Advantages of Partial Stroke Testing (PST)

Given that Partial Stroke Testing (PST) of the valve and it attached devices have been implemented successfully, you should be able to achieve the following benefits:

  • Increase the SIL (lower PFDavg) of the valve, keeping the Proof Test Interval constant
  • Lengthen the Proof Test Interval of the valve, keeping the SIL constant
  • Combination of the above
  • Eliminates the need for a second ESD valve in some cases

However, it is imperative that performing a PST of a critical or safety (ESD) valve does not cause a spurious trip of the process, due to a failure in the device performing the PST. Most spurious trips occur from solenoid valve (SOV) failures, and not by failures related to the valve itself.

As such, the PST device should have internal diagnostics, be fully fault tolerant, and fail safe. Ideally, it should be capable of repair online without by-passing or disabling its safety function.

In addition, it should prohibit over-stroking of the valve (because of a sluggish response), which could also initiate a spurious trip of the process due to excessive valve closure.

Author - Dr. Lawrence Beckman