As CraigB says, the basic is to understand ASME VIII requirements. This Code asks to consider two required (I prefer to say minimum) gasket pressures, one for seating condition and one for design condition. This is the "y-m" ASME VIII approach. Even this simple model is not so easy to be explained; there is a flange ring rotation under the pressure, and App2 rules try to adjust the area of compressive sealing in order to count-at list in part- this rotation. So the areas for "seating" and "operational" gasket stress are different(I think this makes sense when the difference between seating -zero pressure- and design is substantial... but this is another story...)
The Code doesn’t require checking the actual gasket pressure under operational conditions because the Code is not focused on flange checking. It is true also the model considered is poor and, in fact, it doesn’t give you the chance to correctly count the gasket pressure in operational conditions. Of course, this doesn’t mean is something wrong with the Code, you must understand that the Code rules are focused on design and are leading to good results in practice (some critics saying the rules are too conservative).
Some words about the real distribution of the gasket pressure/stress/load. When loaded due to pressure and external moments, the outer edge gasket compression increases on the compression side and reduces on the tension side. It may be a lift-off of the gasket surface along the inside circumference.
The operating gasket stress will be removed from the seating gasket stress when the gasket is stiff, while it may remain closed to the seating gasket stress when the gasket is soft. "Soft" or "stiff" are to be counted- in this case- in relation with the flange and bolts stiffness. The relationship between the stiffness of the gasket, bolts and flange is what a FEA model can show you. Also 13445 Annex G considers a calculation intended to provide this result.
Edited by mariog (12/13/08 02:28 PM)
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