Raised face with a spiral wound gasket is the default industrial flange joint, and it is the right answer most of the time. Ring type joint is the answer when pressure, temperature or consequence makes a soft gasket unacceptable. The two are not ranked, they are different tools, and RTJ gets specified in places where it makes life harder for no benefit.
How each one seals
Raised face
A narrow annular face stands proud of the flange, which concentrates the bolt load onto a smaller area than a full face joint would. A gasket, usually spiral wound, sits on it and is compressed. The gasket deforms to take up surface imperfections and the seal is made by that compression.
Raised faces carry a deliberate serrated finish, and it is there to grip the gasket. A face machined too smooth actually seals worse, because the gasket has nothing to bite into.
Ring type joint
A precision groove is machined into the face and a solid metal ring sits in it. Tightening the bolts wedges the ring against the angled groove walls until the ring yields locally. The seal is metal against metal, formed by plastic deformation.
Because it is a yielded metal seal, there is nothing to creep, relax, extrude or burn out. It is also single use, because that deformation is permanent.
Where RTJ wins
- High pressure. Above the range where a spiral wound gasket is comfortable, the metal seal is the only practical answer. Wellhead service at 10,000 and 15,000 psi is entirely ring joint.
- High temperature. No filler material to degrade, so the joint survives temperatures that destroy a soft gasket.
- Fire exposure. A metal seal does not burn. On hydrocarbon service that is a safety case, not a preference.
- Thermal cycling. Once correctly assembled, the joint tolerates cycling well because there is no gasket relaxation.
- Zero tolerance for leakage. Toxic or lethal service, where the consequence of a weep is not a housekeeping issue.
Where RTJ costs you
- You must spread the joint to change the ring. The ring sits captive in the groove, so you cannot slide it out sideways the way you can pull a sheet gasket. The pipe has to move axially. In a rigid, fully welded run with no flexibility, that can mean cutting pipe to change a gasket.
- Groove damage is a machining job. Scratch a raised face and you can often dress it. Damage a ring groove and it is a re-machining operation with a thickness check, or a new flange.
- Rings are single use, always. Reusing one is a leak waiting to happen.
- Assembly skill. Alignment and even bolt tightening matter more than on a raised face joint. A cocked ring joint does not seal.
- Cost. The flange is more expensive to make and the assembly takes longer.
- Not available in Class 150. Under ASME B16.5, ring joint facing starts above Class 150.
The access question decides more jobs than the pressure question. Before specifying RTJ, ask how the ring gets changed in ten years. If the answer involves cutting pipe, consider whether a raised face joint with a spiral wound gasket would actually serve the plant better.
Side by side
| Factor | Raised face, spiral wound | Ring type joint |
|---|---|---|
| Seal mechanism | Gasket compression | Metal yielding into a groove |
| Pressure ceiling | High | Very high, to 20,000 psi |
| Fire resistance | Depends on filler | Excellent |
| Gasket change | Slide it out sideways | Must spread the joint axially |
| Face damage repair | Often dressable in place | Re-machining, with a thickness check |
| Reusable gasket | No, but cheap and easy | No, and the joint is harder to open |
| Assembly difficulty | Routine | Needs care and even tightening |
| Class 150 available | Yes | No |
| Cost | Lower | Higher |
Choosing
- If the pressure class or the standard dictates it, as on API 6BX wellhead work, the decision is already made.
- If the service is fire-critical or lethal, RTJ, and design the pipe with enough flexibility to spread the joint.
- If it is ordinary process duty within spiral wound capability, raised face. Do not buy maintenance difficulty you have no use for.
- If the line is Class 150, the question does not arise. Raised face, or step up a class if you genuinely need a metal seal.
See ring type joint flanges for groove and ring detail, and selecting R, RX and BX gaskets.