What Is an API Ring Gasket?
An API ring gasket is a solid metal sealing ring designed specifically for API 6A flanged connections used on wellhead equipment, Christmas trees, and high-pressure upstream piping. API ring gaskets include the BX and RX styles, which differ from standard ASME B16.20 RTJ gaskets in geometry, groove design, and pressure-energization mechanism.
When to Use API Ring Gaskets
API ring gaskets are mandatory on:
- API 6A wellhead and Christmas tree flanges rated 2,000 to 20,000 psi
- API 6BX flanges (BX-type gaskets only)
- API 17D subsea production equipment
- API 16A drilling equipment flanges (BOP stack connections)
- High-pressure flowline connections in upstream oil and gas
Standard R-type RTJ gaskets per ASME B16.20 are designed for refinery and process plant flanges. API ring gaskets handle the higher pressures, shock loads, and pressure surges encountered in wellhead and drilling applications.
BX vs RX Ring Gaskets
| Feature | BX Ring | RX Ring |
|---|---|---|
| Standard | API 6A | API 6A |
| Flange type | API 6BX flanges only | API 6B flanges (backward compatible with R-grooves) |
| Cross-section | Square/octagonal with pressure hole | Octagonal, pressure-energized |
| Pressure range | 5,000-20,000 psi | 2,000-15,000 psi |
| Pressure energization | Yes (radial pressure hole) | Yes (hollow center expands under pressure) |
| Interchangeability | BX grooves only | Fits R-grooves and RX grooves |
| Reusability | No (single use) | No (single use) |
Both BX and RX gaskets achieve a pressure-energized seal: as internal pressure increases, the gasket expands into the groove, increasing contact stress on the sealing surfaces. This self-energizing action provides superior sealing compared to conventional R-type rings, which rely solely on initial bolt load.
API 6A Ring Gasket Dimensions
API 6A defines ring gasket dimensions by ring number and rated working pressure. Sample BX ring dimensions:
| Ring Number | Bore (in) | Pressure Rating (psi) | OD (mm) | Height (mm) |
|---|---|---|---|---|
| BX-150 | 1-13/16 | 5,000 | 52.37 | 15.88 |
| BX-152 | 2-1/16 | 5,000-20,000 | 60.32 | 15.88 |
| BX-154 | 2-9/16 | 5,000-20,000 | 73.02 | 17.48 |
| BX-156 | 3-1/8 | 5,000-20,000 | 87.31 | 19.05 |
| BX-160 | 4-1/16 | 5,000-20,000 | 111.12 | 22.23 |
| BX-164 | 7-1/16 | 5,000-15,000 | 187.33 | 31.75 |
Note: API 6A uses bore size designations (e.g., 2-1/16”, 3-1/8”) rather than NPS pipe sizes.
Material Grades
| Material | Hardness (HRc max) | Application |
|---|---|---|
| Low-carbon steel (AISI 1018) | 56 HRB | General service, non-corrosive |
| AISI 4130 | 22 HRc | Standard wellhead service |
| Incoloy 825 | 25 HRc | Sour gas (H2S) per NACE MR0175 |
| Inconel 625 | 30 HRc | Subsea, severe corrosion |
| Inconel 718 | 35 HRc | High-strength, high-temperature |
All API ring gaskets must be softer than the flange groove material to ensure the gasket deforms before the groove. Hardness testing and material certification per API 6A are mandatory.
API ring gaskets work with high-strength stud bolts torqued or tensioned per the equipment manufacturer’s flange assembly procedure.
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