A partial stroke test (PST) moves a shutdown valve a few percent of its travel and back, usually from the safety PLC or a smart positioner, without stopping the process. It reveals failure modes such as a seized stem, a stuck actuator or a solenoid that does not vent, but not the failures that only show at full travel or at the seat.
Modelling
Split λDU into three parts by the test that reveals them:
- PSTC · λDU, revealed by the partial stroke (interval T_PST, e.g. 3 months),
- the share revealed only by the full proof test,
- the share revealed only at renewal (if proof test coverage is below 100 %).
Each part contributes roughly (its rate) × (its interval) / 2.
Worked example
Valve λDU = 2 × 10⁻⁶ per hour, perfect full proof test yearly, partial stroke every 3 months with a coverage of 60 % of λDU.
| Part | Rate | Interval | Contribution |
|---|---|---|---|
| Revealed by partial stroke | 1.2 × 10⁻⁶ | 2,190 h | 1.3 × 10⁻³ |
| Revealed only by full test | 0.8 × 10⁻⁶ | 8,760 h | 3.5 × 10⁻³ |
| PFDavg | ≈ 4.8 × 10⁻³ |
Without the partial stroke the valve gave 8.8 × 10⁻³. The PST almost halved PFDavg, and it can also justify a longer full proof test interval, which is often the real economic reason for it.
Conditions for taking credit
- The PST coverage must come from an analysis of the valve's failure modes (often in the device FMEDA for a given PST method), not from a general claim.
- Failed partial stroke tests must be acted on like any detected dangerous failure.
- A PST does not replace the full proof test: seat leakage, full travel and stroke time at process conditions still need it.
- Check the spurious trip risk of the PST itself and the process tolerance to a small valve movement.
In First Out AI
Enter the partial stroke interval in months and its coverage in percent of λDU in the group details (Pro). The SRS draft lists the PST in the proof testing section, and the proof test draft reminds that the PST does not replace the full stroke.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.