API 6A Type 6BX 20,000 psi Weld Neck Flange — Dimensions & BX Ring Numbers

API Specification 6A (aligned with ISO 10423) is the American Petroleum Institute's specification for wellhead and Christmas tree equipment. Among much else it defines the two flange families used on wellheads and trees — Type 6B and Type 6BX — with rated working pressures from 2,000 up to 20,000 psi. This page covers the Type 6BX weld neck flanges rated 20,000 psi.

Unlike an ASME class number, an API 6A rating is the actual rated working pressure: a 20,000 psi (≈1,380 bar) flange is designed to operate at that pressure within its temperature class. It is the highest standard rating in API 6A's flange tables and the domain of HPHT (high-pressure, high-temperature) wells. Hydrostatic test pressures above the rating apply to assembled equipment per the specification.

6BX is API 6A's high-pressure flange design. It seals on a pressure-energized BX metal ring gasket and is designed for face-to-face makeup — the raised faces contact so the bolting is supported by the flange faces rather than the gasket. That makes 6BX more compact than an equivalent 6B design and is why it is the only type specified at 10,000 psi and above.

Type 6B (rated 2,000–5,000 psi) uses R or RX ring gaskets in the ASME RTJ tradition, with a standoff gap between the flange faces after makeup. Type 6BX (2,000–20,000 psi) uses the pressure-energized BX gasket and makes up face-to-face, giving a stiffer, more compact joint. Above 5,000 psi only 6BX exists — there is no 6B at 10,000, 15,000 or 20,000 psi.

A BX gasket is a pressure-energized metal ring with a lens-like cross-section: internal pressure pushes it harder against the groove faces, improving the seal as pressure rises. Each flange size takes a specific ring — the chart's last column gives it, BX-151 through BX-159 for the 20,000 psi series. BX gaskets deform on makeup and should not be reused.

Nine nominal sizes: 1-13/16, 2-1/16, 2-9/16, 3-1/16, 4-1/16, 7-1/16, 9, 11 and 13-5/8 inches — the classic wellhead bore family. The largest, 13-5/8 in at 20,000 psi, is a massive forging with a 1,160 mm outside diameter and twenty 80 mm stud holes, used on HPHT wellhead and BOP-stack connections.

Size is the nominal bore in inches; all other values are millimetres. B is the maximum bore, OD the flange outside diameter, T the flange thickness, BC the bolt circle, N the number of studs, H the bolt hole size, and the last column the BX gasket ring number. The remaining letters — C, Q, G, K, J1–J3 and R — are the face, ring-groove and hub dimensions identified on the drawing above the chart.

API 6A defines material classes AA through HH: AA–CC for general service (carbon, low-alloy and stainless steels) and DD–HH for sour service in line with NACE MR0175 — the class sets minimum yield, impact and hardness requirements. Typical weld neck flange materials are AISI 4130/4140 low-alloy steel, F22, 410 and 17-4 stainless, and F51/F53 duplex, selected to the well's pressure, temperature and H2S conditions.

PSL — Product Specification Level, from PSL 1 to PSL 4 — sets the depth of quality control: material testing, NDE, weld examination, traceability and documentation increase with each level. Wellhead flanges are commonly specified PSL 2 or PSL 3 (PSL 3G with gas testing on assembled equipment); the level should be stated on your enquiry along with the material class.

Letter classes define the design temperature envelope — for example K (−60 °F to 180 °F), P (−20 °F to 180 °F), U (0 °F to 250 °F) and X/Y extending to 350/650 °F for HPHT service. The class matters because it drives impact-test temperatures for the material. State the temperature class with your order; K–U cover most surface wellhead duty.

Yes, when ordered in the sour-service material classes (DD, EE, FF or HH), which invoke NACE MR0175/ISO 15156 — controlled hardness, qualified materials and impact requirements. Sour HPHT wells typically combine a sour material class with PSL 3 and a low-temperature class; we supply accordingly with full certification.

The N and H columns give the stud count and hole size per flange — from 8 studs at 1-13/16 in up to 20 studs of 80 mm holes at 13-5/8 in — on the bolt circle BC. High-strength stud bolts with heavy hex nuts are used, and at 20,000 psi controlled bolt tensioning (not just torque) is standard practice to achieve the even preload the face-to-face makeup requires.

On a 20,000 psi joint the connection must be as strong and as inspectable as the pipe or spool it terminates. The weld neck's integral tapered hub and full-penetration butt weld deliver that — the weld is volumetrically examined and the whole load path is forged, with no threads or seal welds. Threaded and other end connections are limited to the low-pressure end of the API 6A range.

No. The systems are unrelated: API 6A flanges are rated by actual working pressure with their own dimensions and BX/R/RX gaskets, while ASME B16.5 uses class ratings — and tops out around 6,170 psig cold (Class 2500) where API 6A continues to 20,000 psi. Never attempt to mate flanges across the two systems; use a proper crossover spool designed for the service.

On HPHT wellheads and Christmas trees, tubing and casing spools, choke and kill lines, BOP stack connections, frac trees and well-test packages — anywhere surface pressure control equipment sees pressures beyond the 10,000 and 15,000 psi ratings. They are the standard connection at the top of the API 6A pressure range.

We manufacture to the API 6A Type 6BX dimensional requirements from certified forgings, with dimensional inspection against the specification, EN 10204 3.1 mill certificates, heat-number traceability, PMI, hardness testing (including NACE limits for sour classes) and UT/MPI examination on request. Third-party witness inspection by LR, BV, DNV, SGS or TÜV is welcomed; project-specific PSL documentation is quoted case-by-case.

Yes — the full 6BX family from 2,000 to 20,000 psi and the 6B family from 2,000 to 5,000 psi, in weld neck and blind versions, are charted on this site (see the series table above). Test flanges, adapter and crossover spools and flanges to your drawing are quoted case-by-case.

Size and quantity, type (6BX) and rated working pressure (20,000 psi), end connection (weld neck — with pipe/spool wall for the weld prep), API 6A material class (AA–HH), temperature class, PSL level, any NACE/sour requirement, and the certification and inspection you need. Tesco Steel & Engineering is an ISO 9001:2015 certified manufacturer and exporter in Mumbai, India.