A valve datasheet may state “inspection and testing: API 598” and “seat leakage: Rate A.” This is not automatically contradictory. API 598 can define the test route, while an rate identifies the closure-leakage acceptance. Problems arise when the order does not say which requirement governs.

As of November 2026, the current published editions are API 598, 11th Edition, February 2023, and:2015, Edition 4, confirmed in 2025. Always state the edition in an RFQ because older project documents may reference earlier versions.

The Short Answer

API 598 and both address valve inspection and pressure testing, but they organize the requirements differently.

API 598 is widely used with API valve product standards. It covers factory examinations and pressure tests, including shell, backseat where applicable, and closure tests. Acceptance depends on valve type, seat construction, size, test medium, and the applicable product standard.

ISO 5208 covers pressure-boundary integrity, closure tightness, and structural adequacy. It also identifies leakage rates such as Rate A. It is used with the relevant valve product standard; if that standard differs, the product standard takes precedence.

Neither standard is universally “stricter.” The answer depends on the exact valve and the complete test requirement.

Key Differences at a Glance

What Does Rate A Mean?

Rate A is commonly specified when no visible leakage is required during the defined closure test. However, “Rate A” alone is not a complete test instruction. The result still depends on the specified test medium, pressure, holding time, test direction, valve size, seat design, and governing product standard.

Other rates permit measured leakage within defined limits. A supplier therefore should not say only that a valve “passes” without identifying the rate and test conditions.

Does API 598 Mean Zero Leakage?

Not in every case. API 598 acceptance depends on the valve design and seat type. Resilient-seated and metal-seated valves may have different permitted results, and applicable product standards can add requirements.

A buyer should not convert “API 598 compliant” into a blanket promise of bubble-tight performance. The standard name does not replace the acceptance details.

Shell Testing and Seat Testing Prove Different Things

A shell test checks the pressure-containing boundary of the valve under specified factory conditions. A closure or seat test checks leakage through the closed obturator and seat interface. Passing one does not automatically prove the other.

Factory acceptance is not a lifetime guarantee of zero in-service leakage. Performance can change with cycling, temperature, contamination, corrosion, installation stress, and seat wear.

How to Write a Clear Valve Test Requirement

A useful RFQ or inspection and test plan should state:

1. Testing standard and edition.

2. Governing valve product standard.

3. Required shell, backseat, and closure tests.

4. Test medium, pressure, holding time, and direction.

5. Required leakage rate or maximum allowable leakage.

6. Valve size, pressure class, body material, and seat construction.

7. Whether test results must be measured, recorded, and witnessed.

8. Which document governs if requirements conflict.

An illustrative clause could read:

“Pressure testing shall follow API 598, 11th Edition. Closure acceptance shall meet Rate A where permitted by the governing product standard. Test medium, pressure, duration, and direction shall follow the approved inspection and test plan. Actual leakage shall be recorded. The project specification and governing product standard shall take precedence in case of conflict.”

This wording still requires project-specific engineering review; it is not a universal specification.

One Final Caution

API 598 and are not the correct leakage standards for every valve. Control valves and pressure-relief valves commonly use other standards. Confirm the valve category before copying a test clause from another project.

The practical rule is simple: do not ask which standard is stricter until the valve type, product standard, edition, test conditions, and leakage acceptance are all defined. A complete requirement prevents more inspection disputes than a standard number alone.