false trips and extend the time between final control element proof tests.
Afton opened with some benefits of partial stroke testing including higher levels of safety integrity, reduced false trip rate, less capital equipment, and less yearly manual proof test expense. Concerns include higher false trip rate, increased capital expense, and a degraded PST result. Safety instrumented systems are a balance between safety and availability.
Bill noted that the only way to guarantee that a safety system won’t have a false trip shut the plant down is not to install a safety system. He walked through a high-pressure trip condition with a Rosemount 3051S SIS transmitter, DeltaV SIS logic solver, ASCO solenoid and Fisher Controls GX actuator for the safety instrumented function (SIF). This design is SIL 1 with a mean time to fail spurious (MTTFS) of 90 years.
Adding a partial stoke test to the SIF lowers the MTTFS from 90 years to 24 years and remains suitable for SIL 1 reduction. This is the source of concern for many process manufacturers on using partial stroke tests in their safety functions.
Adding a DVC6000 SIS digital valve controller extends the capability to SIL 2 and MTTFS to 96 years. Adding automatic PSTs provides dynamic action on the safety valve and reduces the probability of dangerous failure modes.
One common concern heard is a degraded PST result may require the plant staff to take down the process prematurely to investigate—can’t afford the downtime. Bill noted it’s much better to know that there is a problem, and have a plan to address it in a planned manner. Other concerns were the speed of safety valve opening time. I addressed how volume boosters can address this concern in a post, Addressing Safety Valve Opening Times with Partial Stroke Tests.