Technical guide

API 6A vs API 6D: What's the Difference?

Updated 1 November 2026 · Safinat Lulua

Short answer: API 6A (ISO 10423) covers wellhead and Christmas tree equipment, including gate valves, chokes, spools and flanges, at rated working pressures from 2,000 to 20,000 psi. API 6D (ISO 14313) covers pipeline and piping valves, such as ball, gate, plug and check valves, rated in ASME pressure classes 150 to 2500. Equipment upstream of and including the wellhead choke is normally API 6A, and equipment downstream in flowlines, gathering systems and pipelines is normally API 6D.

Scope

API 6A applies to equipment that contains well pressure at the surface, such as casing and tubing heads, hangers, Christmas tree valves, chokes, adapters, crosses, tees and API 6B/6BX flanges. API 6D applies to valves in pipeline transportation and in piping systems, covering gathering lines, trunk lines, compressor and pump stations, and metering skids.

Key differences

	API 6A	API 6D
Typical equipment	Wellheads, trees, gate valves, chokes	Ball, gate, plug and check valves
Pressure rating	2,000 / 3,000 / 5,000 / 10,000 / 15,000 / 20,000 psi	ASME Class 150, 300, 600, 900, 1500, 2500
Quality levels	PSL 1–4 and PR1/PR2	Optional supplementary requirements (Annexes)
Material classes	AA–HH, with sour classes DD–HH	Per purchaser; NACE MR0175 for sour
End connections	API 6B/6BX flanges, studded, hubs	ASME B16.5/B16.47 flanges, butt-weld
Fire test	API 6FA	API 6FA or API 607

Where the boundary sits

On a typical producing well, everything from the casing head up to and including the production choke on the tree is API 6A. The flowline that leaves the choke, and the valves on manifolds, headers and pipelines downstream, are usually API 6D or ASME B16.34 valves. High-pressure flowlines and test manifolds near the wellhead sometimes stay on API 6A, so confirm with the project specification.

Frequently asked questions

Can an API 6D valve be used on a wellhead?

Generally no. Wellhead and tree equipment must be API 6A because of its psi-based rating, PSL quality levels and wellhead-specific end connections. Some operators accept API 6D valves on low-pressure wellhead flowlines, but only where the project specification allows it.

Is 5,000 psi API 6A the same as ASME Class 2500?

No. They are separate rating systems with different flange dimensions and test pressures. An API 5,000 psi flange (6B) does not mate with an ASME Class 2500 flange.

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