A gasket is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression. Flanged joints are made by bolting together two flanges with a gasket between them to provide a seal. The gasket’s function is to seal two different surfaces held together by one of several means, the most common being screw-threaded devices such as bolts.
The charecteristics of Good Gasket
It will seal the fluid system.
A good gasket should be chemically inert to resists the system fluid to avoid chemical reaction with fluid system.
Should deform enough to flow into the imperfections on the gasket seating surfaces to provide intimate contact between the gasket and the seating surfaces.
Withstands system temperatures without serious damaging the fluid system properties.
Should be resilient and resists creep enough to maintain an adequate portion of the applied load.
A good gaskets has sufficient strength to resist crushing under the applied load, and maintain its integrity when being handled and installed.
Should not corrode the gasket seating surfaces.
There are limits on the degree of flange surface imperfection that can be sealed successfully with a gasket. Large nicks, dents, or gouges must be avoided, since a gasket cannot properly seal against them.
The surface finish of a flange is described as follows:
1. Roughness : Roughness is read in millionths of an inch (or meter) as the average of the peaks and valleys measured from a midline of the flange surface.
2. Lay – Lay : is the direction of the predominant surface-roughness pattern. Example: multidirectional, phonographic spiral serrations, etc.
3. Waviness : Waviness is measured in thousandths or fractions of an inch. Basically, it is the departure from overall flatness.
Typical roughness readings can be from 125 to 500 micro-inches for serrated flanges and 125-250 micro-inches for non-serrated flanges. Fine finishes, such as polished surfaces, should be avoided. Adequate “bite” in the surface is required to develop enough friction to prevent the gasket from being blown out or from extruding or creeping excessively.
The lay of the finish should follow the midline of the gasket. For example, concentric circles on a round flange, or a phonographic spiral. Every effort should be made to avoid lines across the face, such as linear surface grinding, which at 180º points will cross the seal area at right angles to the gasket, allowing a direct leak path.
Waviness is seldom a problem under normal conditions. There are two areas that must be watched, however, since excessive waviness is very difficult to handle.
The first area is glass-lined equipment where the natural flow of the fused glass creates extreme waviness. Often the answer here is to use thick and highly compressible gasketing.
The second area of concern is warped flanges. If warpage is caused by heat or internal stresses, re-machining is generally sufficient. However, warpage due to excessive bolt loads or insufficient flange thickness results in what is generally called bowing.
The solution is to redesign for greater flange rigidity. Sometimes backer plates can be added to strengthen the design without replacing the parts. Another step would be to add more bolts. When this is done, usually smaller bolt diameters are possible, thus adding more bolt stretch and better joint performance.
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