GOOD AND TIGHT FOR THE RIGHT TIGHT, TIGHTER IS BETTER: A DESCRIPTION OF ASME MODERN FLANGE DESIGN R obert W illiams WestermannBG August 21, 2017 Abstract The Design by Method approach found in the ASME Code allows for the selection of bolting materials with the intent of securing the joints as tightly as possible. During operation, stresses in the structure of piping runs will settle and the incurred strain will be transmitted to the weakest member of the assembly, the bolted flange. In such cases a tighter joint may not only reconcile those strains, but when employing a properly selected and fitted gasket, improve the fatigue resistance and volumetric laminar flow resistance of the joint.
I. B olted - flanged J oint Piping runs can be assembled in multiple ways; with fully welded assemblies, threaded pipes and connectors, or bolted flanges. The bolted flanged joint offers an unparalleled ease in the assembly and disassembly department of large bore piping systems, allowing for a variety of equipment and fittings to also be mounted. By its very nature, the flanged joint is the weakest link in a piping structure. When under normal operating conditions, a joint leak may be the result of inadequate design, materials, or assembly. On the other hand, leaks may also develop as a result of abnormal operating conditions, such as: A. Excessive working loads B. Fatigue caused by cyclic loads C. Sudden hammer loads D. Excessive dead loads E. Excessive or transient temperatures
F. Material creep
With the exception of fatigue and hammer loads, each of these conditions will produce a constant strain on the joint and its components causing a loss of contact load between the flange face and gasket. Fatigue and hammer loads cause a material to behave in a manner unlike those found when operating under laboratory conditions where the application of loads are slow and controlled. A flanged joint is composed of the following elements; flanges, bolts, a gasket, and (if part of the design) the wall of the pipe or vessel. A minimum axial load must be maintained on the joint to prevent unseating and leakage. With the use of a gasket between the flanges, a minimum axial load is necessary in keeping the gasket well seated.
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