Applying the ASME Codes: Plant Piping & Pressure Vessels (Mister Mech Mentor, Vol. 2)
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Except in small pipe sizes, usually smaller than 2-1∕2 in. NPS, which may have threaded screw connections, and low-end water piping, which may be joined by grooved flexible or split-ring devices, true mechanical pressure piping is joined to pieces of equipment by means of gasketed, bolted engineered flanges.
For example, we have pumps, compressors, pressure vessels and valve bodies, all made with integral flanges in a special series of standard size-strength-bolting patterns, such as American National Standards Institute (ANSI)∕ASME Standard B16.5 and the American Petroleum Institute (API) Standard Std-605 patterns. To form a suitably strong and pressure-tight joint that will not leak the fluid contents, the pipe end mating the equipment flange must have a companion flange attached to it, which matches the specification and mates with the pattern of the equipment flange. Steel bolts compress a special gasket between the machined faces of the flanges, and the gasket blocks fluid flow under pressure. There are many types of gaskets, for the many different pressure-temperature-chemical exposure environments they are intended to withstand. And as long as the design pressure-temperature rating specified in the applicable Flange Standard is not exceeded, and the whole flange assembly is properly installed, the bolted flange-gasket sandwich joint will not leak or break.
It is important that all persons affected by the use of flanged joints be aware that the integrity of the joint is adversely affected by external pipe-stress loads, such as bending moments, which always exist to some degree. The engineer must factor in such loads, and may have to reduce actual fluid-pressure loading accordingly, or go to a higher pressure-rating flange for the desired fluid max pressure, so that the flanged joint in toto will not be overstressed.