{"id":15078,"date":"2026-11-01T16:55:28","date_gmt":"2026-11-01T08:55:28","guid":{"rendered":"https:\/\/raymonvalve.com\/?p=15078"},"modified":"2026-11-01T16:56:23","modified_gmt":"2026-11-01T08:56:23","slug":"partial-stroke-testing-esd-valves","status":"publish","type":"post","link":"https:\/\/raymonvalve.com\/es\/blog\/partial-stroke-testing-esd-valves\/","title":{"rendered":"Partial Stroke Testing for ESD Valves: Test Coverage, Diagnostic Limits, Proof-Test Relationship and Acceptance Evidence"},"content":{"rendered":"
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Engineering Guide \u00b7 ESD Valve Testing \/ Functional Safety<\/p>\n\n\n\n

Partial stroke testing (PST) for ESD valves<\/strong> verifies a defined portion of an emergency shutdown valve assembly\u2019s movement and response without commanding the complete safety stroke. It may reveal sticking, sluggish actuation or faults in the exercised control path, but a successful PST does not prove full closure, seat tightness or the safety integrity level of the complete SIF. Engineers should compare the tested path with the dangerous failure modes being credited, document acceptance limits and preserve measured response evidence before relying on PST in a proof-test strategy.<\/p>\n\n\n\n

\"Engineering
Engineering schematic showing a conceptual PST path; not evidence of a tested installation.<\/figcaption><\/figure>\n\n\n\n

Engineering decision:<\/strong> Specify what the test exercises, which failures are potentially detected, how abnormal results are handled and which additional full-stroke or leakage checks remain necessary. The presence of a PST device alone does not establish SIL performance.<\/p>\n\n\n\n

1. What Is Partial Stroke Testing for an ESD Valve?<\/h2>\n\n\n\n

An ESD valve is designed to move to a specified safe position after a shutdown demand. A normally open, fail-closed isolation valve may remain open for long periods, allowing faults in the valve or operating mechanism to develop unnoticed. A controlled PST introduces limited travel to assess selected operability and diagnostic conditions while avoiding a complete commanded safety stroke.<\/p>\n\n\n\n

Online PST may be feasible under an approved operating arrangement, but its process impact and risk of unintended movement must be assessed. A partial stroke is not automatically non-disruptive.<\/p>\n\n\n\n

For an overview of the valve and accessory interfaces discussed here, see Raymon Valve\u2019s industrial ball valves for automated isolation<\/a>; that product-category information does not establish PST or SIF compliance.<\/p>\n\n\n\n

How the test works<\/h3>\n\n\n\n
    \n
  1. Confirm authorization, operating conditions and the approved PST procedure.<\/li>\n\n\n\n
  2. Issue the defined test command through the configured PST path.<\/li>\n\n\n\n
  3. Measure movement, response and available diagnostic variables.<\/li>\n\n\n\n
  4. Confirm return to the required normal operating state.<\/li>\n\n\n\n
  5. Record pass, fail or inconclusive disposition with alarms and exceptions.<\/li>\n<\/ol>\n\n\n\n

    This is a description of test principles, not an operating instruction or permission to manipulate a live safety system. The approved site procedure governs test execution.<\/p>\n\n\n\n

    \"Illustrative
    Illustrative PST response trend; values and acceptance thresholds are equipment-specific.<\/figcaption><\/figure>\n\n\n\n

    Test boundary and PST method<\/h3>\n\n\n\n

    The valve body, stem, closure member, actuator, solenoid, pneumatic or hydraulic supply, position feedback, PST device and shutdown command path are not necessarily all exercised in one test. The scope depends on whether the design uses mechanical restriction, travel limiting, position control or solenoid-associated PST and on where the command and feedback signals are connected.<\/p>\n\n\n\n

    A PST initiated through an independent positioner may not test the same shutdown solenoid or logic path used during a genuine ESD demand. Specify the included components and excluded paths in the test documentation.<\/p>\n\n\n\n

    Document the tested path before assigning coverage<\/h3>\n\n\n\n

    For the project acceptance record, distinguish the command source<\/strong>, actuated path<\/strong>, measurements<\/strong> y excluded functions<\/strong>. An independently commanded positioner test, for instance, must not be credited as checking a separate ESD solenoid or logic output unless the test architecture actually exercises that route. Similarly, a position feedback indication does not independently verify final-element mechanical integrity if the measurement shares the fault being assessed.<\/p>\n\n\n\n

    Illustrative specification issue:<\/strong> A project requires the shutdown solenoid to respond on demand, but the proposed PST is commanded only through a separate positioner port. The test may confirm movement while leaving the shutdown solenoid path unexercised. The buyer should request the pneumatic schematic, command-path description and an additional approved test for any excluded safety-critical path. This is a hypothetical configuration, not a Raymon Valve field case.<\/p>\n\n\n\n

    Test-boundary decision:<\/strong> Record whether the shutdown solenoid, pneumatic supply path, actuator spring, valve stem, closure member, position indication and return-to-service confirmation are exercised, monitored, excluded or require separate verification. Do not use the words \u201ccomplete ESD loop tested\u201d without system-level evidence.<\/p>\n\n\n\n

    2. What Failures Can PST Detect?<\/h2>\n\n\n\n

    PST is useful when a selected fault would affect movement within the tested range and the installed sensors can reliably identify that change. Detection is conditional on test method, process loading, measurement resolution and the failure-mode model.<\/p>\n\n\n\n

    Control-path qualification:<\/strong> A solenoid or shutdown-path fault is potentially detectable only if the PST actually operates the relevant path and records a response sensitive to that fault. Position-sensor faults are likewise not automatically detected by a position-based test: a failed or miscalibrated sensor can mask the actual movement unless independent or suitably diagnostic feedback is available. Review the installed wiring, pneumatic\/hydraulic schematic and test command source before assigning fault coverage.<\/p>\n\n\n\n

    Failure mode \/ condition<\/th>PST detection potential<\/th>Additional evidence or boundary<\/th><\/tr><\/thead>
    Failure to initiate movement<\/td>Often detectable<\/td>Representative command and reliable movement feedback<\/td><\/tr>
    Stem or shaft sticking<\/td>Potentially detectable<\/td>Fault must affect the tested travel<\/td><\/tr>
    Increasing packing or bearing friction<\/td>Condicional<\/td>Sensitive trend measurements and comparable baseline<\/td><\/tr>
    Actuator response failure<\/td>Condicional<\/td>Only exercised actuator\/control functions evaluated<\/td><\/tr>
    Insufficient full-stroke torque or thrust<\/td>Not established<\/td>Full-range margin review and relevant operating conditions<\/td><\/tr>
    Solenoid or shutdown path fault<\/td>Configuration-dependent<\/td>Verify whether the solenoid and shutdown path are exercised<\/td><\/tr>
    Position feedback malfunction<\/td>Condicional<\/td>Independent feedback check or calibration as applicable<\/td><\/tr>
    Final travel obstruction<\/td>Not reliably detected<\/td>Applicable full-stroke verification<\/td><\/tr>
    Seat leakage after closure<\/td>Not demonstrated<\/td>Applicable seat-tightness\/leakage verification<\/td><\/tr>
    Degradation beyond tested travel<\/td>May remain hidden<\/td>Additional inspection or broader testing<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
    \"Matrix
    Qualitative failure detection matrix; actual coverage depends on assembly and test method.<\/figcaption><\/figure>\n\n\n\n

    For example, limited rotation of an ESD ball valve may demonstrate that movement begins, while failing to reveal high friction near final seating, a defective seat or inadequate actuator margin under maximum relevant differential pressure. A response within the PST acceptance envelope is evidence of that test<\/em>, not full isolation performance.<\/p>\n\n\n\n

    Example: an apparently successful test with residual shutdown risk<\/h3>\n\n\n\n

    Consider a hypothetical<\/em> normally open fail-closed quarter-turn isolation valve. The approved PST moves the closure member a limited distance and confirms a return to its initial position. The record demonstrates that movement occurred through the tested path, but it does not establish the actuator’s available margin at the most demanding shutdown differential pressure, the ability to reach final seating, or the required closed-seat leakage performance. Those are separate verification questions. This is an illustrative decision example, not a Raymon Valve project or certified test result.<\/p>\n\n\n\n

    3. Diagnostic Coverage: Meaning and Limits<\/h2>\n\n\n\n

    Diagnostic coverage concerns the portion of relevant dangerous failures classified as detectable by a defined diagnostic arrangement. In a simplified failure-rate classification, DC = \u03bbDD<\/sub> \/ (\u03bbDD<\/sub> + \u03bbDU<\/sub>)<\/strong>, where DD and DU denote dangerous detected and dangerous undetected failure-rate components. This expression is not a way to infer a numerical PST coverage value for an unspecified assembly.<\/p>\n\n\n\n

    Scope of the diagnostic-coverage expression:<\/strong> This ratio describes a defined population of dangerous failure modes with a consistent detected\/undetected classification. It does not show which faults a specific PST actually observes, whether detection is credited immediately, or whether a periodic PST is modeled as an online diagnostic or a partial proof test. The selected reliability method must define failure rates, test effectiveness, diagnostic or PST interval, detection latency and fault restoration assumptions. Do not substitute partial travel percentage for coverage.<\/p>\n\n\n\n

    Travel percentage is not diagnostic coverage<\/h3>\n\n\n\n

    A particular percentage of valve travel does not imply the same percentage of dangerous failure modes detected. Failure mechanisms are not uniformly distributed over travel. A test may reveal sticking near the starting position yet provide little evidence about final closure or seating.<\/p>\n\n\n\n

    Coverage must follow the actual architecture<\/h3>\n\n\n\n

    Consider valve design, seat and packing arrangement, actuator sizing margin, utility conditions, solenoid circuit, sensors, resolution, test direction, process loads, test frequency, failure-rate basis and how faults are classified and repaired. A manufacturer’s generalized coverage statement should not be transferred to a different package without assessing these assumptions.<\/p>\n\n\n\n

    Whether PST is modeled as an online diagnostic or as a partial proof test requires consistent treatment of detection time, interval, failure categories and repair. Avoid double-counting detection credit.<\/p>\n\n\n\n

    Keep diagnostic detection and proof-test credit separate<\/h3>\n\n\n\n

    Los t\u00e9rminos diagnostic coverage<\/em>, PST detection coverage<\/em> y proof-test coverage<\/em> are not interchangeable across calculation methods. A periodic PST may be modeled as a diagnostic check or as a partial proof test depending on the safety assessment, fault handling and reliability method. The analysis must explicitly state which dangerous failure modes are revealed, when faults become known, what happens after an alarm, and what proportion remains for the subsequent full proof-test procedure. Do not add two coverage percentages without verifying their underlying failure-mode sets and assumptions.<\/p>\n\n\n\n

    Reliability review example:<\/strong> A supplier may provide a diagnostic-coverage estimate for a specific actuator and PST device. If the project changes the valve seat design, travel limit or pneumatic circuit, that figure may no longer represent the delivered assembly. The responsible engineer should reconcile the supplied reliability data with the as-built configuration, fault response and proof-test model before using it in SIF verification.<\/p>\n\n\n\n

    4. PST vs Full Stroke Proof Testing<\/h2>\n\n\n\n

    PST and full-stroke checks address different portions of the shutdown function. Full movement is often necessary to assess travel through the complete range, but even a full-stroke test is not automatically a complete SIF proof test; the approved procedure must encompass the appropriate function and failures.<\/p>\n\n\n\n

    For a separate discussion of shell and closure testing, see valve pressure and seat testing<\/a>; pressure and seat tests serve a different acceptance purpose from PST.<\/p>\n\n\n\n

    Comparaci\u00f3n<\/th>Partial stroke testing<\/th>Full stroke \/ proof testing<\/th><\/tr><\/thead>
    Movement<\/td>Defined limited travel<\/td>Full specified travel where included<\/td><\/tr>
    Movement initiation<\/td>Evaluated if commanded and measured<\/td>Evaluated<\/td><\/tr>
    Final seating<\/td>Not demonstrated<\/td>May be evaluated with proper procedure<\/td><\/tr>
    Seat tightness<\/td>Not established<\/td>Requires applicable leakage check<\/td><\/tr>
    Solenoid \/ logic path<\/td>Depends on PST arrangement<\/td>Depends on approved proof-test scope<\/td><\/tr>
    Failure coverage<\/td>Selected mechanisms<\/td>Broader but procedure-dependent<\/td><\/tr>
    Evidencia de aceptaci\u00f3n<\/td>PST trends, configuration and disposition<\/td>Approved proof-test measurements and functional evidence<\/td><\/tr>
    Complete SIF verification<\/td>No<\/td>Only if relevant function and requirements are covered<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
    \"Comparison
    Full valve travel does not automatically demonstrate all proof-test requirements.<\/figcaption><\/figure>\n\n\n\n

    Proof-test relationship and PFDavg<\/h3>\n\n\n\n

    PST can detect some dangerous failures before a real demand and may inform average probability of failure on demand (PFDavg) calculations. Its contribution depends on detectable failure subsets, failure rates, detection and repair time, PST interval, full-proof-test interval and coverage, common-cause assumptions and system architecture.<\/p>\n\n\n\n

    For a highly simplified low-demand case, PFDavg \u2248 \u03bbDU<\/sub> \u00d7 T \/ 2 illustrates the influence of dangerous undetected failure rate and test interval. It is not<\/strong> a complete calculation for a partially tested ESD package or whole SIF.<\/p>\n\n\n\n

    Limits of the simplified PFDavg relationship:<\/strong> The illustrative expression assumes a low-demand context, approximately constant dangerous undetected failure rate, periodic restoration of the modeled faults at the relevant proof test, and a simplified single failure population. It omits partial-test coverage allocation, repair downtime, diagnostic detection latency, imperfect proof testing, common-cause failures and multi-element SIF architecture. A PST interval is the time between partial tests; a full proof-test interval is the time between approved broader proof tests; detection latency and fault restoration time are separate assumptions. None of these may be interchanged in a project-specific PFDavg calculation.<\/p>\n\n\n\n

    Can PST justify a longer full proof-test interval?<\/h3>\n\n\n\n

    Only with a documented and approved reliability assessment. Confirm that the credited failures are actually detectable, residual undetected failures remain addressed, fault repair assumptions are realistic and modified intervals satisfy the applicable SIF requirements. Any changes should follow the site’s functional safety and management-of-change process.<\/p>\n\n\n\n

    Evidence available<\/th>Decision for proposed interval change<\/th><\/tr><\/thead>
    PST device installed; no validated failure-mode evidence<\/td>No interval extension supported by installation alone<\/td><\/tr>
    Vendor coverage figure without applicable test-path and equipment data<\/td>Insufficient: reconcile claimed coverage with installed assembly<\/td><\/tr>
    Documented failure modes, relevant reliability data, test frequency and restoration assumptions<\/td>Eligible for SIF-specific reassessment, not automatic approval<\/td><\/tr>
    Revised SIF verification accepted by responsible engineering and change-control process<\/td>Implement only the approved interval and test plan<\/td><\/tr>
    Unresolved failed\/inconclusive PST or unreliable records<\/td>Investigate impairment; do not claim interval-extension credit<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    Importante:<\/strong> A successful PST, nominal diagnostic coverage percentage or actuator safety manual alone does not demonstrate that the entire SIF achieves its required SIL.<\/p>\n\n\n\n

    5. ESD Valve PST Acceptance Criteria<\/h2>\n\n\n\n

    Acceptance criteria must be approved before testing and tied to the actual valve assembly, method, process restrictions and instrumentation. A controller’s green PASS indication alone is not an adequate technical record.<\/p>\n\n\n\n

    Travel, response and restoration<\/h3>\n\n\n\n

    Identify normal position, test direction, permitted movement range, available sensor resolution and criteria for stopping or invalidating a test. Monitor the applicable starting position, command status, movement initiation, partial travel achieved, return response, alarms and verified post-test position. Where supported, actuator pressure and timing trends add diagnostic context.<\/p>\n\n\n\n

    No universal stroke percentage, response-time limit or pressure threshold is assumed. Acceptance values must be specified from approved equipment documentation, commissioning baselines or project engineering assessment.<\/p>\n\n\n\n

    Three outcome categories<\/h3>\n\n\n\n