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New gasket constants to replace the ASME Code m and y are currently being developed by the Pressure Vessel Research Council (PVRC) and ASME. The current m and y are difficult to replicate for non-asbestos gaskets and do not consider joint leakage. Because of the Clean Air Act and environmental considerations in general, it was decided to revisit the design parameters of the bolted joint. The new approach to bolted joint design makes the tightness (or, lack of leakage) of the joint a design parameter.
In a manner similar to the traditional ASME Code method, the design bolt load for a joint is calculated for operating and seating requirements from the new constants Gb, a and Gs and the required tightness class associated with the minimum tightness. Gb and a, gives the gasket seating load and are similar to y in the present Code. Gs is associated with the operating stress and is similar to the m value in the Code.
The proposed ASME constants Gb, a, and Gs give a design bolt load obtained by interpretation of leakage test data as plots of gasket stress Sg, vs. a tightness parameter, Tp. Tp is the pressure (in atmospheres) normalized to the atmospheric pressure required to cause a helium leak rate of 1 mg/sec for a 150 mm OD gasket in a joint. Since this is about the same as the OD of an NPS 4 joint, the pressure to cause a leak of 1 mg/sec of helium for that joint is its tightness. A standard test procedure, the PVRC Room Temperature Tightness Test (ROTT) has been designed to produce the constants Gb, a and Gs. Low values for Gb, a and Gs are desirable while a higher value of Tp means a tighter joint.
The new gasket factors Gb, a and Gs, are available for for Durlon® gasket materials. While the torque tables for Durlon® gasketing use these new gasket constants as a guide, generally the recommended bolt stress range of 40% to 60% of bolt yield is higher and takes precedence for our recommended torque values.
|9600 w/ DurCore||1/8″||1.5||833||204||0.316||1.00E-07|