API 598 seat-leakage acceptance criteria: what the test does and does not prove

1. What API 598 covers

API 598 provides a basis for valve inspection, pressure testing and allowable leakage criteria. That makes it an acceptance document, not a complete maintenance programme.

The test evidence should identify the valve, test basis and recorded result. The applicable standard text remains the authority for the exact acceptance requirements.

Operators typically keep this evidence with the valve handover record. It gives a useful starting point for later comparison, when the records remain traceable.

The valve integrity management guide shows where acceptance evidence sits within the whole asset life cycle.

2. API 598 does not use ISO-style leakage classes

API 598 and ISO 5208 both address valve leakage testing, but their frameworks are distinct. API 598 provides allowable leakage criteria for its test basis.

ISO 5208 defines leakage-rate classes and corresponding test requirements. Do not attach an ISO class label to an API 598 result without a stated ISO 5208 test basis.

Do not convert one result into the other from a valve description alone. Acceptance needs the named test basis, conditions and result.

Our ISO 5208 leakage-rate guide explains how its classes should be read without implying equivalence.

3. What a passing factory result proves

A passing result shows that the tested valve met the stated acceptance basis during that test. This is valuable evidence, but its time boundary is narrow.

The result does not promise zero future leakage. It does not show how contamination, wear, storage, installation or service later affected sealing surfaces.

It also does not prove that every field test should copy the factory method. Installed systems have different boundaries, operating limits and safety controls.

1. Confirm which standard and edition formed the acceptance basis.
2. Keep the tested valve identity linked to the result.
3. Record the test conditions and accepted outcome.
4. Avoid adding a leakage class that the record never states.

4. What an in-service result needs

An installed valve needs evidence suited to its current isolation duty. The method, boundary and acceptance basis should be agreed before testing starts.

Downstream pressure rise, continuing flow and noise are screening observations. They indicate possible passing but can have other causes.

Common alternatives include incomplete travel, stop settings, equalisation, another flow path and instrumentation error. A proper seat or isolation test must resolve the question.

The passing valves guide explains this screening and confirmation sequence. The ASME B31.8 overview covers programme-level operating decisions.

5. Keep acceptance and maintenance claims separate

Operators typically store factory acceptance and in-service condition as separate records. Each record should state what was tested and what remains unknown.

A changed field result does not erase the earlier acceptance evidence. It shows that condition must now be assessed at the installed asset.

Where maintenance is considered, suitability gates come first. Injection fittings must be sound, design and service must suit injection, OEM guidance applies, and operations must permit the work.

Our field services can survey, treat and retest suitable installed valves. Severe or uncertain conditions need proper assessment, isolation, repair or replacement.