API Flange Specifications: 6B vs 6BX — Differences, Applications & Selection

Type 6B flanges are the traditional API flange design, specified in API 6A for working pressures up to 5,000 PSI. They are also used in API 6D pipeline applications.

Key Characteristics:
- Use a flat-bottom ring groove with an R or RX type ring gasket
- The gasket is compressed between the flange faces as bolts are tightened
- Flange faces are flat — no raised face
- Bolting is through-bolted (stud bolts with nuts on both ends)
- Available in sizes from 2-1/16″ to 30″

Applications:
- Wellhead components rated 2,000, 3,000, and 5,000 PSI
- Pipeline flanges per API 6D
- Lower-pressure production manifolds
- Flowline connections

6B flanges are the workhorse of the industry for moderate-pressure applications.

Type 6BX flanges are a high-pressure flange design introduced to address the limitations of 6B flanges at higher working pressures. They are specified in API 6A for pressures from 5,000 to 20,000 PSI.

Key Characteristics:
- Use a BX type ring gasket with a tapered sealing surface
- The gasket creates a pressure-energized seal — internal pressure forces the gasket lips tighter against the groove walls
- Flanges have a raised face with a deeper, more precisely machined ring groove
- The flange design includes a metal-to-metal make-up shoulder that prevents over-compression of the gasket
- Available in sizes from 1-13/16″ to 21-1/4″

Applications:
- Wellhead components rated 5,000 PSI and above
- High-pressure Christmas tree connections
- Choke and kill manifold connections
- Subsea wellhead connections

6BX flanges are mandatory for high-pressure and critical service applications.

The ring gasket is the sealing element in a flanged connection. API 6A specifies three gasket types:

Type R Gaskets:
- Oval or octagonal cross-section
- Used with 6B flanges
- Made from soft iron, low-carbon steel, or corrosion-resistant alloys
- Simple, reliable, not pressure-energized

Type RX Gaskets:
- Modified octagonal cross-section with pressure-energized design
- Used with 6B flanges for improved sealing at higher pressures
- Self-sealing under pressure — the higher the pressure, the tighter the seal
- Interchangeable with R gaskets in 6B flange grooves

Type BX Gaskets:
- Trapezoidal cross-section with pressure-energized lip design
- Used exclusively with 6BX flanges
- NOT interchangeable with R or RX gaskets
- Each BX number corresponds to a specific flange size and pressure rating
- Must be ordered by the correct BX number (BX-151 through BX-172)

Critical Rule: Never substitute gasket types. A 6BX flange requires a BX gasket — using an R or RX gasket will result in connection failure.