Gaskets

If it were technically and economically feasible to manufacture perfectly smooth and polished flanges, and if we could maintain these surfaces in permanent contact, there would be no need for gaskets. This technical and economic impossibility results from:

• • Size of the vessel and/or the flanges.
• • Difficulty in maintaining these surfaces perfectly smooth and the flange faces perfectly parallel to each other during the handling and/or assembling of the vessel or piping.
• • Corrosion and/or erosion of the sealing surfaces.

To overcome these difficulties, gaskets are used as a sealing element between the flange faces. When a gasket is seated between flange surfaces, it "flows", filling the imperfections between them and providing the necessary seal. Therefore, in order to obtain adequate sealing, we must consider four factors: gasket seating stress, sealing force, material selection and surface finish.

Gasket seating stress:

We must provide adequate pressure to seat the gasket correctly; so it will be able to flow and fill the flange imperfections. This seating stress must be limited in order to prevent crushing of the gasket by excessive compression.

Sealing force:

There must be enough residual pressure on the gasket in order to keep it in sufficient contact with the flange surfaces, thus avoiding leakage.

Material selection:

The gasket material must be able to resist the internal pressure as well as the chemical properties of the fluid to which it is subjected during operation.

Surface finish:

There is a recommended flange surface finish for each style of gasket. Surface finish which is not compatible with the gasket is one of the primary causes of leakage.

Forces in a Flanged Joint

The Figure 1 shows the major forces in a flanged joint.

• – Blowout Force: originated by the internal pressure; it attempts to blow out the gasket.
• – Separation Force: also caused by the internal pressure; it tries to separate the flanges. It is also known as hydrostatic force.
• – Bolt (studs) Force: it is the total load exercised by the bolts or studs.
• – Sealing Force: it is the force, which presses the flanges against the gasket.

Main reasons for gasket failure

• 1. Temperature incompatible with the gasket.
• 2. Pressure incompatible with the gasket.
• 3. Chemical attack.
• 4. Thermal cycling incompatible with the gasket.
• 5. Incorrect measurament of the gasket.
• 6. The surface finish of the flanges is incompatible with the gasket.
• 7. Flange material incompatible with the gasket.
• 8. Special characteristics of the equipment.
• 9. Inadequate gasket assembly.