If you work on wellhead or Christmas tree equipment for more than a few months, “API 6A” stops being a document you occasionally reference and becomes the language you think in — pressure classes, material classes, PSL levels. For anyone newer to the specification, or anyone who wants a single clear reference to hand a colleague, this post walks through what API 6A actually covers and why each part of it exists. It’s adapted from an internal technical briefing our engineers put together — you can download the full PDF version at the end of this post.
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A specification with 60+ years behind it
API Specification 6A was first published by the American Petroleum Institute in the early 1960s, building on API’s earlier well-control valve standards dating back to 1939. It’s been revised through 21 editions since, with the current 21st edition published in 2018. Since roughly the mid-1990s, API 6A has been progressively harmonized with the international standard ISO 10423, so equipment built to one is recognized against the other on projects worldwide — useful to know if you’re specifying equipment for an international operator or working alongside engineers who reference ISO 10423 by habit.
- 1939 — API’s earliest well-control valve standardization work begins
- Early 1960s — API 6A is first published as its own specification
- Mid-1990s — Harmonization with ISO 10423 begins
- 2018 — 21st edition published, the current edition today
What API 6A actually covers
API 6A sets out the design, materials, testing, inspection, marking and documentation requirements for wellhead and Christmas tree equipment used across drilling, completion, workover and production of oil and gas wells. In practice, that spans three broad groups of equipment:
- Wellhead housings — casing heads, casing spools, tubing heads and tubing head adapters. See our Wellhead Equipment 101 post for a closer look at these components.
- Trees and flow control — Christmas trees, crosses, tees, gate and check valves, chokes and actuators. We cover the tree side specifically in What Is a Christmas Tree in Oil and Gas?
- Connections — wellhead connectors and fittings that join equipment while holding pressure.
If you’re not sure where the line falls between “wellhead” and “Christmas tree” specifically, we untangle that distinction in Wellhead vs. Christmas Tree: What’s the Difference?
The API Monogram: what it confirms, and what it doesn’t
Manufacturers licensed under the API Monogram Program may apply the API mark to equipment built to Spec 6A, under a quality management system that API itself audits. For a purchaser, the mark is a useful signal — but it isn’t a substitute for specifying the equipment correctly in the first place.
What the monogram confirms: the product was manufactured under an audited quality system, to the design, material and testing requirements of the specification edition it references.
What the purchaser still has to define: the PSL, performance requirement level, material class, temperature class and rated working pressure needed for the specific well and site. The monogram tells you the manufacturer did their part correctly — it can’t tell you whether you asked for the right equipment in the first place.
Product Specification Levels: PSL 1 through PSL 4
The Product Specification Level sets how much design verification, material testing, inspection and documentation a piece of equipment must carry. Requirements build up from PSL 1 to PSL 4 — it’s worth being clear that the PSL does not change the equipment’s pressure rating; that’s a separate parameter entirely (more on that below).
- PSL 1 — baseline requirements: dimensional, material and functional testing at the essential level.
- PSL 2 — adds further material testing and documentation on top of PSL 1.
- PSL 3 — increases inspection, non-destructive examination and traceability further still.
- PSL 4 — the most stringent level: full testing, documentation and traceability for critical, high-consequence service.
Ratings: rated working pressure and temperature class
Equipment is built to one of six standard pressure classes, in pounds per square inch: 2,000 / 3,000 / 5,000 / 10,000 / 15,000 / 20,000 psi. Test pressures and hold times for a given rating are set out in API 6A’s own qualification tables. HPHT wells — typically above 15,000 psi and/or 350°F — call for additional design and qualification attention beyond the standard classes. We go deeper into how these classes actually work in API 6A Pressure Classes Explained.
Temperature class is the other half of the rating, covering eight standard classes from Arctic cold to high operating temperature:
| Class | Range (°F) | Range (°C) | Typical service |
|---|---|---|---|
| K | −75 to 180 | −60 to 82 | Arctic, extreme cold ambient |
| L | −50 to 180 | −46 to 82 | Cold ambient |
| N | −50 to 140 | −46 to 60 | Cold ambient, lower operating temperature |
| P | −20 to 180 | −29 to 82 | Temperate ambient |
| S | 0 to 140 | −18 to 60 | Warm ambient |
| T | 0 to 180 | −18 to 82 | Warm ambient, higher operating temperature |
| U | 0 to 250 | −18 to 121 | High operating temperature |
| V | 35 to 250 | 2 to 121 | High ambient and operating temperature |
Materials: classes AA through HH, and sour service
A two-letter code sets the material requirement for pressure-containing parts (first letter) and pressure-controlling parts (second letter), and whether the equipment is qualified for general or sour service under NACE MR0175 / ISO 15156. We cover what “sour service” actually means and why it matters in What Is NACE MR0175 and When Do You Need Sour Service Materials? — the short version below:
| Class | Service | Materials (containing / controlling) |
|---|---|---|
| AA | General | Carbon / low-alloy steel, both parts |
| BB | General | Carbon / low-alloy steel — stainless steel |
| CC | General | Stainless steel, both parts |
| DD | Sour | Sour-qualified carbon / low-alloy steel, both parts |
| EE | Sour | Sour-qualified carbon / low-alloy steel — sour-qualified stainless steel |
| FF | Sour | Sour-qualified stainless steel, both parts |
| HH | Sour | Corrosion-resistant alloy, both parts |
Performance requirements: PR1 and PR2
Alongside PSL, API 6A defines two performance requirement routes for demonstrating a design actually meets the specification:
- PR1 — Design validation. The baseline route: verification by design calculation or finite element analysis, confirming the design meets the specification’s requirements. This is exactly the kind of work we cover in What Is FEA and Why Does It Matter for Pressure Equipment?
- PR2 — Qualification testing. Adds physical performance qualification testing of a representative product, following the standard’s own Annex F requirements — referenced for valves specifically as API 6AV1.
Exact test pressures, cycles and hold times are set out in API 6A’s own qualification tables, and are confirmed against the current edition for each specific product before test planning begins.
Core equipment covered
- Casing head & spool — support and seal each casing string, forming the base of the wellhead stack.
- Tubing head & adapter — supports the tubing string and transitions from casing head to tree.
- Christmas tree, crosses & tees — houses the valves and fittings that control flow from the producing well.
- Gate & check valves — isolate or direct flow, opening and closing against full working pressure.
- Chokes — control flow rate and pressure drop at surface. See our How Does a Choke Valve Work? post, or size one directly with our Choke Bean Sizing calculator.
- Wellhead connectors — join sections of equipment while holding a reliable pressure seal.
Markings and traceability
Every item of API 6A equipment carries a permanent mark so it can be identified and traced throughout its service life. The full marking requirements sit within the standard itself; the essentials are the manufacturer’s name or mark (and the API Monogram where licensed), nominal size and rated working pressure, material class, temperature class, Product Specification Level, and a unique serial number.
What to confirm before you specify equipment
Before any equipment can be correctly specified against API 6A, six parameters need to be set from the actual well and site conditions:
- Rated working pressure — one of the six standard psi classes.
- Temperature class — set by site ambient and service temperature.
- Material class — general or sour service, per NACE MR0175 / ISO 15156.
- PSL and PR level — matched to the consequence of failure for this equipment.
- End and outlet connections — type, size and orientation needed for the stack.
- Trim & bore size — sized to the tubing, casing and flow requirements.
Skip any one of these and you’re not really specifying API 6A equipment — you’re hoping a supplier fills in the gaps correctly on your behalf.
Where Rapid Global supports this work
- Specification support — helping confirm the right PSL, PR level, material class and temperature class for a given well.
- Connection & seal design — threaded and metal-to-metal connections used across wellhead and tree assemblies.
- Material & sour-service assessment — checking material selection against NACE MR0175 / ISO 15156 requirements.
- FEA verification — simulation-based evidence to support PR1 design qualification before build.
See our Design & Engineering and FEA & Simulation services for more on how we support API 6A specification work end to end.
Download the full technical briefing
This post is adapted from our internal technical briefing, Introduction to API 6A: Specification for Wellhead and Christmas Tree Equipment — a structured, visual reference covering everything above in a single downloadable PDF, including simplified schematics of a wellhead stack and Christmas tree.
Download the free PDF briefing →
Have a specific wellhead or tree package you need help specifying? Talk to an engineer.