Flange Gaskets and Bolt Sequence: What Actually Decides Sealing

When a flanged joint weeps, the gasket is blamed first. Yet if the gasket is the right standard and the right material, the cause is usually not the gasket but how bolt load is distributed around it. A gasket is a passive part: give it even compression and it seals; crush one side and starve the other and it leaks. That distribution is the one thing the fitter controls.

Why the pattern is crosswise

As soon as one bolt is tightened, the flange deflects locally around it. Tighten its neighbour next and the deflection walks around the circle, compressing the gasket progressively from one side. The flange behaves like a lid pressed down at a single corner — the far side lifts slightly.

A cross, or star, sequence prevents that. After one bolt, the next is the one diametrically opposite, then the axis halfway between them, then its opposite. The flange is pulled down evenly from two ends at every step and the gasket keeps its centre. The sequence matters more as bolt count rises; on a large flange, random tightening will not give a sound seal.

Staged, not in one pass

The second rule is to build torque in stages. Take one bolt straight to final torque and it will lose load the moment its neighbours move the flange. In practice this order works:

1. Bring all bolts up to finger tight and check that the faces are parallel.
2. Work the cross pattern up to a small fraction of final torque; the gasket seats.
3. Repeat the pattern at an intermediate stage.
4. Repeat it again at final torque.
5. Finish with one clockwise pass at final torque around every bolt.

That last pass is often skipped and is the most informative one: a bolt that offers no resistance tells you the gasket near it is not sufficiently compressed.

Bolts are not an alignment tool

If the faces are not parallel — the line was sprung into place, the axes are offset, or the gap does not suit the gasket — bolts can still pull the joint shut. Bolts are strong. But that does not fix the error, it only makes it invisible. The force used to align the flanges stays as permanent stress in the pipe, the weld and the equipment nozzle, and when the line heats up it adds to thermal stress.

The correct response is to slacken off, let the line go free and find where the offset comes from. Support adjustment, a corrected pipe length, or a flexible element sized for the movement — the answer is one of those, not a longer spanner.

Gasket material follows the fluid

Gasket selection starts with one question: what is inside, and at what temperature? On a cold water circuit many materials will do. Introduce steam, thermal fluid or chemicals and the range narrows fast; the material must not soften, must not react with the fluid, and must not extrude under load. Face type matters here too: a raised face applies high stress in a narrow band, a flat face spreads it widely, and one material will not behave identically on both.

In short: pick the gasket for the fluid, tighten the bolts in the right order and in stages, and never align the line with bolts. Do all three and the flanged joint becomes the least troublesome part of the plant.