Valve Testing in the Field: Partial Stroke Testing and Proof Testing

Field testing of installed valves verifies that valves continue to perform their intended functions after installation and throughout their service life. Two important field testing methods for automated safety valves are partial stroke testing (PST) and full proof testing. These tests are particularly important for emergency shutdown valves and other safety instrumented system (SIS) components whose operation is infrequent during normal process operation and whose failure to actuate on demand could have serious safety consequences. Regular testing provides confidence that the valve will function when required and detects degradation before it leads to a dangerous undetected failure.

Partial Stroke Testing

Partial stroke testing (PST) moves an on-off valve a limited fraction of its full stroke (typically 10 to 20 percent of travel toward the closed position) and then returns it to the fully open position, without interrupting the process protected by the valve. The PST verifies that the valve is not stuck in the fully open position and can begin to move, that the actuator is receiving the control signal and developing pressure, and that the stem and closure element mechanics are functional. PST detects the most common dangerous failure mode for safety valves—failure to move from the open position—with high coverage while minimizing the process impact compared to full stroke testing. Smart positioners and partial stroke test devices provide automated PST capability with position monitoring and trip detection.

• PST stroke: typically 10-15% of full travel toward closed, then return to full open

• PST frequency: typically quarterly to monthly depending on SIL requirements and diagnostic coverage

• Diagnostic coverage: fraction of dangerous failure modes detected by PST, typically 50-90%

• False trip avoidance: PST must not inadvertently close the valve completely during the test

• IEC 61511: SIS standard that credits PST diagnostic coverage in safety integrity calculations

Full Proof Testing

Full proof testing verifies the complete operation of the safety valve from the full open to the fully closed position and back, confirming that the valve achieves its required shutoff. Full proof testing provides the highest diagnostic coverage (typically approaching 100 percent for most failure modes) but requires either taking the protected process out of service or providing an alternate means of process isolation during the test. In some processes, proof testing requires a planned shutdown, limiting test frequency to annual turnarounds. In others, process design provides redundant isolation that allows one valve to be proof tested while the process remains in service through alternate paths. IEC 61511 requires that proof test intervals be set short enough to maintain the required safety integrity level given the valve's failure rate and the diagnostic coverage of available tests.

As-Found and As-Left Testing

As-found testing documents the condition of the valve before any maintenance is performed, providing data on actual in-service valve condition and degradation rates. As-found data reveals whether the valve would have performed its safety function at the time of the test and is used to update failure rate assumptions in safety integrity calculations. As-left testing documents the valve condition after maintenance to verify that all maintenance activities were completed correctly and the valve meets its performance specification before returning to service. Comparing as-found and as-left data over multiple maintenance cycles reveals trends in valve performance degradation, informing decisions about maintenance intervals, spare parts management, and potential valve replacement before service life is exhausted.