What Is API 598?
API 598 is an American Petroleum Institute standard that specifies inspection and pressure testing requirements for industrial valves, defining the test procedures, test pressures, test durations, acceptance criteria, and documentation requirements that verify a valve’s pressure boundary integrity and sealing performance before it is approved for shipment and installation. It is the primary valve testing standard referenced by API 600, API 602, API 6D, and most other API product standards, making it the universal testing framework within the valve standards overview hub.
Key Takeaways
- API 598 governs inspection and pressure testing of valves — it applies to gate, globe, check, ball, plug, and butterfly valves across all nominal sizes and pressure classes, providing a single consistent testing framework that is referenced by multiple API product design standards without each product standard needing to define its own independent testing procedures.
- Defines shell and seat leakage testing procedures — shell hydrostatic testing verifies the pressure-containing body and bonnet can withstand 1.5 times the rated pressure without leakage or permanent deformation; seat leakage testing verifies the closure element seals to the applicable leakage class when the valve is closed against the test pressure.
- Establishes acceptance criteria and marking requirements — API 598 defines quantitative leakage acceptance criteria for both metal-seated and soft-seated valves, expressed as maximum allowable leakage rates in drops per minute or cubic centimeters per minute, with different acceptance levels for different valve types and seat materials.
- Commonly referenced by API 600, API 602, and API 6D — these product standards define the valve’s design and construction requirements but delegate testing procedure specification to API 598, which is updated independently to reflect current testing practice without requiring simultaneous revision of every referencing product standard.
How It Works
Shell Hydrostatic Testing Requirements
The shell hydrostatic test is the fundamental verification that the valve’s pressure-containing envelope — body, bonnet, gland, and all pressure boundary joints — can safely contain the rated working pressure without leakage or structural deformation. API 598 specifies the shell test pressure as 1.5 times the pressure-temperature rated working pressure of the valve at the ambient test temperature, where the rated pressure is determined from the applicable ASME B16.34 pressure-temperature table for the valve’s body material group and pressure class. For a Class 600 carbon steel valve (ASTM A216 WCB, rated at 98.6 bar at 38°C per ASME B16.34), the shell hydrostatic test pressure is 1.5 × 98.6 = 148 bar — this test pressure is applied to the internal cavity of the valve with both end connections open to allow air purging, and the valve held at or near the fully open position to expose both body cavity and bonnet cavity to the test pressure. The complete ASME B16.34 pressure-temperature rating framework used to calculate API 598 shell test pressures is addressed in the what is ASME B16.34 reference. The minimum test duration at the shell test pressure is specified in API 598 Table 2 as a function of nominal valve size — 15 seconds minimum for NPS 2 and below, 60 seconds minimum for NPS 3 through NPS 8, and 120 seconds minimum for NPS 10 and above — the minimum duration being the time during which the test pressure is maintained and the valve is observed for leakage with no pressure drop permitted. The acceptance criterion for the shell test is zero visible leakage — no drop formation, no moisture on external surfaces, no pressure decay during the hold period.
Seat Leakage Testing Criteria
Seat leakage testing verifies the sealing performance of the valve’s closure element against the body seats when the valve is in the fully closed position — a fundamentally different measurement from the shell test, which verifies the pressure boundary rather than the sealing interface. API 598 defines two distinct seat leakage acceptance levels based on seat construction: soft-seated valves (with PTFE, PEEK, nylon, or other resilient seat inserts) must achieve zero measurable leakage — no visible drops or bubbles — when tested at the specified seat test pressure; metal-seated valves (with metal-to-metal contact between the closure element and body seats) are permitted a defined maximum leakage rate proportional to nominal valve size, recognizing that metal seats cannot achieve the perfect surface conformance of soft seats and therefore cannot be held to zero leakage. For metal-seated gate and globe valves at the seat test pressure (1.1 times rated pressure per API 598), the maximum allowable seat leakage in drops per minute is 0.18 × DN (in mm) per side — for example, an NPS 4 (DN 100) metal-seated gate valve is permitted a maximum of 18 drops per minute per seat. The detailed seat leakage test procedures and acceptance criteria applicable to production testing are addressed in the valve pressure testing procedure reference and the hydrostatic testing standard reference.
Relationship With Product Standards
API 598’s role as a shared testing standard referenced by multiple product design standards is one of its most important structural features — it creates a consistent testing framework across the full range of API valve standards without requiring each product standard to maintain its own independent and potentially divergent testing requirements. API 600 specifies that bolted-bonnet steel gate valves shall be shell tested and seat tested per API 598, with test pressures derived from the ASME B16.34 rated pressure of the specified valve body material and pressure class — full details are addressed in the what is API 600 reference. API 602 specifies that compact forged steel gate, globe, and check valves shall meet the same API 598 shell and seat test requirements — full details are addressed in the what is API 602 reference. API 6D specifies that pipeline valves shall meet API 598 testing requirements with the additional API 6D requirement for high-pressure closure testing and operational testing not covered by API 598 — full details are addressed in the what is API 6D reference.
Main Components
Visual Inspection and Marking
API 598 requires visual inspection of every valve before pressure testing — the inspection verifies that the valve is free from external defects (cracks, cold shuts, scabs in cast surfaces), that all machined surfaces meet the specified surface finish requirements, that all required markings are correctly applied to the valve nameplate or body, and that the valve moves freely through its full travel range without binding or excessive friction. API 598 marking requirements specify that each valve shall be marked with its nominal size, pressure class or pressure rating, body material designation, flow direction indicator (where applicable), and manufacturer’s name or trademark. Material traceability is a parallel requirement — the body and bonnet material markings must link to the material test certificate for the specific heat of material used, enabling verification that the pressure-containing parts were manufactured from qualified material meeting the applicable ASTM specification’s chemical and mechanical property requirements. The complete EN 10204 material certificate framework that documents the material qualification required to support API 598 traceability is addressed in the what is EN 10204 3.1 reference.
Fire, Emission, and Optional Testing
API 598 defines the core shell and seat testing requirements for production valve qualification but explicitly excludes fire-safe testing and fugitive emission testing from its scope — these are separate qualification tests conducted on prototype valve designs rather than production testing of every manufactured valve. Fire-safe performance testing verifies that a valve design retains acceptable metal seat sealing integrity after external fire exposure, and is governed by what is API 607 — a prototype type test rather than a production test. The complete fire-safe certification qualification process is addressed in the fire-safe certification reference. Fugitive emission performance testing verifies that a valve design’s stem seal system meets specified emission class limits under thermal and pressure cycling, and is governed by ISO 15848 — also a prototype type test rather than a production test. The complete fugitive emission testing framework is addressed in the fugitive emission testing and what is ISO 15848 references. API 598 does permit optional tests that project specifications may invoke in addition to the mandatory shell and seat tests — low-pressure air seat test at 0.6 bar (applicable for valves where low differential pressure sealing is a critical requirement), and backseat test at 1.1 times rated pressure for gate and globe valves (verifying that the fully-open backseat position seals the stem for under-pressure packing replacement).
Certification and Documentation
API 598 requires that the results of all required tests — shell hydrostatic test pressure, duration, and result; seat leakage test pressure, duration, test medium, and leakage measurement; and backseat test result where applicable — be documented in a test record that is delivered with the valve as part of its certification documentation package. The test record must identify the valve by serial number or heat number, confirm that test pressures met API 598 requirements for the valve’s nominal size and pressure class, record the actual leakage measured during seat testing, and be signed by the manufacturer’s authorized representative confirming compliance. The complete documentation package framework applicable to API 598-compliant valve deliveries is addressed in the valve certification documents reference. The procedures for verifying that a delivered valve’s test certificates meet API 598 requirements — including reviewing test pressures against calculated rated pressures, confirming test durations meet Table 2 minimums, and verifying seat leakage measurements against the applicable acceptance criteria — are addressed in the how to verify valve compliance reference.
Advantages
Safety Assurance and Standardization
API 598’s value to the industrial valve industry lies in providing a single, consistent, independently updatable testing standard that all API product standards can reference — eliminating the duplication and potential inconsistency that would result from each product standard defining its own testing requirements independently. A purchasing engineer specifying “API 600 with API 598 testing” receives unambiguous, quantitative testing requirements — specific test pressures, specific test durations, specific leakage acceptance criteria — without needing to interpret or supplement the testing requirements from the design standard. This standardization enables meaningful comparison between valve offers from different manufacturers: every valve claiming API 598 compliance has been subjected to the same test conditions and evaluated against the same acceptance criteria, making the test certificate a reliable quality assurance document rather than a manufacturer-specific claim. The documented shell hydrostatic test result is the primary assurance that the valve’s pressure boundary does not contain manufacturing defects that would cause in-service failure — without this test, a casting or forging defect might not be detectable by visual inspection and dimensional verification alone. For the comparison between API 6D and API 600 testing requirements — both of which reference API 598 as their testing standard — refer to the API 6D vs API 600 reference.
Typical Applications
Oil, Gas, Refining, and Power Industries
API 598 testing is effectively universal in industrial valve supply for oil, gas, petrochemical, and power generation service — it is referenced by API 600, API 602, and API 6D, which together cover the majority of steel valve purchases in these industries, making API 598 testing mandatory for virtually all valve positions in these sectors when these design standards are specified. In European and internationally regulated projects where both API product standard compliance and PED CE marking are required, API 598 test certificates form part of the technical documentation supporting the PED conformity assessment — the hydrostatic pressure test record provides documented evidence of pressure boundary integrity required by PED Essential Safety Requirement 7.4. The integration of API 598 testing within European regulatory compliance is addressed in the what is PED 2014/68/EU reference. In automated valve systems where quarter-turn valves are supplied with ISO 5211 actuator mounting flanges, API 598 testing verifies the valve’s pressure integrity before the actuator is mounted and the assembly is shipped — ensuring that any pressure boundary defect is detected at the valve level rather than after actuator installation. The ISO 5211 actuator mounting standard applicable to automated valve assemblies is addressed in the what is ISO 5211 reference.
Frequently Asked Questions
What does API 598 cover?
API 598 covers the inspection and pressure testing requirements for industrial valves — visual inspection criteria, shell hydrostatic test procedure and acceptance criteria, seat leakage test procedure and acceptance criteria for both soft-seated and metal-seated valves, optional backseat and low-pressure air test procedures, and documentation requirements for test results. It applies to gate, globe, check, ball, plug, and butterfly valves across all nominal sizes and pressure classes, and is referenced by API 600, API 602, API 6D, and other API product standards as their testing specification.
Is API 598 mandatory?
API 598 has no inherent legal mandatory status — it is a voluntary technical standard published by the American Petroleum Institute. It becomes effectively mandatory when referenced by a product standard (API 600, API 602, API 6D) that is specified in the valve purchase order, or when explicitly invoked in the project valve specification or purchase order testing requirements. In practice, API 598 testing is a contractual requirement on virtually all industrial valve purchases for oil, gas, petrochemical, and power generation service, making non-compliance commercially unacceptable even where it has no legal force.
Does API 598 define fire-safe testing?
No — API 598 explicitly excludes fire-safe testing from its scope. API 598 covers production testing of every manufactured valve to verify pressure boundary integrity and seating performance; fire-safe testing is a prototype qualification test conducted once on a representative valve design to demonstrate that the design retains metal seat sealing after fire exposure. Fire-safe prototype testing is governed by API 607 for quarter-turn valves, API 6FA for pipeline valves, and BS 6755 Part 2 in some specifications — these are separate standards from API 598 and are invoked independently in project specifications.
What is the difference between shell and seat testing?
Shell testing verifies the structural integrity of the valve’s pressure-containing boundary — body, bonnet, gland, and all joints between pressure-containing parts — by pressurizing the internal cavity to 1.5 times rated pressure with the closure element open and accepting no visible leakage through any pressure boundary component. Seat testing verifies the sealing performance of the valve’s closure element against the body seats by pressurizing one end of the closed valve to the seat test pressure (1.1 times rated pressure for liquid seat tests) and measuring the leakage past the closed seats at the opposite end, accepting no measurable leakage for soft seats and a defined maximum leakage rate for metal seats.
Conclusion
API 598 is the universal production inspection and pressure testing standard for industrial valves in oil and gas service — its standardized shell hydrostatic test procedure, quantitative seat leakage acceptance criteria for both soft-seated and metal-seated valves, visual inspection requirements, and documentation framework provide the consistent quality assurance baseline that makes API valve test certificates meaningful and comparable across manufacturers. Its role as the testing reference standard for API 600, API 602, API 6D, and other product standards means that understanding API 598 testing requirements is prerequisite to interpreting compliance claims and test documentation for virtually any steel valve in industrial process service. Engineers requiring a comprehensive framework that integrates API 598 within the full landscape of valve design, dimensional, testing, and certification standards should consult the valve standards overview hub as the governing reference for all valve testing standards navigation.