Full travel does not prove tight shut-off

Evidence class one: position indication

Position evidence describes the command, actuator or indicator state. Examples include a local pointer, stem position, limit switch or control-system feedback. Each can show that the operating train reached a set point.

The evidence can still be wrong. Linkage slip, poor switch setting, lost motion and display faults can separate indication from the internal closure member. A closed lamp is therefore an instrument reading, not an isolation certificate.

Compare local and remote indications where both exist. Check whether the operating history shows a complete stroke. The actuator torque guide covers faults that can interrupt travel while controls still appear normal.

Evidence class two: mechanical closure

Closure evidence asks whether the ball, gate or plug reached its intended closed position. Travel marks, turns, stop settings and actuator movement help answer this question. Design records provide the expected relationship between operator travel and valve travel.

Reaching a stop is not always full closure. The operator may meet a gearbox stop, debris obstruction or wrong actuator setting. Excess force can damage stems, keys or gearing without improving isolation.

When closure remains uncertain, inspect the operating train under an approved plan. Do not force the valve. Our hard-to-operate valve guide explains why rising resistance needs diagnosis before more torque.

Evidence class three: seat integrity

Seat integrity evidence asks whether leakage across the closed valve is acceptable under defined test conditions. A controlled seat test provides this evidence. Pressure decay, a double-block-and-bleed check or another approved method may be suitable.

Downstream pressure rise, flow indication and noise are screening signs. Heating, trapped pressure, connected branches, instrument error and another passing boundary can create false results. Confirm the suspected seat leak with a method that defines the test volume and boundaries.

Standards such as API 598 and ISO 5208 describe seat leakage testing and acceptance language. Field observations cannot be converted directly into a leakage class without the required test basis. See our field seat-leak testing comparison.

Build an isolation statement from all three

A sound isolation statement records each evidence class separately. First record indicated position. Then record evidence of mechanical closure. Finally record the seat test method, conditions and result.

1. Indication: what the local and remote systems reported.
2. Closure: what showed that the closure member completed its intended travel.
3. Integrity: what controlled test assessed leakage across the seat.

If one class is missing, say so. Do not replace missing seat evidence with a stronger claim about travel. The passing valve guide explains repair choices after leakage is confirmed.

Where isolation is safety-critical, use the site's isolation plan and verification rules. A valve condition assessment can organise the evidence, but operations owns the isolation decision.

A declaration of tight shut-off should name the test pressure direction and acceptance basis. It should also state who accepted the result for the intended isolation duty.

Where the valve forms part of a wider boundary, document the other isolation points and any dependence on their condition.

Use plain status labels in records. Write indicated closed, mechanically closed and seat tested separately. Add the date and test conditions. This stops later readers from treating one green signal as proof of all three.