Flange Gasket Selection - Practical Guide

The gasket is the cheapest part of a piping system and the one most likely to leak. A wrong choice voids the pressure rating of the whole flange assembly, forces unscheduled shutdowns, or in the worst case releases hydrocarbons to atmosphere. This guide covers the three gasket families, the selection criteria that actually matter, and recommendations by service.

Selection depends on three properties: seating stress (y) - the minimum compressive stress needed to make the seal initially; gasket factor (m) - a multiplier on internal pressure giving the residual stress that must remain in service; and the P-T rating - the maximum pressure and temperature combination the material can tolerate without creeping, extruding, or decomposing. ASME Section VIII Appendix 2 uses m and y to size bolting; manufacturer-published values are authoritative.

The three families: soft non-metallic (CNAF, elastomers like EPDM/NBR/FKM, PTFE, flexible graphite) for low-to-moderate pressure and temperature; semi-metallic (spiral-wound, kammprofile, double-jacketed) for general industrial service up to high pressure and temperature; and metallic ring-type joints for the highest-pressure and highest-temperature applications. Facing compatibility matters - a spiral-wound gasket needs a flat or raised face, not a ring-joint groove.

Related references: flange dimensions (ASME B16.5/B16.47) and bolt-hole orientation.

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