The Short Answer
ASME B16.34 is the base design standard for steel valves: it defines pressure-temperature ratings, wall thickness, materials, and dimensions for flanged, threaded, and welded-end valves. API 6D is a product standard for pipeline and piping valves (gate, globe, ball, check) that builds on ASME B16.34 ratings and adds requirements for design, materials, testing, and documentation that pipeline service demands. You do not choose between them — an API 6D valve is an ASME B16.34 valve with additional pipeline-grade requirements.
What ASME B16.34 Defines
ASME B16.34, Valves — Flanged, Threaded, and Welding End, is the foundational standard:
- Pressure-temperature ratings for material groups (e.g., Group 1.1 carbon steel, Group 2.1 316 stainless, Group 3 alloy steels). The rating tables give the maximum allowable working pressure at each temperature for each pressure class (150, 300, 600, 900, 1500, 2500).
- Minimum wall thickness for body and bonnet per class and size.
- Materials — a limited list of cast, forged, and bar materials with their pressure-temperature data.
- Design requirements — stem, bonnet, seat, and shell design rules; minimum stem diameter; bolting.
- Dimensions for flanged, threaded, and butt-welding end valves.
- Testing — shell and seat test requirements reference the applicable test standard.
How ratings work: the rating is the maximum pressure the valve can sustain at the design temperature. Ratings decrease as temperature rises (e.g., a Class 300 carbon-steel valve rated 740 psig at 100°F drops to about 610 psig at 500°F). Never apply the class number as if it were a pressure limit at all temperatures.
What API 6D Adds
API 6D, Specification for Pipeline and Piping Valves, covers gate, globe, plug, ball, and check valves for pipeline and piping systems. It does not replace ASME B16.34 ratings; it adds:
- Design requirements beyond B16.34: full and reduced bore definitions, extended body, by-pass and drain/vent requirements, anti-static devices, fire-safe design per API 607, and block-and-bleed capability where specified.
- Material requirements and traceability: materials of construction for pressure-retaining parts must be traceable; NACE compliance is frequently specified.
- Testing: API 6D specifies hydrostatic shell testing and seat testing, and adds optional tests — gas testing, high-pressure gas testing, fugitive-emission testing, and fire testing — depending on the purchase order.
- Documentation and inspection: much stricter documentation requirements (data sheets, traceability, test reports, nameplate marking).
- Acceptance criteria: tighter criteria on seat leakage and zero-leakage options.
- Coating and shipping requirements for pipeline service.
How They Interact in Practice
| Requirement | ASME B16.34 | API 6D |
|---|---|---|
| Pressure-temperature rating | Yes — the authority | Adopts B16.34 ratings |
| Wall thickness | Yes — minimums | Yes (meets or exceeds B16.34) |
| Materials | Limited list with ratings | Yes, plus traceability requirements |
| Fire-safe design | Not covered | Yes (API 607) |
| Anti-static | Not covered | Yes (for non-metallic sealed ball/plug) |
| Hydrostatic shell test | Required | Required (per API 598 or API 6D) |
| Seat leakage test | Required | Required, plus optional tight classes |
| Documentation | Standard | Enhanced traceability and reports |
| Typical application | General industrial valves | Pipelines, transmission lines, critical process |
Common Rating Mistakes
- Using the class number as the pressure rating. Class 300 does not mean “300 psi forever.” Check the rating table for the actual design temperature.
- Ignoring temperature derating. Ratings drop with temperature; a valve chosen at ambient may be undersized at operating temperature.
- Applying ASME B16.34 ratings to non-listed materials. Only the materials in B16.34 (or materials with B16.34-consistent data) may use those tables. A proprietary alloy needs its own verified rating data.
- Assuming an API 6D valve is automatically fire-safe. Fire-safe testing (API 607) is a specified option in API 6D; state it in the purchase order or it may not be included.
- Using B16.34 flanged dimensions for pipeline valves without checking API 6D end-to-end dimensions. API 6D face-to-face and end-to-end dimensions differ in some sizes from B16.34; the pipeline spec governs.
What to Put in a Purchase Specification
To avoid ambiguity, specify explicitly:
- Valve to ASME B16.34 (ratings and wall thickness), additionally to API 6D for pipeline service.
- Pressure class: Class 600.
- Design conditions: 1500 psig @ 400°F.
- Fire-safe to API 607 (if required).
- Seat leakage: API 6D Class A (zero leakage) or Class B.
- NACE MR0175 compliant (if sour service).
- Fugitive-emission tested to ISO 15848-1 (if required).
Regional Equivalents Worth Knowing
- ISO 14313 was the international adoption of API 6D (now largely aligned in ISO 14313 / API 6D harmonized editions).
- EN 12516 provides European pressure-temperature ratings for valve bodies — see our article on EN 12516 vs. ASME B16.34 for the differences.
- GB/T 12224 is the Chinese standard for valve pressure-temperature ratings, similar in concept to ASME B16.34.
Conclusion
ASME B16.34 and API 6D are complementary layers, not competitors. Use B16.34 for the rating, wall thickness, and material basis; add API 6D when the valve must meet pipeline-grade design, testing, and documentation requirements. Always check the actual pressure-temperature table at the design temperature, and state fire-safe, leakage, NACE, and fugitive-emission requirements explicitly — none of them are automatic.