A leak is usually explained as "a bad gasket". The gasket is normally the consequence; the real cause sits one step earlier.
Root causes
- 1. Insufficient or uneven tightening. The most common cause: full torque in one pass, a skipped star pattern or bolts left loose all crush the gasket unevenly.
- 2. Wrong gasket selection. Filler unsuited to service temperature, the wrong ring material, or a gasket type that does not match the face form.
- 3. Misalignment. Parallelism and offset corrected by pulling with bolts load the gasket unevenly and lock stress into the pipe.
- 4. Surface damage. Radial scratches, corrosion pits, old gasket residue.
- 5. Thermal cycling and creep. Preload is lost on lines that heat and cool; without a torque check after the first heat up, leaks appear later.
- 6. Design loads. Thermal expansion, equipment layout or support errors put a moment on the flange, and the gasket opens on one side.
- 7. Pressure surge. Sudden valve closure or water hammer creates instantaneous load beyond the preload.
What to do when a leak is found
| Step | Action |
|---|---|
| 1 | Make the line safe; never tighten bolts under pressure |
| 2 | Check bolt torque and look for loose studs |
| 3 | On dismantling, inspect the gasket and face; the marks show the cause |
| 4 | Classify surface damage and remachine if needed |
| 5 | Fit a new gasket and correct the alignment |
| 6 | Tighten in stages and re-check after the first heat up |
Gasket marks are diagnostic: one sided crushing points to misalignment, uneven depth to unbalanced torque, and burn marks to a temperature mismatch.
Recurring leaks
If the same joint keeps leaking, the problem is in the design rather than the assembly. Line flexibility, support arrangement and equipment loads should then be reviewed.
Related: flange bolt torque, how to select a flange gasket. For gasket and stud sets: enquiry form.
FAQ
Can bolts be tightened while leaking?
No. Tightening under pressure is unsafe; the line must be made safe first.
What do gasket marks show?
One sided crushing indicates misalignment, uneven depth unbalanced torque, and burn marks a temperature mismatch.
What if the same flange keeps leaking?
The cause is likely in pipe stresses and design rather than assembly; review flexibility and supports.
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