What Is API 6BX Flange? 6BX Ring Gasket

API 6BX refers to a specific flange type within the API 6A standard that uses BX-type ring gaskets for sealing. The “6B” prefix originally came from API Specification 6B (which defined wellhead flanges before being merged into API 6A), and the “X” suffix indicates the BX ring gasket groove. API 6BX flanges are the standard for high-pressure wellhead equipment at 5,000 psi working pressure and above.

Key Specifications

Property	Detail
Governing standard	API 6A (ISO 10423)
Historical standard	API 6B (now merged into 6A)
Gasket type	BX ring (pressure-energized, square cross-section with vent hole)
Pressure range	5,000 to 20,000 psi (345 to 1,379 barg)
Size range	1-13/16 in to 18-3/4 in bore
Material classes	AA through HH per API 6A
Temperature classes	K (-60degC) through V (180degC)
Flange type	Weld neck with BX groove
Test requirements	PR1 and PR2 (including fire test) available

API 6B vs 6BX Designation

The legacy terminology still appears on equipment data sheets and purchase orders:

Designation	Gasket Type	Pressure Range	Status
API 6B	R or RX ring gaskets	2,000-5,000 psi	Legacy (lower pressure wellhead)
API 6BX	BX ring gaskets	5,000-20,000 psi	Current standard for HP wellhead

Both are now covered under API 6A. The “6B” and “6BX” labels persist as shorthand for the gasket groove type rather than separate specifications.

Common Mistake

API 6BX flanges are NOT compatible with standard ASME B16.5 RTJ flanges. The BX ring gasket has a different profile (square vs. oval/octagonal), and the groove geometry does not match ASME B16.20 R-type or RX-type grooves. Never substitute one gasket type for the other.

BX Ring Gasket Details

Feature	BX Ring	R/RX Ring (ASME B16.20)
Cross-section	Square with pressure passage	Oval (R) or octagonal (R, RX)
Pressure energization	Yes (vent hole allows internal pressure to boost seal)	RX only (R is not pressure-energized)
Operating pressure	5,000-20,000 psi	Up to 10,000 psi typical
Groove profile	Square-edged	Rounded or angular
Reusability	Single-use	Single-use
Material	Low-carbon steel, Incoloy, Inconel	Soft iron, 304 SS, 316 SS, Inconel

Applications

Equipment	Typical Rating
Surface wellhead	5K-15K
Christmas tree connections	5K-20K
Choke and kill manifold	10K-15K
BOP (blow-out preventer) connections	10K-20K
Subsea wellhead (via API 17D)	5K-15K

Pro Tip

Each BX ring has a unique number (e.g., BX-152, BX-156) corresponding to a specific flange size and pressure rating. The ring number is stamped on the ring itself. Always verify the BX ring number matches both the flange drawing and the API 6A specification table before installation. Using the wrong ring number will result in a seal failure.

Read the full guide to flanges