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At what interval PST is performed for ESDs and what is the reference here?

For Emergency Shutdown (ESD) valves, the Partial Stroke Test (PST) interval is not fixed by any international standard. The testing frequency is determined by the required SIL, reliability calculations, failure rates, manufacturer's recommendations, and the site's proof test philosophy.
Typical practice, we follow is:

High-demand SIL 2/SIL 3 shutdown valves - Monthly to Quarterly
Lower criticality ESD valves - Every 6 to 12 months

A typical engineering process to determine the interval is:
1. Determine the required SIL of the Safety Instrumented Function.
2. Obtain valve failure data (e.g., from FMEDA or reliability databases).
3. Decide the full proof test interval (often aligned with plant turnaround, e.g., 4–6 years).
4. Use SIL verification software (such as exSILentia, SILSolver, or equivalent) to determine the PST interval needed to achieve the target PFDavg.
5. Validate that the chosen PST interval is practical from an operations and maintenance perspective.

Nothing fixed by any standard.
The primary references in this case are:
IEC 61511
Requires periodic proof testing of Safety Instrumented Functions (SIFs).
States that proof test intervals shall be established based on the required SIL and reliability analysis.
Does not prescribe a fixed PST interval.
IEC 61508
Provides the basis for proof testing and calculating PFDavg.
Recognizes partial stroke testing as a means of improving diagnostic coverage where justified.
ISA TR84.00.03
One of the principal technical references specifically covering PST.
Explains how PST intervals can be selected and incorporated into SIL verification calculations.
Discusses achievable diagnostic coverage and common implementation methods.

Attaching a useful document on this.