Oval vs Octagonal RTJ Gasket
Both oval and octagonal are R-style ring type joint (RTJ) gaskets defined in ASME B16.20. They share the same ring numbers and fit the same R-grooves. The difference is in cross-sectional geometry, which affects sealing contact, gasket stress distribution, and performance at high pressures.
| Feature | Oval RTJ | Octagonal RTJ |
|---|---|---|
| Cross-section shape | Elliptical (curved) | Eight-sided (flat contact faces) |
| Sealing contact | Line contact (tangent to groove) | Surface-to-surface (flat on flat) |
| Contact area | Smaller | Larger |
| Seating stress | Higher (concentrated) | Lower (distributed) |
| Seal quality | Good (conforms to minor groove imperfections | Better) higher contact area improves leak resistance |
| Groove compatibility | R-groove only | R-groove (preferred) |
| Manufacturing | More difficult to machine | Easier to machine (flat faces) |
| Pressure-energized | No | Slightly (flat faces press harder under internal pressure) |
| Reuse | Never | Never |
| Cost | Slightly higher | Standard |
How They Seat in the Groove
An oval RTJ gasket contacts the groove on two opposing arcs; essentially tangent lines where the curved gasket surface meets the curved groove walls. This creates a narrow, high-stress seal. The oval shape conforms well to grooves with minor imperfections or out-of-roundness, but the small contact area means that any surface damage to the groove can compromise the seal.
An octagonal RTJ gasket seats on two flat faces that match the groove sidewalls. The wider contact band distributes bolt load over a larger area, producing a more reliable seal. Octagonal rings also benefit from a slight pressure-energizing effect: as internal pressure rises, the flat faces are pushed harder against the groove walls.
When to Use Each Type
| Application | Recommended Type | Reason |
|---|---|---|
| New construction (general oil and gas) | Octagonal | Better seal, easier to manufacture |
| Older flanges with R-grooves machined for oval | Oval | Groove profile may not fully support octagonal contact |
| High-pressure service (Class 1500-2500) | Octagonal | Larger sealing surface handles higher bolt loads |
| API 6A wellhead connections | Per API 6A ring designation | Often BX-style, not R-style |
| Replacement in existing system | Match original specification | Changing type may not fit groove profile |
ASME B16.20 Requirements
ASME B16.20 provides separate dimensional tables for oval and octagonal cross-sections. Both are assigned the same R-number for a given flange size and class. The standard specifies:
- OD, ID, and height for each ring number
- Surface finish of 63 micro-inches Ra or better on sealing faces
- Material hardness at least 30 HB softer than the flange groove
- Ring must be free of tool marks, nicks, and scratches on seating surfaces
RTJ gaskets of either type are single-use. After bolt-up and service, the ring has permanently deformed into the groove. Reusing a deformed ring guarantees a leak. New gaskets are required for every flange break, regardless of how briefly the joint was in service.
Material Considerations
Both oval and octagonal RTJ gaskets use the same material grades: soft iron, low-carbon steel, SS 304, SS 316, SS 347, Inconel 625, and Incoloy 825. The material must be softer than the flange groove so that the gasket deforms preferentially. For carbon steel flanges (ASTM A105), soft iron or low-carbon steel rings are standard. For stainless flanges, stainless rings of matching or compatible grade are specified.
Proper installation requires controlled bolt torque applied in a star pattern using calibrated stud bolts and hex nuts.
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