API 598 vs MSS SP-61 vs FCI 70-2 Class VI: Which Forged Valve Seat Leakage Test Do You Need?

A forged valve seat leakage test specification that says “Class VI per API 598” looks precise in an RFQ, but it combines two different acceptance systems. API 598 is an inspection and pressure-testing standard for industrial valves. ANSI/FCI 70-2 defines seat-leakage classes for control valves under specified test conditions. MSS SP-61 provides another pressure-testing framework for valve shells and seat closures.

The three documents are not interchangeable labels for “no leakage.” A measurable purchase requirement must identify the valve function, standard and edition, test medium, pressure or differential pressure, test direction, duration, leakage limit and reporting method.

Quick Answer

For a forged gate, globe or check valve used for isolation, specify the applicable product standard and an isolation-valve pressure-test standard such as API 598 or MSS SP-61, as required by the project. For a throttling control valve, ANSI/FCI 70-2 may be used to define a seat-leakage class.

Do not write only “Class VI,” “zero leakage” or “API 598 tight shutoff.” Those phrases omit the conditions needed to reproduce and accept the test. The purchase order and approved inspection and test plan (ITP) should contain one complete test matrix.

API 598, MSS SP-61 and FCI 70-2 Answer Different Questions

As checked in August 2026, API lists API Standard 598, Valve Inspection and Testing, as the 11th edition dated February 2023, with a 12th edition under development. MSS lists SP-61-2019 for pressure testing of valve shells and seat closures. The Fluid Controls Institute lists ANSI/FCI 70-2-2021 for control-valve seat leakage testing. Because editions change, the purchase order should always state the required edition instead of relying on “latest.”

Document	Primary purpose	Typical purchasing question it answers	What it does not establish by itself
API 598	Inspection and pressure testing of valves	Which shell, closure and other production tests apply, and what acceptance basis is used?	The complete service duty, trim selection or every project-specific supplementary test
MSS SP-61	Pressure testing of valve shells and seat closures for full-open and full-closed valve types	What shell and closure tests and acceptance criteria apply under this specification?	An automatic equivalence to every API 598 requirement
ANSI/FCI 70-2	Classification and testing of control-valve seat leakage	Which leakage class and defined test conditions apply to a control valve?	A universal isolation-valve shutoff requirement or “absolute zero” leakage
ASME B16.34	Pressure-temperature ratings, materials, dimensions, examination, testing and marking for covered valve constructions	What general design and rating framework applies to the valve?	A complete order-specific seat-leakage matrix

This distinction matters for forged valves because the same RFQ may contain an API 602 product description, an ASME pressure class, an API 598 inspection reference and a copied “Class VI” note. A list of standards is not yet an executable test instruction.

For product-family context, see CLDG’s types of forged steel valves. For the separate relationship between Class, PN, temperature and material, use the valve pressure-rating guide.

Why “Class VI per API 598” Is Ambiguous

The phrase is ambiguous for three reasons.

First, it mixes a control-valve leakage classification with an isolation-valve inspection reference. The buyer may intend a soft-seated, bubble-tight isolation valve, a metal-seated control valve, or simply a stricter-than-standard closure test. Those are different engineering requirements.

Second, a leakage limit cannot be assessed without test conditions. Gas and liquid tests do not produce directly comparable results. The pressure differential, flow direction, seat type, nominal size, port or orifice basis, stabilization time and measurement unit all affect the result.

Third, “zero leakage” is not a measurement method. A visual liquid inspection, a bubble counter, a pressure-decay test and a calibrated flow measurement have different detection limits. If the purchase order does not define the method, two suppliers can test the same valve differently and both claim compliance.

The Nine Fields Every Purchase Order Should Lock

Use the following matrix before quotation approval. It converts a vague adjective into a repeatable acceptance test.

Required field	What to state	Why it matters
1. Valve duty	Isolation, non-return, throttling or control	Determines whether an isolation-valve or control-valve leakage framework is appropriate
2. Standard and edition	For example, API 598 11th edition, MSS SP-61-2019 or ANSI/FCI 70-2-2021	Prevents silent changes between editions
3. Test item	Shell, backseat, high-pressure closure, low-pressure closure or specified seat-leakage class	Separates pressure-boundary integrity from closure performance
4. Test medium	Water, air, nitrogen or another approved fluid	Leakage behavior and detection method depend on the medium
5. Test pressure basis	Exact pressure or the approved rating/standard rule used to calculate it	Prevents confusion between operating pressure, Class rating and test pressure
6. Direction and side	Upstream side, downstream side, both directions or cavity arrangement	Essential for directional seats, check valves and piston-effect designs
7. Duration	Holding time after the target pressure has stabilized	Starting the timer during pressurization shortens the effective test
8. Acceptance limit	Permitted leakage with unit and measurement method	Makes “tight shutoff” auditable
9. Records and witness status	Gauge IDs, calibration, medium temperature, result, repair/retest status, Hold/Witness/Review point	Creates traceability for release

The valve data sheet should also identify seat and trim materials. A soft seat and a metal seat should not inherit the same unqualified leakage wording merely because the body size and pressure class match.

A Factory Test Workflow That Produces Defensible Evidence

CLDG’s controlled pressure-test procedure provides several practical controls that are often absent from short RFQs. They are useful examples of what an approved ITP should make visible.

1. Review the basis before testing. Confirm valve identification, drawing status, seat materials, applicable standard, test pressure source and purchaser-specific requirements.
2. Select a suitable calibrated gauge. CLDG’s factory procedure places the target pressure within 25% to 75% of the gauge range. A gauge selected only because it can survive the pressure may give poor resolution.
3. Control the test medium. The controlled procedure uses test fluid in the range of 5°C to 40°C. For austenitic stainless-steel pressure boundaries, its water-quality control limits chloride content to below 20 ppm.
4. Remove trapped air during hydrostatic testing. Trapped gas stores energy and can distort the test. The fixture must not apply external loads that mask or create leakage.
5. Do not use sealing compounds to pass the test. Temporary sealants would invalidate the evidence unless explicitly permitted for the intended design and procedure.
6. Start the holding time after reaching and stabilizing the specified pressure. Pressurization time is not holding time.
7. Inspect the correct boundaries. Shell testing examines the pressure boundary and static joints. Closure testing examines the seat interface under the specified direction and conditions. Backseat testing, when applicable, is a separate item.
8. Record failures and retests. A failed valve should be stopped and segregated. Any repair must be followed by the required repeat test, with both the original disposition and final result traceable.

The exact pressure and duration must come from the approved order-specific basis. They should not be copied from an old catalogue, a different valve type or a supplier’s unrelated test certificate.

A Practical ITP Matrix for Forged Isolation Valves

The following structure works for small-bore forged gate, globe and check valves without pretending that one numerical pressure fits every rating, material and standard edition.

ITP stage	Parameter to approve	Objective evidence	Typical participation
Document review	Valve tag, design standard, inspection standard and editions, seat/trim materials, approved drawing	Approved data sheet and drawing	Manufacturer review; EPC or owner review as specified
Gauge and medium check	Gauge range and calibration validity, medium, temperature and water quality where applicable	Calibration record and pre-test checklist	Review or witness point
Shell test	Pressure basis, medium, stabilization and duration, visible-leakage acceptance	Signed test record; photo or video only if required	Witness or hold point for selected valves
Closure/backseat test	Test side and direction, pressure, duration, seat type, leakage unit and limit	Measured result and acceptance statement	Witness or hold point as specified
Repair and retest	Nonconformance disposition, repair method and repeat-test scope	NCR/repair record plus new test result	Review before release
Final release	All records complete and valve drained, dried, protected and capped	Manufacturing record book or release dossier	Manufacturer, EPC, owner or third party per approved ITP

Not every order needs four-party witnessing. The participation level should follow service criticality, project specification and the agreed ITP. The important point is that Hold, Witness and Review points are assigned before testing, not reconstructed after a dispute.

Three Common Specification Failures

1. “API 598, Class VI” on a Forged Gate Valve

This wording does not say whether the buyer wants the standard API 598 closure acceptance, a separate ANSI/FCI 70-2 test, or a project-specific gas leakage limit. The correction is to identify the valve duty and write each requested test as a separate row.

2. “Zero Leakage” Without a Unit

The requirement cannot be verified consistently. Replace it with the applicable standard and edition, medium, differential pressure, direction, duration, allowable leakage, unit and measurement method.

3. Comparing Quotations with Different Test Bases

One supplier may include only production testing, another may include witnessed low-pressure gas testing, and a third may price a control-valve leakage class. A bid comparison should normalize the test matrix before comparing price and delivery.

For control-valve sizing work, CLDG’s Cv and Kv calculator can support the flow calculation, but sizing coefficients do not define shutoff leakage. Capacity and seat tightness must be specified separately.

Preguntas frecuentes

Is Class VI the same as zero leakage?

No. Class VI is a defined control-valve leakage class tested under specified conditions. It should not be rewritten as an unlimited “absolute zero” claim or automatically applied to every isolation valve.

Does API 598 require zero seat leakage for every valve?

No single phrase covers every construction. Acceptance depends on the applicable test, valve and seat type, size, test conditions and the edition or purchaser requirements. Quote the actual acceptance requirement instead of using “API 598 tight shutoff” as shorthand.

Can MSS SP-61 replace API 598?

Only when the purchaser and governing specification accept that basis. The two standards should not be treated as interchangeable without a clause-by-clause project review.

Which standard should be used for a control valve?

ANSI/FCI 70-2 is the recognized control-valve seat-leakage classification and test standard in this comparison. The full valve specification still needs design, materials, rating, actuator, fail action, sizing and other project requirements.

What test records should a buyer request?

At minimum, request valve identification, standard and edition, test item, medium, pressure, duration, direction, allowable leakage, actual result, gauge identification and calibration status, date, inspector approval, and any repair/retest history.

The Procurement Rule to Remember

API 598, MSS SP-61 and ANSI/FCI 70-2 are not three names for the same test. Select the document that matches the valve’s function, then convert it into an order-specific matrix. When the medium, pressure, direction, duration, leakage limit and evidence are fixed before manufacturing, the factory, inspector and buyer are measuring the same thing.

Engineering References

• API Standards Plan — API Standard 598, Valve Inspection and Testing
• MSS SP-61-2019 — Pressure Testing of Valves
• Fluid Controls Institute — ANSI/FCI 70-2-2021
• FCI 70-2 informational sheet — control-valve seat leakage
• ASME B16.34 — Valves: Flanged, Threaded, and Welding End