Flange, Gasket and Bolting Reference
Joints leak for the same handful of reasons every time. This page is the practice we apply, written out.
Facings have to match
A raised face mating with a raised face, a flat face with a flat face, and a ring joint with the matching groove. Any other combination is a leak waiting for pressure, no matter how much torque is applied.
The facing also has to be clean and undamaged. Paint on a facing is a compressible layer the gasket cannot seal against, and a radial score across the sealing surface is a path from the bore to atmosphere.
Gaskets follow the temperature
A spiral wound gasket covers a wide range and is the usual choice on a steel valve body. Soft sheet gaskets suit lower pressure and temperature. Ring joint gaskets are used where a soft gasket is not reliable.
A gasket that has been compressed does not recover, so it is replaced whenever the joint is opened. Reusing one is the most reliable way to produce a weep on reassembly.
Bolting practice
Load the joint evenly: a crossing pattern, in several increments, finishing at the specified torque. Tightening round the circle in one pass can leave the far side under-loaded even when every bolt reads correctly.
On a hot line, re-tighten after the first thermal cycle, because the initial cycle relaxes the gasket and redistributes the load. Where the joint sees repeated cycles, the gasket material is the item to review.
Joint Checklist
| Facing | Match RF to RF, FF to FF, RTJ to RTJ | Mismatched facings leak regardless of torque |
| Facing condition | Clean, flat, unpainted, unscored | Paint and radial scores are leak paths |
| Gasket | Suited to the medium and the temperature | Replace whenever the joint is opened |
| Bolting | Material selected for the joint temperature | Not inherited from the flange material |
| Sequence | Crossing pattern, several increments, final torque | Even load is the objective |
| After first heat-up | Re-tighten to procedure | Standard practice on hot lines |
Questions this tool raises
Because the initial thermal cycle relaxes the gasket and redistributes the bolt load. Re-tightening to procedure after the first cycle is standard practice.
No, and it should not be assumed. Bolts operate at the joint temperature and lose strength with it, so the bolting is selected separately for the service.
Use the gasket manufacturer's figure for the gasket and size, and load the joint evenly in a crossing pattern. Where the joint is critical, control the bolt elongation rather than the torque.