{"id":2860,"date":"2026-10-10T22:03:31","date_gmt":"2026-10-10T22:03:31","guid":{"rendered":"https:\/\/petrostreet.com\/main\/?p=2860"},"modified":"2026-10-10T22:03:36","modified_gmt":"2026-10-10T22:03:36","slug":"partial-stroke-testing-pst-of-emergency-shutdown-valves","status":"publish","type":"post","link":"https:\/\/petrostreet.com\/main\/partial-stroke-testing-pst-of-emergency-shutdown-valves\/","title":{"rendered":"Partial Stroke Testing (PST) of Emergency Shutdown Valves"},"content":{"rendered":"\n

1. Introduction<\/strong><\/strong><\/h4>\n\n\n\n

Emergency Shutdown (ESD) valves are critical elements of Safety Instrumented Functions (SIFs) designed to isolate hazardous inventories, stop process feed, prevent escalation of abnormal operating conditions, and mitigate major accident hazards. In oil and gas facilities, petrochemical plants and refineries, these valves may remain in the fully open position for years, awaiting a demand to close. During this period, corrosion, deposits, actuator deterioration, packing friction, mechanical seizure, solenoid valve malfunction and other failure mechanisms may compromise their ability to perform their safety function.<\/p>\n\n\n\n

Full Stroke Testing (FST) is traditionally used to demonstrate that an ESD valve can travel through its complete operating range and reach its required safe position. However, performing FST while a process unit is operating may interrupt production, affect process stability, or require isolation and shutdown arrangements that are only practical during a turnaround.<\/p>\n\n\n\n

Partial Stroke Testing (PST) provides a way to test selected elements of the shutdown valve assembly without moving the valve through its entire operating range. Properly designed and managed, PST can detect a proportion of dangerous undetected failures, improve confidence in ESD valve availability, and support a risk-based extension of the interval between full stroke tests.<\/p>\n\n\n\n

Key principle:<\/strong> PST can help justify delaying FST, but it does not automatically replace FST or authorize extending the proof-test interval beyond the validated Safety Instrumented System (SIS) design.<\/p>\n\n\n\n

2. International codes and standards<\/strong><\/strong><\/h4>\n\n\n\n

The following standards provide the principal framework for implementing PST and managing its effect on functional safety.<\/p>\n\n\n\n

    \n
  • IEC 61511-1 \u2014<\/strong> Functional safety for the process industry sector; lifecycle management, proof testing and verification that test intervals support the required SIL.<\/li>\n\n\n\n
  • IEC 61508 \u2014<\/strong> Broader functional-safety framework for reliability, dangerous failure probabilities and systematic capability.<\/li>\n\n\n\n
  • ISA-TR96.05.01-2017 \u2014<\/strong> Partial Stroke Testing of Automated Valves; practical guidance on PST methods, limitations, valve movement and safety-function impact.<\/li>\n\n\n\n
  • ISA-TR84.00.03-2023 \u2014<\/strong> Automation Asset Integrity of Safety Instrumented Systems; inspection, testing, maintenance and lifecycle assurance.<\/li>\n\n\n\n
  • API RP 554 \u2014<\/strong> Process instrumentation and control practices; consult the applicable edition and relevant provisions.<\/li>\n\n\n\n
  • API RP 14C \u2014<\/strong> Analysis, Design, Installation, and Testing of Basic Surface Safety Systems for Offshore Production Platforms; applicable where offshore systems fall within its scope.<\/li>\n<\/ul>\n\n\n\n

    Check applicable editions, local regulations, company engineering practices and valve-manufacturer instructions before implementation. Standards do not establish one universal PST interval or guarantee a fixed percentage of failure detection for every valve.<\/p>\n\n\n\n

    3. How PST works<\/strong><\/strong><\/h4>\n\n\n\n

    PST moves an ESD valve a predefined percentage toward its safe position while the process remains in operation, then returns it to its normal position. The test is typically initiated through a smart valve positioner, dedicated PST controller or approved automated test system.<\/p>\n\n\n\n

    For example, an ESD valve normally held fully open may be commanded to close by 10\u201320% of its travel before being returned to fully open. This is an illustrative range only; actual movement must be selected through engineering assessment, not adopted as a universal default.<\/p>\n\n\n\n

    A meaningful PST should verify actual valve movement rather than merely confirming that a command was issued or a limit-switch signal changed. Useful parameters include:<\/p>\n\n\n\n

      \n
    • Actual travel achieved against the target travel.<\/li>\n\n\n\n
    • Time taken to initiate and complete the partial movement.<\/li>\n\n\n\n
    • Breakaway friction, actuator pressure and travel profile, where measurable.<\/li>\n\n\n\n
    • Position feedback and return to the normal operating position.<\/li>\n\n\n\n
    • Diagnostic alarms, abnormal deviations and any process disturbance.<\/li>\n<\/ul>\n\n\n\n

      The test must not disable the ESD function or compromise the valve’s ability to close on demand. A controlled test procedure, operating authorization and defined abort criteria are essential.<\/p>\n\n\n\n

      4. Advantages of PST in operating plants<\/strong><\/strong><\/h4>\n\n\n\n
        \n
      • Early detection of dangerous failures.<\/strong> PST can identify certain failures that prevent or impede movement, including a sticking valve, excessive friction, actuator problems and some solenoid or control-system faults, depending on test configuration.<\/li>\n\n\n\n
      • Reduced exposure to undetected failure.<\/strong> Regular testing can reveal certain faults earlier than waiting for a turnaround. The safety benefit depends on test coverage, frequency, reliability of the method and prompt repair.<\/li>\n\n\n\n
      • Reduced production disruption.<\/strong> A carefully engineered online PST may avoid the production interruption associated with a complete valve stroke.<\/li>\n\n\n\n
      • Potential extension of the FST interval.<\/strong> Where supported by documented failure-mode coverage and SIL verification, PST information may permit a longer interval between full stroke tests without exceeding the allowable probability of failure on demand.<\/li>\n\n\n\n
      • Better maintenance planning.<\/strong> Trends in travel time, actuator pressure, friction and repeat failures can help identify deteriorating valves and plan repairs before a turnaround.<\/li>\n<\/ul>\n\n\n\n

        These benefits are conditional. PST does not establish that the valve can travel its full distance, achieve the required shutdown time, or provide the required final-seat leakage performance.<\/p>\n\n\n\n

        5. Using PST to delay full stroke testing until the next turnaround<\/strong><\/strong><\/h4>\n\n\n\n

        The principal opportunity is to use PST as a complementary proof-testing activity that reduces the accumulation of dangerous undetected failures between full proof tests. The decision should be based on the valve’s failure modes and the SIF’s calculated average probability of failure on demand (PFDavg), rather than a general rule that successful PST permits FST to be postponed.<\/p>\n\n\n\n

        5.1 Establish the existing FST requirement<\/strong><\/strong><\/h5>\n\n\n\n

        Establish the approved proof-test interval from the SIF safety requirements specification, SIL verification, manufacturer recommendations and applicable company standards. Determine whether the interval is driven by the final element, other SIF components, or the overall SIF reliability requirement. A turnaround interval of five years does not automatically mean an ESD valve may safely remain without FST for five years; the original SIL verification may require a shorter interval.<\/p>\n\n\n\n

        5.2 Determine PST coverage<\/strong><\/strong><\/h5>\n\n\n\n

        Perform a failure modes and effects analysis (FMEA), or use suitable Failure Modes, Effects and Diagnostic Analysis (FMEDA) data, to classify dangerous undetected failure modes into:<\/p>\n\n\n\n

          \n
        • Failures detectable by PST.<\/li>\n\n\n\n
        • Failures detectable only by FST or another specific test.<\/li>\n\n\n\n
        • Failures requiring separate leakage, response-time, inspection or diagnostic checks.<\/li>\n<\/ul>\n\n\n\n

          Do not assume all valve failures are detectable by PST. Coverage depends on valve type, actuator, solenoid arrangement, test method, process service and acceptance criteria. The estimate must be justified for the actual application.<\/p>\n\n\n\n

          5.3 Recalculate the required test interval<\/strong><\/strong><\/h5>\n\n\n\n

          A simplified illustration for a low-demand SIF, assuming constant dangerous failure rates, independent failures and ideal periodic testing, is:<\/p>\n\n\n\n

          PFDavg \u2248 (\u03bbD,PST \u00d7 TPST \/ 2) + (\u03bbD,FST \u00d7 TFST \/ 2)<\/strong><\/p>\n\n\n\n

          Where \u03bbD,PST is the dangerous failure rate associated with failures detectable by PST; \u03bbD,FST is the dangerous failure rate associated with failures not detected by PST but detectable by FST; TPST is the PST interval; and TFST is the full stroke proof-test interval.<\/p>\n\n\n\n

          This is illustrative and simplified. A complete SIF verification may need to account for proof-test coverage, common-cause failures, repair times, diagnostic effectiveness, test-induced failures, bypass exposure and the contributions of sensors and logic solvers.<\/p>\n\n\n\n

          5.4 Formal approval of the extended interval<\/strong><\/strong><\/h5>\n\n\n\n

          Before changing the FST interval, the responsible functional-safety authority should confirm that:<\/p>\n\n\n\n

            \n
          1. The proposed interval satisfies the required SIL and safety requirements.<\/li>\n\n\n\n
          2. PST coverage and test effectiveness are documented and defensible.<\/li>\n\n\n\n
          3. Failure modes not covered by PST have suitable test or inspection arrangements.<\/li>\n\n\n\n
          4. Manufacturer restrictions and mechanical limitations are addressed.<\/li>\n\n\n\n
          5. The revised interval is approved through functional-safety management and change control.<\/li>\n\n\n\n
          6. The maximum interval is entered in the proof-test schedule and maintenance management system.<\/li>\n<\/ol>\n\n\n\n

            This provides a defensible basis for aligning FST with the next turnaround where justified. It is not a blanket exemption from proof testing.<\/p>\n\n\n\n

            6. Actions following PST failure<\/strong><\/strong><\/h5>\n\n\n\n

            A failed PST should be treated as a potential impairment of a safety-critical final element until its significance is established.<\/p>\n\n\n\n

            Recognize and record the failure.<\/strong> Record the valve tag, test conditions, achieved travel, response time, actuator pressure, alarms and diagnostic results. Distinguish an actual mechanical failure from an instrument, feedback or test-system fault.<\/p>\n\n\n\n

            Assess the safety function immediately.<\/strong> Notify operations, instrumentation and control, maintenance, and process-safety or functional-safety representatives. Determine whether the ESD function can still be relied upon and apply the site’s impairment and compensating-measures procedure.<\/p>\n\n\n\n

            Diagnose and repair.<\/strong> Investigate the actuator, solenoid valve, instrument air or hydraulic supply, positioner, coupling, valve stem, internal deposits and mechanical friction. Arrange maintenance, isolation or shutdown as required by risk assessment.<\/p>\n\n\n\n

            Retest and update records.<\/strong> After corrective action, perform necessary functional tests and confirm that the valve meets acceptance criteria. Record failure mode, root cause where appropriate, repair, test results and effects on the proof-test strategy.<\/p>\n\n\n\n

            Operational decisions after a failed PST<\/strong><\/strong><\/h5>\n\n\n\n
              \n
            • If the safety function cannot be demonstrated:<\/strong> apply the approved SIF impairment procedure, establish compensating safeguards and determine whether continued operation is acceptable. Shutdown may be required.<\/li>\n\n\n\n
            • If a test-system or feedback fault is suspected:<\/strong> investigate and verify the valve’s actual condition independently. Do not classify the PST as successful merely because the fault appears electrical or diagnostic.<\/li>\n\n\n\n
            • If the valve is repaired:<\/strong> repeat the relevant tests, verify restoration of the safety function and close out the impairment through the authorized process.<\/li>\n\n\n\n
            • If failures recur:<\/strong> perform root-cause analysis and review the PST method, test interval, valve suitability, maintenance strategy and FST interval.<\/li>\n<\/ul>\n\n\n\n

              A failed PST should also trigger a review of the assumptions used to justify extended FST intervals. If the failure reveals a previously unrecognized failure mode, the existing PST coverage estimate may no longer be valid.<\/p>\n\n\n\n

              7. Other effects and limitations of PST<\/strong><\/strong><\/h4>\n\n\n\n
                \n
              • Process disturbance.<\/strong> Partial valve closure can reduce flow, alter downstream pressure, affect compressor or pump operation, change reactor conditions or interfere with minimum-flow requirements. Permitted movement must be established through process engineering assessment, particularly for compressor anti-surge, reactor feed and critical cooling services.<\/li>\n\n\n\n
              • Spurious trips and test-induced failures.<\/strong> Poorly configured PST logic, incorrect bypass management or faulty feedback can generate alarms, unnecessary trips or unintended valve movement. Validate the sequence and protect against accidental initiation.<\/li>\n\n\n\n
              • Mechanical damage and wear.<\/strong> Repeated movement can affect packing, actuator components and valve internals. Engineering may vary the test position within approved limits where appropriate.<\/li>\n\n\n\n
              • False confidence in tight shutoff.<\/strong> A valve that successfully moves 15% may still be unable to complete its stroke or achieve the required seat leakage. This is particularly important where isolation prevents hazardous inventory transfer, toxic releases or escalation of a major accident.<\/li>\n\n\n\n
              • Maintenance backlog and reliability implications.<\/strong> A high PST completion rate is not sufficient evidence of good safety performance if failed tests remain unresolved, overrides are prolonged or FST dates are repeatedly deferred without reassessment.<\/li>\n<\/ul>\n\n\n\n

                8. Best industrial practices for oil and gas, petrochemical and refining facilities<\/strong><\/strong><\/h4>\n\n\n\n
                  \n
                • Prioritize by risk.<\/strong> Start with ESD valves whose failure could materially affect required SIL or major-accident risk reduction. Consider service severity, valve history, operating environment and turnaround interval.<\/li>\n\n\n\n
                • Validate the test method.<\/strong> Demonstrate that the test produces measurable movement and detects intended failure modes. Use appropriate position feedback, travel-time monitoring and acceptance limits.<\/li>\n\n\n\n
                • Select suitable applications.<\/strong> Exercise particular caution with dirty, polymerizing, crystallizing, corrosive, erosive or plugging services, and where tight shutoff is a critical safety requirement.<\/li>\n\n\n\n
                • Control bypasses and overrides.<\/strong> Any bypass, inhibit or override must be authorized, indicated, time-limited and managed under site SIS procedures. PST must never unintentionally inhibit a shutdown demand.<\/li>\n\n\n\n
                • Use trend data.<\/strong> Monitor travel deviation, response time, actuator pressure, test failures, repeat defects and overdue proof tests. Investigate deterioration rather than simply repeating a failed test.<\/li>\n\n\n\n
                • Close the failure loop.<\/strong> Capture PST failure modes in the maintenance system and reliability database. Feed results into FMEA\/FM\u200b\u200bEDA assumptions, SIL verification and future maintenance intervals.<\/li>\n\n\n\n
                • Audit interval extensions.<\/strong> Review extended FST intervals periodically and whenever process conditions, valve design, failure history or safety requirements change.<\/li>\n<\/ul>\n\n\n\n

                  9. Conclusion<\/strong><\/strong><\/h4>\n\n\n\n

                  Partial Stroke Testing is a valuable component of the functional-safety strategy for ESD valves in continuously operating oil and gas, petrochemical and refining facilities. It provides opportunities to identify certain dangerous failures online, reduce the period during which those failures remain undetected, improve maintenance planning and potentially defer full stroke testing until a planned turnaround.<\/p>\n\n\n\n

                  PST is a supplement to, not an automatic replacement for, full stroke proof testing.<\/strong> It cannot establish full travel, complete closure, seat tightness or the required full-stroke shutdown time unless those properties are verified through suitable additional tests.<\/p>\n\n\n\n

                  The most defensible approach is to determine PST coverage through failure-mode analysis, quantify its contribution to SIF reliability, confirm that the revised FST interval meets the required SIL, and formally approve any extension through functional-safety management.<\/p>\n\n\n\n

                  Where these conditions are satisfied, PST can help plants achieve a practical balance between operational continuity and safety assurance\u2014allowing full stroke testing to be scheduled around turnarounds without relying on an unverified assumption that successful partial movement proves the entire ESD function.<\/p>\n","protected":false},"excerpt":{"rendered":"

                  1. 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Ahmad","author_link":"https:\/\/petrostreet.com\/main\/author\/hussnainahmad983gmail-com\/"},"uagb_comment_info":0,"uagb_excerpt":"1. 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