Why Flanges Leak After Start-Up, and How to Stop It

1. Not enough bolt load

If the studs are not stretched enough, the gasket is never compressed enough to seal at operating pressure. This often happens when torque values come from an old table instead of a calculation for the actual stud, gasket and lubricant, or when the torque was reduced to avoid crushing a gasket.

Fix: calculate the target bolt stress for the joint and tighten to it with a controlled method. See controlled bolt torquing.

2. Uneven bolt load

Tightening studs around the circle, or in one pass, leaves the gasket compressed more on one side. The joint may hold cold and leak as it heats up.

Fix: a cross pattern over several passes, with gap checks. Our tightening sequence guide explains the method. On large joints, tensioning loads studs together.

3. Damaged flange faces

A radial scratch across the sealing face gives the fluid a direct path past the gasket, whatever the bolt load. Pitting, erosion from an earlier leak and dents from dropped tools do the same.

Fix: inspect the face whenever the joint is open, and machine it back to finish with on-site flange facing.

4. Gasket problems

A gasket of the wrong type, size or rating, a reused gasket, or one that has been crushed by too much load will not seal reliably. Gaskets are also easily damaged during handling.

Fix: check the gasket against the specification before fitting, never reuse it, and set the bolt load within its limits.

5. Misalignment

When pipe or equipment pulls the flanges out of line, part of the bolt load is used just to force the faces together, and the gasket ends up loaded unevenly.

Fix: correct the alignment before tightening rather than using the bolts to pull the joint together.

6. Relaxation and thermal cycling

After assembly, the gasket and studs relax slightly, and heating and cooling make the flange, studs and gasket expand at different rates. A joint assembled with a marginal load can lose enough load to leak after a few cycles.

Fix: a sufficient starting load, a check pass at full load, and, where your procedure calls for it, a further check after a few hours.

What to check first when a flange leaks

1. Was the joint tightened to a calculated value, and is there a record?
2. Was a cross pattern and several passes used?
3. What did the face look like when the joint was open?
4. Is the gasket the specified type, and was it new?
5. Is the joint aligned without force?

The answers usually point straight to the cause. The record from a flange management scope answers the first two immediately.