Seven steps from parts check to final inspection, in the order a fitter works, to stop flange leaks at assembly.
Most flange leaks are not design failures. They are assembly failures: a dirty face, a wrong gasket, dry threads or a rushed tightening pattern. This checklist puts the steps in the order a fitter works through them. Use it alongside your site procedure, not instead of it.
1. Confirm the parts
- Flange size, class and facing match on both sides
- Gasket type, size and material match the job sheet
- Studs and nuts are the specified grade and length
- Washers, if called for, are the right type
Wrong parts are surprisingly common in a busy shutdown. Check labels and markings, not just appearance.
2. Inspect the faces
Clean both faces with a non-scratching tool. Look for radial scratches, pitting, dents and old gasket residue across the sealing area. Radial marks are worse than circular ones because they run straight from inside to outside. Damage beyond minor marks needs repair or a decision from the engineer, not a thicker gasket.
3. Check alignment
The flanges should sit parallel and centred with bolts going in freely. Do not use the bolts to pull badly misaligned pipe together. That load stays in the joint and in the piping, and it fights the gasket for its whole service life.
4. Lubricate the bolts
Apply a suitable thread lubricant to the stud threads and the nut face that turns against the flange. Friction eats most of the torque you apply. Dry or rusty threads can mean far less clamping force for the same wrench reading. Use the lubricant the torque values were calculated for.
5. Fit the gasket
Centre the gasket. Never use grease or tape on a gasket unless the maker says so. Snug the nuts by hand so the gasket is lightly held.
6. Tighten in a cross pattern
- Number the bolts and work across the flange, like tightening wheel nuts
- First pass at a low fraction of final torque
- Raise the value in steps over several passes
- Finish with circular passes at full value until the nuts stop turning
This brings the faces together evenly. Going once around at full torque usually crushes one side of the gasket and leaves the other loose.
7. Final checks
- Gap between faces even all the way round
- Studs protrude through the nuts by roughly the same amount
- Tag the joint with gasket type, date and who did it
- Watch it during start-up and re-check if your procedure calls for it
When it still leaks
Do not just keep tightening. Overloading can crush the gasket or yield the bolts and make things worse. Depressurise, open the joint and look for the cause. Often it is a reused gasket: see can gaskets be reused. Or the wrong gasket for the service: see choosing a gasket.
For the bigger picture, read flange types.