I saw an email string with a question asking if it’s possible to detect all possible failure modes in a safety valve if very intensive and intrusive tests, such as full-stroke testing, leak testing, etc. are performed. Would it be conceivable to use 100% proof test coverage when performing full repair upon detection of a failure?
Emerson’s Mike Boudreaux noted that many people would not claim 100% proof test coverage, because they cannot fully test all failure scenarios in operating conditions. He noted that it’s common to use 90+% proof test coverage for valves. If one needs 100% coverage then they would need to ensure that they are not only testing all of the dangerous failure modes, but also that they are testing them properly without possibility of human errors.
Mike noted that several Fisher valves and actuators have been certified to the IEC 61508 international functional safety standard. Here is an Exida functional safety assessment report for GX Series valves and actuators, Series ED, ET, EZ, and HP valves, Type 657 and 658 actuators, Vee-Ball Series V150, V200, and V300 valves, and Type 1051 and 1052 actuators. He noted that safety manuals on these products provide insight into the certification for these valves and the related proof test procedures.
In an earlier post, Credit for Partial Stroke Tests in Verification Process, I discussed how partial stroke testing could extend the diagnostic coverage for the safety valve and actuator.
Emerson’s Riyaz Ali shared his process safety experience in the email thread and noted that full-stroke test coverage cannot be claimed at 100%, until and unless the valve is re-built with a proper seat. Generally, mechanical items unlike electronics follow a bathtub curve.
Assuming that SIS shutdown on-off valves do not have as much energy throttling compared with basic process control system (BPCS) valves, one would expect the SIS shutdown valve to have better erosion protection. Still, its seat area needs to be re-built and certified to be as good as its original state, in order to claim 100%. Riyaz counsels to be very cautious in using 100% proof test coverage. He also notes that many leading companies with whom he’s worked have never claimed more than 95%, even after the valve seat has been re-built.
For those of you who do the PFDavg calculations for your organization, what proof test coverage percentage do you use?
