What Is an RTJ Gasket? Ring Joint Types
A ring type joint (RTJ) gasket is a solid metal sealing ring machined to precise dimensions and designed to seat in grooves cut into RTJ flange faces. RTJ gaskets achieve a metal-to-metal seal by plastic deformation of the ring under bolt load, making them the standard choice for high-pressure and high-temperature flanged connections in oil and gas production, refining, and subsea systems.
When to Use RTJ Gaskets
RTJ gaskets are mandatory or preferred in these situations:
- ASME pressure classes 600 through 2500 (many operators require RTJ from Class 900 upward)
- API 6A wellhead and Christmas tree flanges (upstream applications)
- Subsea and offshore installations where leak-free performance is critical
- Sour gas service per NACE MR0175 where gasket integrity cannot be compromised
- High-temperature applications above the limits of spiral wound gaskets
RTJ flanges have machined grooves that accept specific ring numbers. The ring deforms into the groove under bolt load, creating a pressure-energized seal that tightens as internal pressure increases.
RTJ Gasket Styles
ASME B16.20 defines three styles of ring joint gaskets:
| Style | Cross-Section | Interchangeability | Key Features |
|---|---|---|---|
| R (oval or octagonal) | Oval or octagonal | Fits R-grooves only | Original design; oval seats on curved surfaces, octagonal on flat surfaces |
| RX | Pressure-energized variant of R | Fits R-grooves (backward compatible) | Hollow cross-section expands under pressure for tighter seal |
| BX | Octagonal with pressure passage | Fits BX-grooves only (not R-grooves) | Designed for API 6BX flanges; highest pressure ratings |
The oval vs octagonal choice within the R-style affects sealing performance and reusability.
Dimensions and Ring Numbers
Each combination of flange size and pressure class corresponds to a specific ring number (e.g., R-24 for NPS 2 Class 600). The ring number defines the OD, ID, height, and groove dimensions.
| Ring Number | NPS | Class | OD (mm) | ID (mm) | Height (mm) |
|---|---|---|---|---|---|
| R-16 | 1 | 600/900 | 41.28 | 30.18 | 6.35 |
| R-24 | 2 | 600/900 | 60.32 | 46.02 | 7.92 |
| R-35 | 4 | 600 | 87.30 | 66.68 | 9.52 |
| R-39 | 4 | 900 | 95.25 | 73.02 | 11.13 |
| R-45 | 6 | 600 | 117.48 | 95.25 | 11.13 |
| R-46 | 6 | 900 | 130.18 | 104.78 | 12.70 |
Full dimensions are tabulated in ASME B16.20 and API 6A.
RTJ Gasket Materials
RTJ gaskets are machined from soft iron, low-carbon steel, or stainless/alloy steel. The gasket must always be softer than the flange groove to ensure the ring deforms rather than the flange.
| Material | Hardness (HB max) | Typical Application |
|---|---|---|
| Soft iron | 90 | Low-pressure, low-temperature |
| Low-carbon steel (AISI 1018) | 120 | General oil and gas service |
| SS 304 | 160 | Corrosive service, moderate temperature |
| SS 316 | 160 | Marine, chemical service |
| SS 347 | 160 | High-temperature service |
| Inconel 625 | 175 | Subsea, severe corrosion |
| Incoloy 825 | 170 | Sour gas, H2S |
Comparison with Other Gasket Types
| Feature | RTJ | SWG | Soft (PTFE/Graphite) |
|---|---|---|---|
| Max pressure | Class 2500+ | Class 2500 | Class 300 typical |
| Sealing mechanism | Metal-to-metal deformation | Compressed filler layers | Surface compression |
| Reusability | No (single use) | No | No |
| Flange face | RTJ groove | Raised face (RF) | Raised face or flat face |
| Cost | High | Medium | Low |
RTJ gaskets require controlled bolt torque to achieve proper ring deformation without damaging the flange grooves. Each gasket is single-use; once deformed, it cannot be reseated. Always use new rings during flange reassembly.
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