Cut Gaskets for Pipe Flanges

A flange gasket is not chosen; it is derived. Its outside diameter, bore, bolt circle and hole pattern all follow from the flange the joint is made on - the flange standard, the nominal size and the pressure class - and the two common families are dimensioned in ASME B16.21 for ASME B16.5 flanges and EN 1514-1 for EN 1092-1 PN-designated flanges.

Two patterns are cut, and both are shown in the photograph. A full-face gasket covers the whole flange face and is drilled for the bolts; it is used on flat-face flanges, particularly cast iron and plastic, where clamping outside the bolt circle keeps the flange from being bent. An inside-bolt-circle ring gasket stops short of the bolts and is used on raised-face flanges, where the load has to be concentrated on the raised face to seal it.

ASME B16.5 and EN 1092-1 flanges are not interchangeable: their bolt circles, hole counts and gasket dimensions differ, and a gasket cut to one standard will not fit the other even at the same nominal bore. When ordering, state the flange standard, nominal size, pressure class or PN, whether full-face or ring is wanted, the gasket material and thickness, and the medium and temperature the joint will see.