Sheet gaskets
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Frequently asked questions
What sheet gasket materials exist and where are they used?
Common types are non-asbestos compressed fibre for steam, oil and water at moderate temperature; rubber sheet (NBR, EPDM) for low pressure; PTFE for corrosive chemicals; and graphite for high temperature. The choice depends on medium, temperature and pressure together, not one factor.
Which dimensions are needed to cut a flange gasket?
You need inside diameter, outside diameter, number of bolt holes, pitch circle diameter and hole diameter, or simply the flange standard and nominal size, for example DN50 PN16 or JIS 10K 50A. The gasket ID should not be smaller than the pipe bore, or it will protrude into the flow.
Should I choose a thick or thin gasket?
A thin gasket (about 1.5 to 2 mm) holds its compressive stress better and is less likely to blow out, so it is the default for flat, smooth flanges. Use a thicker one only when the flange is warped or pitted and needs to take up unevenness.
Why does a new gasket leak once the line is hot?
Usually the bolts were tightened unevenly or too lightly, and not retightened after the gasket relaxed. Tighten in a star pattern over several increasing passes, and for compressed fibre retorque after the first heat cycle as the sheet maker advises. Never reuse an old gasket and avoid putting jointing compound on it.
How should sheet gaskets be stored so they stay usable?
Keep sheets flat or rolled on a large diameter, never folded, in a dry place out of sunlight. Cut gaskets should lie flat or be stacked in bags, not hung by one bolt hole, because they warp easily. Compressed fibre gaskets absorb moisture, so keep the packaging sealed.
Should I apply sealant or grease to a sheet gasket when fitting?
Generally not. Sealant and grease make the surface slippery, so the gasket can be pushed out under pressure and struggles to hold its seating stress. If easy removal later is needed, use only the anti-stick agent the gasket maker recommends, in a very thin film.
Can a gasket be reused once removed?
It is not advisable. After compression and heat, the gasket has taken a permanent set and lost its recovery, so a second tightening will not seal as well as the first. Fit a new gasket every time the flange is opened and remove every trace of the old one from the faces.
How should flanges be prepared before fitting a gasket?
Scrape off the old gasket with a plastic or brass scraper, avoiding tools that scratch the face. Look for radial scratches, since a mark across the sealing face is a direct leak path. Both flanges must be parallel and concentric before tightening, and bolts must not be used to pull misaligned flanges into place.
