This blog post was adopted from Jim Drago, PE, Senior Manager, Marketing Intelligence, Garlock Sealing Technologies, Palmyra, New York, USA article entitled “Initial gasket compression is key to safe, reliable flange joints”
A question that we are often asked is “what pressure rating is the gasket material I need good for?” Obviously, the concern is for the gaskets ability to maintain a good seal and not “blow-out” According to Jim Drago the answer to that questions depends on a number of variables.
Blow-out resistance is not primarily a function of gasket tensile strength, but rather a number of other factors, including clamping force, flange surface texture, temperature, gasket creep resistance, joint rigidity and internal pressure.
1) The clamping force of the bolts transferred through the flanges produces compressive stress (force per unit area) on the surface of the gasket. Different types of gaskets require different minimum compressive stresses in order to effect and maintain a seal.
2) Surface-finish determines how effectively flanges grip a gasket. It is especially important to bear this in mind when dealing with soft, sheet-type gaskets. A serrated flange surface, as specified by ASME B16.5 section 18.104.22.168 will “bite” into a gasket – holding it in place and preventing it from splitting and shearing as it would with a
3) Temperature and gasket creep are directly related. When the temperature increases, flange bolts relax as their yield strength decreases. Gasket materials become thinner without an increase in compressive load. Bolt relaxation and gasket creep effectively reduce the clamping force on a gasket, increasing the probability of a leak.
4) Flange rigidity affects how bolt load will be transferred to a gasket.
5) Internal system pressure develops forces within a joint that concurrently work to pull apart the flanges and push out the gasket.
Finally, In the case of soft-sheet gaskets, compressive stress and not the tensile strength of the material is the major determinant of flanged joint reliability. Therefore, gasket installation demands the utmost diligence. The use of torque wrenches and following gasket manufacturers’ guidelines will go a long way to assuring reliable joints for improved worker safety and plant productivity.
To read the full article, including testing data, go to