Valve Leakage Classes: API 598, ISO 5208, FCI 70-2, ANSI FCI 70-3

Valve seat leakage is meaningful only with the governing standard and edition, valve type, seat design, test medium, pressure, direction, duration, temperature, measurement method and acceptance rate. API 598, ISO 5208, ANSI/FCI 70-2 and MSS SP-61 serve different product and test scopes; their rates and classes must not be treated as interchangeable.

Quick comparison

API 598 is an inspection and testing standard for covered valves and contains product-dependent test requirements and allowable leakage. ISO 5208 specifies pressure testing of metallic industrial valves and closure tightness rate designations, used with the applicable product standard. ANSI/FCI 70-2-2021 defines control-valve seat leakage classes under its test conditions. MSS SP-61 covers pressure testing of specified valves within its own scope. Choose the standard from the product standard and purchase specification—not from the desired marketing term.

API 598: valve inspection and testing

API's standards plan lists API Standard 598, Valve Inspection and Testing, 11th edition (2023). API 598 does not provide the same Class I–VI system used for control valves, and it should not be summarized with one formula or a few copied bubble counts. Requirements vary with valve type, nominal size, seat material, test medium and direction.

For procurement, state the exact API 598 edition and the valve product standard, then verify shell, backseat where applicable, high- and low-pressure closure tests, duration, test direction, cavity or double-block arrangements, and permitted leakage using the purchased text. Do not assume every seat test is 1.1 times a rating at 38°C or every gas test is 60–100 psi.

ISO 5208: metallic industrial valve pressure testing

ISO 5208:2015 specifies examinations and tests used by a manufacturer to establish pressure-boundary integrity, closure tightness and structural adequacy of the closure mechanism. ISO confirmed this edition in 2025. It is intended to be used with the applicable valve product standard; when that product standard differs, the product-standard requirements apply.

ISO 5208 closure tightness rate designations depend on test conditions and measurement basis. Do not copy a simplified A–G table without the media, units, size basis and valve requirements. 'Rate A' means no visually detectable leakage for the specified test, not a promise of permanent zero leakage in operating service.

ANSI/FCI 70-2: control-valve seat leakage

The current FCI catalog lists ANSI/FCI 70-2-2021, Standard for Control Valve Seat Leakage Testing. It has not been renamed to ANSI/FCI 70-3. The standard provides benchmark seat-leakage classes for control valves under defined laboratory conditions. Control valves are not automatically isolation valves, and a class rating does not establish a safe isolation boundary.

Classes I through VI use different definitions and test methods; they are not a single scale that can be converted directly to ISO 5208 or API 598. Avoid claiming Class VI universally means zero or only soft seats, or that metal seats can never meet a specified rate. Compliance depends on the exact class procedure, valve design, rated capacity or seat dimensions where applicable, medium, pressure, temperature and documented test.

MSS SP-61: use only within its scope

MSS SP-61 is a separate pressure-testing standard for valves. It is not simply a less comprehensive or optional version of API 598, and it should not be assumed to apply to every API 600, API 602 or API 609 valve. Confirm the product standard, purchase order and the current MSS edition, including shell and closure test requirements, media, duration and leakage.

Why classes cannot be converted directly

API, ISO, FCI and MSS may differ in valve scope, test pressure, medium, duration, flow direction, seat loading, allowable leakage expression, size or capacity basis, temperature and measurement technique. A result in bubbles per minute, drops per minute, volumetric rate, percentage of rated capacity or no visible leakage cannot be compared without normalizing the complete test conditions.

Factory tightness also differs from in-service isolation. Process temperature, differential pressure, reverse pressure, contaminants, wear, thermal cycling, actuator settings and pipe loads can change leakage. Never use a factory class alone to authorize line breaking, confined-space entry or maintenance isolation.

How to write the purchase specification

State: valve type, size and class; product standard and edition; seat design and material; leakage-test standard and edition; test medium; high- and low-pressure tests as applicable; test pressure; duration; direction; cavity, double-block or check-valve conditions; allowable rate and units; measurement method; stabilization; calibration; production versus type-test scope; witnessing; records; and any ambient, cryogenic, high-temperature or fugitive-emissions test.

Use the acceptance criterion exactly as written in the governing standard. Do not add an improvised example such as 'air at 6 bar for three minutes' unless the product design and approved procedure support it. More severe pressure or longer duration is not automatically safer and can invalidate comparison or damage soft seats.

Seat material does not guarantee a leakage class

Soft polymer or elastomer seats can provide tight shutoff in compatible service, but pressure, temperature, chemical exposure, permeation, extrusion, particles, fire and cycling limit performance. Metal seats can achieve very low specified leakage with suitable geometry, lapping, load and cleanliness, but may be chosen for temperature, erosion, fire or solids rather than tightness alone.

Do not prescribe one surface roughness, coating or seating force across all valves. Review ball, gate, globe, butterfly, plug and check-valve designs separately. Triple-offset and spring-loaded seats are design features, not guarantees of cryogenic, fire-safe or zero-leakage performance.

Test report review checklist

Check valve identity and serial number, drawing and revision, applicable standards and editions, seat and trim, test medium, fluid temperature, pressure and units, gauges and calibration, stabilization and duration, direction, valve position and actuator settings, observed or measured leakage with units, permitted rate, pass/fail, operator, date, witness, repairs or adjustments, and retest results.

Frequently asked questions

Does bubble-tight mean absolute zero leakage?

No. It normally describes no visible bubbles under a specified test method and observation period. Every measurement method has a detection limit, and the result applies only to the stated conditions.

Can a metal-seated valve meet a very low leakage requirement?

Yes, some designs can meet stringent specified rates under defined tests. Do not assume all metal seats leak significantly or promise permanent zero leakage. Obtain qualification and production-test evidence for the exact design and service.

Is ANSI/FCI 70-2 the same as ANSI/FCI 70-3?

No. FCI's current catalog lists ANSI/FCI 70-2-2021 for control-valve seat leakage testing. The previous article's renumbering claim was incorrect.

How often should an installed valve be leak tested?

Use the regulatory, safety, inspection and maintenance program for the actual service. There is no universal two-to-five-year interval. ESD partial-stroke testing does not measure seat leakage and cannot replace an approved full proof test or isolation test.

Primary references

API standards plan—API 598 edition and title: https://www.api.org/products-and-services/standards/standards-plan

ISO 5208:2015 official scope and current status: https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/06/51/65111.html

FCI current control-valve standards—ANSI/FCI 70-2-2021: https://www.fluidcontrolsinstitute.org/fci-standards-control-valves.asp

MSS standards catalog: https://msshq.org/page/ActiveStandards

Standards are copyrighted and may change. Use the purchased edition specified by the contract.

Contact Wofer Valve

For a leakage-test review, provide the valve type, size and class, product standard, seat and trim, required test standard and edition, medium, pressure, direction, duration, allowable rate, production or type-test scope, witnessing and reporting requirements.

Ted Wang | WeChat/WhatsApp: +86 18267833722 | Email: sales@wofervalve.com | Website: https://www.wofervalve.com