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STYLE	COLOR	COMPOSITION	APPLICATION
790		SYNTHETIC-NBR	FUEL-WATER-LUBRIFICANT-OIL-GAS
840		ARAMIDIC-CSM	ACIDS-CAUSTIC SODA-STEAM
850		ARAMIDIC-NBR	REFRIGERANTS-ALKALIS-OIL-GAS-STEAM
870		ARAMIDIC-NBR	SOLVENTS-STEAM-ACIDS-OIL-GAS-FUEL-ALKALIS
830		CARBO-FIBER NBR	STEAM 270°C-HYDROCARBONS-GAS
910		STABILIZED P.T.F.E.	UNIVERSAL USE - BAM CERTIFICATION
920		STABILIZED P.T.F.E.	GREATER COMPRESSIBILITY - PHARMACEUTICAL
940		STABILIZED P.T.F.E.	OXYGEN SERVICE - CONFORM TO FDA

A gasket is used to create and retain a static seal between two stationary flanges, or other surfaces which connect mechanical assemblies in an operating plant, and containing a wide variety of fluids .
These static seals aim to provide a complete physical barrier against the fluid contained within, and so block any potential leakage path .

To achieve this, the gasket must be able to flow into (and fill) any irregularities in the mating surfaces being sealed, while at the same time be sufficiently resilient to resist extrusion and creep under operating conditions .
The seal is effected by the action of force upon the gasket surface, which compresses the gasket, causing it to flow into any flange imperfections.

The combination of contact pressure between the gasket and flanges, and densification of the gasket material, prevents the escape of the contained fluid from the assembly. As such, gaskets are vital to the satisfactory operation of a broad range of industrial equipment.
On seating, a gasket must be capable of overcoming minor alignment and flange imperfections, such as:

• non-parallel flanges
• distortion troughs / grooves
• surface waviness
• surface scorings
• other surface imperfections

When assembled, a flange gasket seal or “joint” is subject to compressive pressure between the faces of the flanges, usually achieved by bolts under load. In order to ensure the maintenance of the seal throughout the lifetime of the assembly, sufficiently high pressure must remain on the gasket surface to prevent leakage.

Under operating conditions, this pressure will be relieved by hydrostatic end thrust, the force produced by internal pressure which acts to separate the flanges.
The gasket itself is also subject to a side load due to the internal fluid pressure tending to extrude it through the flange clearance space.
To maintain seal integrity, the effective compressive pressure on the gasket (that is, the assembly load minus the hydrostatic end thrust) must be greater than the internal pressure by some multiple, dependent upon the gasket type, manufacturing process involved and level of tightness required.
For soft gaskets, there must also be adequate friction between the gasket and flange faces to help prevent extrusion (blowout) of the gasket from the joint. To allow for any relaxation of gasket compressive pressure, which is normally inevitable, a factor of at least two is usually recommended between the compressive pressure on the assembly and that required to maintain a seal.