1. API 598 vs ISO 5208 vs ANSI/FCI 70-2: Quick Comparison

All three standards are relevant to leakage evaluation, but they establish different testing and acceptance frameworks. The distinction is not just the amount of leakage permitted: it is also how each standard defines the test, the applicable valve type, the measurement procedure and the acceptance criterion.

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API 598:2023 covers inspection and testing for several industrial valve types. ISO 5208:2015 is a pressure-testing framework for metallic valves, used alongside relevant product standards. ANSI/FCI 70-2-2021 focuses on control valve seat leakage testing. Specify the governing standard and edition in the purchase documents rather than treating a leakage label as a standalone requirement.

2. Isolation Valve Leakage vs Control Valve Shutoff: What Changes?

Isolation duty: blocking flow under specified conditions

An isolation valve primarily establishes or interrupts a flow path. Ball, gate, butterfly, plug and certain globe valves may perform this duty depending on the application. Engineers need to define the closed-valve pressure differential, required sealing direction, media compatibility, temperature and permitted internal leakage. The failure consequences and required isolation philosophy also matter.

Passing a factory seat test demonstrates that a particular valve met a defined acceptance criterion at the recorded test conditions. Ingenieurbeispiel: a line valve accepted during an ambient-temperature seat test may later encounter elevated temperature or abrasive particles. The test result cannot resolve whether the seat retains its sealing capability after thermal exposure or wear; the project must check seat temperature limits, solids, cycling and the isolation consequence before assigning barrier duty.

Control duty: modulating flow with a separately defined shutoff requirement

A control valve regulates flow, pressure, temperature or another process variable by changing its opening. It may also be required to limit leakage when commanded closed. A control valve can meet the specified modulating duty while allowing a defined amount of leakage at the seat; flow capacity and characteristic do not establish the shutoff class.

Requiring tighter shutoff may change trim contact geometry, seat materials, required seating load, actuator thrust and cost. Ingenieurbeispiel: if a control valve is quoted for the required leakage class but the stated actuator air supply or maximum shutoff differential is missing, the purchaser cannot conclude the assembled valve will develop sufficient closing force. Request the verified shutoff case and actuator sizing basis; control valve product and selection information provides additional product context.

Seat leakage, shell leakage and fugitive emissions are different

Sitzleckage is internal flow past the closed valve. Shell leakage is leakage across the pressure boundary. Fugitive Emissionen (Leckage) involve unintended external releases, often through stem packing or other interfaces. Acceptable shell-test or emissions results do not establish seat tightness, and a passing seat test does not prove external sealing performance.

Which shutoff requirement belongs in the procurement specification?

Start with the consequence of leakage, not the most demanding-sounding class. The specified closed-valve duty determines what must be demonstrated by the supplier; a product-standard test and an operational barrier assessment address different questions.

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The same pressure class or nominal size does not resolve these differences. A low permissible factory leakage value cannot replace confirmation that the specified valve arrangement is appropriate for the system’s isolation objective.

3. API 598: Seat Leakage Acceptance for Industrial Valves

Scope and product-standard relationship

API 598 addresses inspection, examination and pressure testing of the specified gate, globe, plug, ball, check and butterfly valve types. The governing product standard and project purchase specification must also be reviewed. Applicability to a nonstandard configuration should be agreed upon rather than inferred from the valve’s appearance.

Product standard versus test standard: A project may specify a gate, ball, globe or butterfly valve product standard that invokes API 598 for inspection and testing. The purchaser should verify that relationship in the quoted edition rather than citing API 598 as proof of a valve’s pressure-temperature design suitability.

Resilient-seated vs metal-seated valves

Resilient seats use materials and contact geometry that can conform to mating surfaces. Metal seats rely on engineered metallic sealing contact, where surface finish, seat load, temperature and wear can matter. The applicable API 598 closure acceptance depends on the valve and seat construction, rather than one universal leakage allowance for all valves.

Ingenieurbeispiel: a bid for a soft-seated ball valve and a bid for a metal-seated check valve may both state “API 598 compliant,” while applying different closure acceptance provisions. The problem is not the standard citation itself but the missing valve-specific acceptance basis. Compare each offer against its applicable product standard, seat construction, prescribed test and actual leakage observation before treating the bids as technically equivalent.

For further context on seat construction and material selection, review valve seat and sealing-surface materials.

Test scope and evidence

The purchase specification should address applicable shell, backseat and closure tests; test medium; pressure; duration; closure direction; witness points and exceptions. The supplier’s inspection and test plan should show which tests apply to the offered valve, with traceable recorded results.

4. ISO 5208: Closure-Tightness Testing and Leakage Rates

Scope and relationship to product standards

ISO 5208:2015 specifies examinations and tests used to assess pressure-boundary integrity, closure tightness and the structural adequacy of the closing mechanism of industrial metallic valves. It is intended to be used with the applicable product standard where that standard cites it. If product-standard requirements differ, the product-standard requirements govern according to ISO’s published scope statement. ISO also states that the document does not address the safety aspects of pressure testing; an approved safe test procedure is a separate obligation.

How leakage rates should be interpreted

ISO 5208 designates closure leakage rates, including Rate A. The required rate must be linked to the applicable product standard and agreed purchase specification; other rates may involve a defined permissible measured leakage quantity using the relevant test-fluid and valve-size basis. Do not select a rate from the seat material name or pressure class alone. Ask whether the named closure test, acceptance method and permitted rate correspond to the exact valve on the datasheet.

What Rate A actually means

ISO 5208 Rate A is associated with no visually detectable leakage under the prescribed closure-test conditions. This is not a promise of no molecular leakage at every operating pressure, temperature or duration. Sealing performance can change through thermal cycling, corrosion, particulates, wear or reverse pressure direction.

An engineering specification may cross-reference API 598 and ISO 5208, but a cross-reference is not a general acceptance equivalence. Any proposed correspondence must be verified in the controlled editions for the exact valve and seat construction, applicable product standard, test medium, pressure and leakage criterion. The ISO product-standard precedence described above applies only within ISO 5208’s stated scope. See the ISO 5208:2015 official publication and Raymon Valve’s valve standards resource.

5. ANSI/FCI 70-2: Control Valve Seat Leakage Classes I to VI

ANSI/FCI 70-2-2021 establishes control valve seat leakage classes and production test procedures. It is not a predictive calculation of leakage at untested service conditions. Its classification is not a naming alternative for ISO 5208 leakage rates. A credible Class IV, V or VI offer should identify the trim and seat configuration, test procedure, relevant test differential, leakage criterion and closing-force capability of the quoted assembly.

What the classes describe

The standard’s classification system includes Classes I through VI, but an RFQ should refer to the specific class required and its associated procedure rather than using a generic “tight shutoff” label. In practice, Classes IV, V and VI often arise in control valve discussions; their acceptance criteria are not all expressed using the same basis.

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The table summarizes different calculation and test foundations, not the exact permissible values. Class I test and agreement provisions must be checked against the controlled ANSI/FCI 70-2-2021 edition; a historical-edition description should not be presumed unchanged. Use the controlled standard to establish limits and procedures for a purchase order; do not substitute an internet conversion chart or an unspecified “bubble-tight” statement.

Source applicability: The Fluid Controls Institute’s public information sheet explains why seat-leakage benchmarks and specified tests matter. It does not provide the full Class I–VI acceptance tables, test details or proof of a particular valve assembly’s conformity. For those requirements, use the licensed ANSI/FCI 70-2-2021 standard and traceable supplier documentation.

Why IV, V and VI cannot be compared from labels alone

Different classes use different acceptance and measurement foundations, which may reference rated flow capacity, differential pressure, seat or port dimensions and prescribed observation or measurement arrangements. Ingenieurbeispiel: one supplier may quote Class V and another Class VI for an otherwise comparable control duty. The higher-looking class number does not settle the procurement decision because the test bases differ. Request both suppliers’ proposed leakage acceptance calculations and test conditions, then obtain purchaser engineering approval for any alternative class. Do not calculate an exact allowed rate without the controlled ANSI/FCI edition and the valve-specific inputs.

Shutoff differential pressure and actuator closing force

Closing forces can include pressure-induced unbalanced loads, required seat contact load, packing friction, guiding friction and mechanical resistance. The relevant closing pressure or torque needs to be verified using manufacturer data, actual shutoff differential pressure, available utilities and failure-action requirements. Leakage class alone cannot establish actuator sizing or field shutoff capability.

The applicable publication is ANSI/FCI 70-2-2021, listed by the Fluid Controls Institute.

6. Is ISO 5208 Rate A Equivalent to ANSI/FCI Class VI?

Nein. ISO 5208 Rate A and ANSI/FCI Class VI are not universally equivalent. Rate A uses a specified no-visually-detectable-leakage criterion; Class VI permits a defined limited quantity under its control-valve test procedure. Different fluids, pressure conditions, measurement methods and seat/port parameters prevent a general one-to-one conversion. Neither label alone proves that the valve will provide an approved isolation barrier or the same leakage performance in service.

ISO 5208 Rate A is an observation-based closure-tightness acceptance concept: no visually detectable leakage under the prescribed test conditions. ANSI/FCI Class VI permits a defined limited leakage quantity assessed by the applicable control-valve procedure. A visually observed absence of leakage is not the same as a measured numerical zero, and a Class VI test is not a Rate A test. Neither result should be extrapolated to different service temperatures, differential pressures or operating histories without engineering evidence.

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If a purchase specification requires both pressure testing and a separate control-valve leakage test, it must explain which tests apply and how they are accepted. A separate seat test does not replace required pressure-boundary tests. Do not write “ISO Rate A / Class VI equivalent” without a justified, approved technical basis.

7. How Should Engineers Specify Leakage Acceptance in an RFQ?

The following are hypothetical specification examples, not records of completed Raymon Valve customer projects or claims about specific valve performance.

RFQ specification checklist

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Before comparing commercial quotations, resolve any difference that changes the test basis, seat construction or required shutoff capability.

Example purchase-specification wording

The following is an illustrative RFQ template. Values and standards must be filled in using the approved project datasheet; the brackets are not test requirements by themselves.

Valve duty: [isolation/control/combined]. Applicable valve product standard and edition: [ ]. Leakage test standard and edition: [ ]. Required seat leakage class/rate/limit: [ ]. Test medium: [ ]. Test pressure / differential pressure: [ ]. Test direction(s): [ ]. Test duration and measurement method: [ ]. Valve assembly and actuation during test: [ ]. Required report, witness point and deviation approval: [ ].

How to normalize competing technical bids

When offers describe seat tightness differently, create a technical deviation register before comparing price. These are examples of document review actions, not manufacturer test results.

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Illustrative clarification: If an RFQ specifies an ISO 5208 Rate A closure test but a supplier quotation states only “tight shutoff,” return the offer for clarification: request the applicable standard and edition, test medium, differential pressure, duration, observation method and the proposed acceptance record. Keep the bid technically open until the purchaser accepts the response or approves a documented deviation.

Bid review principle: A supplier’s proposed alternative test may be acceptable only after the purchaser has evaluated its scope, acceptance basis and deviations. A lower price does not remove a contractual testing requirement.

Practical bid-approval checkpoint: Before issuing a purchase order, record the requested leakage class or rate, the supplier-offered criterion, any departure in test conditions, the associated actuator or seat changes, and the name or role of the party approving the deviation. Where evidence is incomplete, status the item as clarification required, not compliant. This avoids a later factory acceptance dispute over a supposedly equivalent shutoff requirement.

8. What Should a Valve Seat Leakage Test Report Include?

“Leak test passed” is not enough if the report does not establish which valve was tested, the specified limits and actual conditions. The required record depth depends on the project’s inspection and test plan (ITP).

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Minimum evidence example: For the actual supplied valve or tag, the acceptance record should identify the test standard and edition, medium, applied differential pressure, test duration, seating direction, applicable acceptance limit, observed or measured leakage result, test date, and sign-off or witness evidence when required by the ITP. A classification or “pass” field without the recorded conditions does not allow a buyer to independently compare different test methods.

Inspectors should match the test record to the valve identification, seat construction and offered actuator or closing method. Check that the actual result was recorded in the same units and observation basis as the purchase acceptance criterion, and that relevant equipment identification and calibration status can be traced where required by the ITP. Example: a report bearing only a “PASS” stamp but omitting the test differential or actual leakage result cannot substantiate a cross-supplier comparison. A witnessed test alone does not establish compliance with every project requirement; deviations still need purchaser approval.

What evidence is still missing after a “pass” result?

A test record can demonstrate the result of a particular factory inspection. It does not, on its own, document pressure-temperature rating suitability, fire-safe qualification, material traceability, non-destructive examinations or fugitive-emission performance. If the project requires these, list them as separate datasheet or ITP deliverables and assess their supporting records individually.

For valve assemblies with actuators, confirm whether the tested configuration and available closing force represent the offered assembly. For multi-directional shutoff requirements, ensure every required direction is included in the approved testing scope.

9. Common Mistakes When Comparing Leakage Requirements

1. Calling Rate A and Class VI equivalent: The two standards do not share a universal one-to-one acceptance basis.
2. Specifying Class VI without assembly review: The valve trim, seating force and actuator capability may be affected.
3. Ignoring medium, pressure and direction: Results obtained under different test conditions are not automatically comparable.
4. Promising field performance from factory testing: Temperature, solids, wear, cycling and corrosion can change leakage behavior.
5. Confusing seat, shell and fugitive leakage: Each requires its own relevant acceptance evidence.
6. Comparing bids before normalizing requirements: Suppliers may offer different test bases, seat constructions or deviations.

10. Final Specification Checklist and Engineering Recommendation

Isolation valve leakage and control valve shutoff classifications are separate specification decisions. API 598, ISO 5208 and ANSI/FCI 70-2 each have a defined purpose and should be applied only within their relevant scopes.

1. Define isolation, control or combined duty.
2. Identify the applicable product and testing standards, including editions.
3. State the exact required leakage rate, class or acceptance limit.
4. Specify medium, pressure, duration and test direction.
5. Verify seat construction and closing-force or actuator capability.
6. Request traceable inspection records and explicitly resolve deviations.

A reliable procurement decision depends on the complete datasheet, actual service conditions, project specifications and documented evidence—not a leakage class label alone.

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