A flanged joint looks simple: two parts, a gasket and a few bolts. Most field leaks start from exactly that assumption. The five rules below cover the mistakes made most often on a first installation.
1. Match the right part to the right pipe
On a weld neck flange the hub bore is machined to the pipe schedule. Putting a SCH 80 hub on SCH 40 pipe leaves a wall mismatch at the weld and inspection will reject it. Size, class, material and schedule must be checked together at ordering and at goods receipt.
2. Do not force alignment
Parallelism and axial offset between two flanges cannot be closed by tightening bolts. A joint pulled into place crushes the gasket on one side and locks permanent stress into the pipe. If there is misalignment, correct the support or the pipe end, not the bolts.
3. Check the face and gasket seating
- No old gasket residue, burrs or radial scratches on the face
- Install the gasket dry and centred
- Match face type to gasket type: spiral wound on RF, full face on FF, metal ring in an RTJ groove
- A gasket is used once; fit a new one whenever the joint is opened
4. Torque in sequence and in stages
Bolts are tightened in a star pattern in at least three stages: about 30%, 60% and 100% of target torque, followed by one clockwise pass. Applying full torque in one go crushes the gasket unevenly and causes leaks.
| Stage | Torque | Purpose |
|---|---|---|
| 1 | ≈30% | Seat the gasket |
| 2 | ≈60% | Balance the load |
| 3 | 100% | Sealing load |
| 4 | 100%, sequential pass | Catch any loose bolt |
5. Check again after the first heat up
On lines that heat and cool, the gasket beds in and bolt load is lost. A torque check after the first run prevents most later leaks and should be part of the installation plan on hot lines.
Bonus: get the stud length right
Blind flanges, isolation sets and thick gaskets change the required stud length. Studs that are too short leave the nut without full thread engagement, and overly long studs cause confusion in the field. Ordering as a set is the practical answer.
Let us put flanges, gaskets and stud sets on one list: enquiry form.
FAQ
In what order are bolts tightened?
In a star pattern, in at least three stages, finishing with one sequential pass to catch any bolt left loose.
Should the gasket be greased?
No. Gaskets are installed dry; grease and adhesives do not improve sealing and increase the risk of slipping.
Is a torque check after heat up necessary?
On thermally cycled lines, yes. The gasket beds in and bolt load drops; without a check, leaks appear later.
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