Gaskets are used to create and maintain a static seal between flanged connections under operating conditions.

The bolting force or clamp force (in case of clamped connections) compress the gasket which flows into the imperfections between the mating surfaces creating a seal. For a properly designed gasket enclosure the following considerations must be taken into account to achieve a satisfactory seal :

Forces acting on Gasket
  • Sufficient force must be available to initially seat the gasket.
  • Sufficient forces must be available to maintain a residual stress on the gasket under operating conditions to ensure that the gasket will be in close contact with the seating surfaces preventing a blow-by or leakage.
  • The gasket material must withstand the pressure exerted on the gasket at the operating temperature as well as resist corrosive attack from the process fluid.

Gasket Seating

There are two major factors to be considered with respect to gasket seating:

  • The gasket material.
  • Surface finish of the gasket seating surface.

ASME Section VIII Division 1 defines minimum design seating stresses for a variety of gasket materials. These seating stresses vary from 0 psi to 26,000 psi depending on gasket material.

For soft gaskets it is necessary to have a relatively rough gasket seating surface finish of about 500 microinches. Sufficient roughness allows the gasket seating surface to bite into the gasket thereby preventing excessive extrusion and increasing resistance to blowout.
For metal gaskets the required surface finish is about 63 microinches. In case of metal gaskets, higher loads are required to flow into the imperfections on the gasket seating surface. This requires the gasket seating surface to be as smooth as possible to ensure an effective seal.

There are three principal forces acting on any gasketed joint. They are:

  • Bolt load for applying the initial compressive load.
  • Hydrostatic end load due to internal pressure that tends to separate the flanges.
  • Internal pressure acting on the portion of the gasket exposed to internal pressure, tending to blowout the gasket.

Industry Codes and Standards

  • ASME B16.20 - Metallic gaskets for pipe flanges - ring-joints,spiral-wound and jacketed
  • ASME B16.21 - Non-metallic flat gaskets for pipe flanges
  • DIN 3754 - Compressed asbestos fiber sheets, dimensions, requirements, tests