Partial Stroke Test PFDavg Calculator
Partial stroke testing (PST) of shutdown valves finds some dangerous failures between full proof tests. This calculator shows the PFDavg with and without PST from the test coverage and both test intervals.
- Without PST: PFDavg = λDU × Tfull ÷ 2 = 0.035
- With PST: PFDavg = C × λDU × TPST ÷ 2 + (1 − C) × λDU × Tfull ÷ 2 = 0.0145
Partial Stroke Test Formula
C is the partial stroke test coverage (the fraction of dangerous undetected failures it reveals), TPST the partial stroke interval and Tfull the full proof test interval. This applies to a 1oo1 valve and ignores repair time and test errors. The coverage should come from an FMEDA for the actual valve and actuator.
Worked example
A valve with λDU = 2 × 10⁻⁶ /h, full tested every 4 years, has PFDavg ≈ 0.035. Monthly partial stroke tests with 60 % coverage cut it to about 0.015, a 2.4 times improvement.
Frequently Asked Questions
What is a partial stroke test?
Moving a shutdown valve a small part of its travel, often 10 to 20 %, to prove it is not stuck, without stopping the process.
What coverage can a partial stroke test claim?
It depends on the valve and actuator FMEDA, commonly in the range of about 50 to 70 % of dangerous undetected failures.
Does PST replace the full proof test?
No. Full proof tests are still needed to find the failures PST cannot detect, such as seat leakage.
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