Forces Acting on a Gasketed Joint Forces Acting on a Gasketed Joint

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Forces Acting on a Gasketed Joint

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Forces Acting on a Gasketed Joint

The Internal Pressure: These are the forces continually trying to unseal a gasketed joint by exerting pressure against the gasket (blowout pressure) and against the flanges holding the gasket in place (hydrostatic end force). See Figure 1
The Flange Load: The total force compressing the gasket to create a seal, i.e., the effective pressure resulting from the bolt loading.

Temperature: Temperature creates thermo-mechanical effects, expanding or contracting the metals, affecting the gasket material by promoting "creep relaxation" which is a permanent strain or relaxation quality of many soft materials under stress.The effect of certain confined fluids may become increasingly degrading as temperature rises and attack upon organic gasket materials is substantially greater than at the ambient temperatures (about 75°F). As a rule, the higher the temperature, the more critical becomes the selection of the proper gasket.

Medium: The liquid or gas against which the gasket is to seal.

General Conditions: The type of flange, the flange surfaces, the type of bolt material, the spacing and tightness of the bolts, etc.

Each of these factors require consideration before an effective gasket material is finally chosen. However, the proper gasket may often be rejected because failure occurred due to a poorly cleaned flange face, or improper bolting-up practice. These details require careful attention, but if complied with will help eliminate gasket blow-out or failure.

There are three principal forces acting on any gasketed joint. They are: Bolt load and/or other means of applying the initial compressive load that flows the gasket material into surface imperfections to form a seal. The hydrostatic end force, that tends to separate flanges when the system is pressurized. Internal pressure acting on the portion of the gasket exposed to internal pressure, tending to blow the gasket out of the joint and/or to bypass the gasket under operating conditions. There are other shock forces that may be created due to sudden changes in temperature and pressure. Creep relaxation is another factor that may come into the picture.

Figure 1 indicated the three primary forces acting upon a gasketed joint which we will consider for this discussion. The initial compression force applied to a gasket seating surfaces regardless of operating condition. Initial compression force must be great enough to compensate for the total hydrostatic end force that would be present during operating conditions. It must be sufficient to maintain a residual load on the gasket/flange interface. From a practical standpoint, residual gasket load must be "X" time internal pressure if a tight joint is to be maintained. This unknown quantity "X" is what is known as the "," factor in the ASME unfired pressure vessel code and will vary depending upon the type of gasket being used. Actually the "m" value is the ratio of residual unit stress (bolt load minus hydrostatic end force) on gasket (psi) to internal pressure of the system. The larger the number used for "m," the more conservative the flange design would be, and the more assurance the designer has of obtaining a tight joint. >>The End

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