Pipe Flanges

Flange Types and Where Each Is Used

The weld neck flange has a tapered hub that transfers stress from the flange to the pipe smoothly and is the standard choice for high pressure, high temperature and severe cyclic service, and for all applications where radiography of the weld is required. The slip on flange is slipped over the pipe and fillet welded at both sides; it is cheaper and easier to align but has lower fatigue life and is not used for severe cyclic service. The socket weld flange is used for small bore lines in the same way as a socket weld fitting. The threaded flange is used where welding is not permitted, typically on small bore utility lines. The lap joint flange uses a stub end, so the flange rotates for bolt hole alignment and the wetted surface is the stub end material, which makes it economical in stainless or exotic material. The blind flange closes a flanged opening and is removable. Selection is governed by the design code, the pressure and temperature class, and whether the joint will be radiographed.

Understanding Flange Classes

The classes 150, 300, 600, 900, 1500 and 2500 in ASME B16.5 are not pressure ratings in psi. They are class designations that define a set of dimensions, and the actual permissible working pressure depends on both the class and the material and the design temperature. For example a class 150 carbon steel flange rated for roughly 19 bar at 38 degrees Celsius derates to about 13 bar at 200 degrees Celsius, and the derating factor differs for stainless steel. The correct procedure is to select the class from the applicable pressure temperature rating table in ASME B16.5 for the specific material and design temperature, then confirm the pipe schedule matches the flange bore. ASME B16.47 covers larger flanges, series A for sizes 26 to 60 inch in the MSS SP-44 tradition and series B for sizes 26 to 60 inch in the API 605 tradition; the two series are not dimensionally interchangeable.