The short answer
ASME B16.34 governs whether the pressure boundary is strong enough; API 6D governs whether the valve can actually be used in the pipeline system. The first is the basic design standard for valve components, the second a complete product specification for pipeline applications that references the first.
How the two standards differ in scope
- ASME B16.34 (Valves—Flanged, Threaded, and Welding End): defines pressure–temperature ratings, minimum wall thickness, end dimensions and material groups for steel valves, covering Class 150 to Class 4500. Its core outputs are the pressure–temperature rating tables (Table 2) and minimum wall thickness calculations. It answers "how much pressure can this body hold at a given temperature".
- API 6D (Specification for Pipeline and Piping Valves, identical to ISO 14313): covers ball, gate, check and plug valves for pipelines and piping systems, and sets out complete requirements for design, materials, welding, inspection and testing. It answers "can this valve be installed on a long-distance pipeline, can it be pigged, can it seal in both directions, how is it pressure-tested, and what documentation ships with it".
What API 6D requires beyond ASME B16.34
- Functional and design requirements: pigging bore, full-bore and reduced-bore provisions, anti-static design, DBB (double block and bleed) and other optional features.
- Testing requirements: shell strength test at 1.5× the rated pressure, high-pressure seat test at 1.1× the rated pressure, low-pressure gas seat test (typically 0.4–0.6 MPa), with acceptance graded to API 598 / ISO 5208.
- Documentation and traceability: material certificates, pressure test reports, NDT (RT/UT/MT/PT) records and serial number traceability.
- Welding and NDT: pressure-retaining welds to ASME IX, NDT to the applicable ASME/ASTM acceptance levels.
- Additional type tests: fire-safe (API 607/6FA), fugitive emissions (ISO 15848-1) and others can be added per project requirements.
How to decide which standard to buy to
- For valves on long-distance pipelines, gathering stations, valve stations and pigging stations: owners usually specify API 6D, sometimes with the API Monogram.
- For general gate and globe valves in refining, power plants and chemical plants: API 600 / 602 / 603 are more common, with seating surfaces selected by API 600 Trim number.
- Butterfly valves are typically to API 609, again with ASME B16.34 as the pressure boundary basis.
- Whichever product standard applies, the pressure–temperature rating must always be checked against ASME B16.34 Table 2 — the allowable pressure for the same Class varies with material and temperature (for example WCB at 38 °C in Class 600 is rated 102 bar, decreasing step by step as temperature rises).
Common misconceptions
- Ordering to B16.34 as if it were a complete product standard: it does not cover testing, pigging, fire safety or documentation, and meeting B16.34 does not mean meeting API 6D.
- Stating "complies with API 6D" without the edition year, and without locking test pressures and the document list in the technical agreement — a frequent source of dispute at acceptance.
- Confusing API 6D (pipeline valve specification) with API 6A (wellhead equipment); the two apply to entirely different scopes.
We recommend stating all four items in the RFQ — product standard + pressure boundary standard + test standard + documentation level — and Vexval can implement them item by item.