Partial Stroke Testing of Safety Instrumented Functions (SIFs)
Safety engineers often grapple with the decision of whether to perform a full test or a partial stroke test on a valve, which serves as the final element of a Safety Instrumented Function (SIF) when determining Safety Integrity Levels (SIL). This article explores the essential considerations and guidelines for making this crucial choice.
Basic Requirements
In accordance with IEC 61511 – Part 1 section A.16.3, a comprehensive discussion of proof testing and inspection of a SIF is provided. This section outlines specific requirements for these procedures, emphasizing the importance of achieving the average Probability of Failure on Demand (PFD) of the SIF and conducting proof testing in real operational conditions. The following key points are highlighted:
- Proof test intervals should align with the target average PFD for the SIF.
- Proof testing should simulate actual operating scenarios and should occur prior to any routine maintenance that may impact or distort the test results.
- Integral tests are preferred, encompassing all components, or they should overlap, such as the Sensor Element/Logic Solver (SE/LS) and Logic Solver/Final Element (LS/FE).
- When a full loop test cannot be conducted due to safety or operational concerns, partial testing is allowed for devices and systems/subsystems of a SIF.
- If opting for partial testing, the procedure must be documented in the test procedures (safety-loop validation report) and should include:
- Full testing of the final element during shutdown.
- Testing the SIF during normal operation, as far as possible, including the output trip relay, shutdown solenoid, and partial valve movement.
Impact of Testing Period on PFD
It is essential to recognize that any limitations on the testing period of the final elements directly affect the PFD of the SIF. These limitations must be factored into the calculation of the average PFD of the SIF. Consequently, a complete SIF loop test should be conducted at predetermined intervals.
Partial Stroke Test vs. Full Test
Partial stroke tests cover only a portion of possible failures and do not offer the same level of diagnostic coverage as full tests. For further insights into this subject, the “Instrument Engineers’ Handbook, Process Control and Optimization, Volume II” (ISBN: 0-8493-1081-4) serves as a valuable resource. In Section 6 of the book titled “Emergency Partial-Stroke Testing of Block Valves,” various block valve component failures are explored, and tables are provided to determine the contribution of failures that could potentially lead to a dangerous valve failure (PFD).
Summary of Table 6.10e: Failures, Failure Modes, and Test Strategy
| Failures | Failure Modes | Full Stroke Test | Partial Stroke Test |
|---|---|---|---|
| Actuator sizing is insufficient to actuate valve in emergency conditions | Valve fails to close (or open) | Can’t be tested | Can’t be tested |
| Valve packing is seized | Valve fails to close (or open) | Suitable | Suitable |
| Valve packing is tight | Valve is slow to move to closed or open position | Suitable | Suitable |
| Air line to actuator crimped | Valve is slow to move to closed or open position | Suitable | Suitable |
| Air line to actuator blocked | Valve fails to move to closed or open position | Suitable | Suitable |
| Valve stem sticks | Valve fails to close (or open) | Suitable | Suitable |
| Valve seat is scarred | Valve fails to seal off | Suitable | Not Suitable |
| Valve seat contains debris | Valve fails to seal off | Suitable | Not Suitable |
| Valve seat plugged due to deposition or polymerization | Valve fails to seal off | Suitable | Not Suitable |
Partial Test Suitability
Partial testing is suitable in the following situations:
- When final elements (valves) are normally open or in an open position.
- When conducting a functional test of the SIF could lead to a full plant shutdown, potentially causing delays in restarting the plant and additional risks.
Conclusion
Based on experience and interactions with various clients, the following recommendations are made:
- All SIFs should undergo full testing during commissioning and startup to ensure comprehensive testing.
- Consult with a process engineer or plant operator to determine whether partial testing or a full test is necessary and assess its impact on normal operations, with reference to the provided table.
- If partial testing is required, revisit the SIL verification calculations. The new PFD will be a combination of PFD(AVG)(PST) and PFD(AVG)(FT), with different proof test coverage and test intervals for each part.
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