Flanged Joint Leaking
The gasketed joint is the part of a valve installation most likely to leak, and the valve usually takes the blame.
Flanged Joint Leaking
The gasketed joint is the part of a valve installation most likely to leak, and the valve usually takes the blame.
What it looks like on site
A weep or drip at the valve flange, often after a thermal cycle or a shutdown.
The causes, in order of likelihood
- Insufficient or uneven bolt load. Tightening by feel, or in a sequence that does not compress the gasket evenly.
- Gasket not suited to the temperature. A soft gasket in a hot joint relaxes, and the bolt load is lost.
- Thermal cycling. The bolts, gasket and flanges expand differently, so the joint that was tight cold weeps hot.
- Damaged or painted facing. A scored facing, or paint on the sealing surface, gives the gasket nothing to seal against.
- Gasket reused. A compressed gasket does not recover, and reusing one is a reliable way to produce a weep.
Bolting procedure is not optional
A flange joint is compressed by the bolts, and the load has to be applied evenly. Tightening round the circle in one pass loads one side first and can leave the opposite side under-loaded even when every bolt reads the correct torque.
The usual practice is a crossing pattern in several increments, with the final pass to the specified torque. Where the joint is critical, the bolt elongation or the gasket compression is controlled rather than the torque alone.
Temperature changes the joint
On a hot line the joint has to be re-tightened after the first thermal cycle, because the initial cycle relaxes the gasket and redistributes the load. A joint that was correct at ambient is often loose once the line is at temperature.
Where the joint is repeatedly leaking after thermal cycles, the gasket material is the item to review. A graphite-based or spiral wound gasket behaves very differently from a soft sheet gasket in exactly this condition.
Facing condition
The gasket seals against the flange facing, so the facing has to be clean, flat and free of damage. Paint on a facing is a leak path, and a radial score across the sealing surface is a path from the bore to the outside.
That is why facings and threads are masked during painting, and why the facing condition is worth checking on receipt rather than after installation.
Leak By Cause
| Item | Detail | Note |
|---|---|---|
| Weeps after the first heat-up | Thermal relaxation | Re-tighten to procedure after the first cycle |
| Weeps repeatedly, hot only | Gasket material not suited to the temperature | Change to a spiral wound or graphite gasket |
| Weeps at one bolt position | Uneven load or a damaged facing | Re-torque in a crossing pattern; inspect the facing |
| Weeps from commissioning | Paint, score or a reused gasket | Clean the facing and fit a new gasket |
| Weeps on several joints at once | Bolting procedure | Introduce a written tightening sequence |
The checks, in order
Confirm the leak is at the joint
Distinguish a flange leak from a body joint or a packing leak.
Check the facing condition
Paint, scoring and corrosion on the sealing surface.
Review the gasket type against the temperature
The usual root cause on a joint that leaks hot.
Check the tightening sequence
Even load, crossing pattern, several increments.
Re-torque after the first thermal cycle
Standard practice on a hot line.
From the works and the projects
Questions this page answers
Because the bolts, gasket and flanges expand differently, and the gasket relaxes. Re-tightening after the first thermal cycle, and choosing a gasket suited to the temperature, are the usual fixes.
No. A compressed gasket does not recover, and reusing one is a reliable way to produce a weep. Replace it whenever the joint is broken.
Yes. Paint is a leak path and a compressible layer the gasket cannot seal against. Facings are masked during painting for exactly this reason.