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Flange Facing : Many facings for flanges are offered by manufacturers, including various tongue and groove types which must be used in pairs. However only four types of facing are widely used. The raised face is used for 80% of all flanges. The ring-joint facing, employed with either an oval-section or octagon section gasket, is used mainly in Petrochemical Industry. The four types of faces are as follows:
Raised Face : It has a raised face with serration on it to hold the gasket firmly in position. This is widely used.
Flat Face : Most common uses are for mating with non-steel flanges on bodies of pumps, valves etc. and for mating with 125# cast-iron valves and fittings. A full size gasket is used to avoid breaking of the flange while tightening.
Ring Joint Facing : It is more expensive facing and used in pairs for high temperature and high pressure service. Both flanges of the pair are alike.
Lap Joint Facing : It has special curvature for accommodating stub end. The combination of flange and stub-end presents similar geometry to the raised face flange and can be used where bending stresses will not occur. This is used for SS pipes, where stub-end is made of SS pipe and flange is made of Carbon Steel.
Flange faces are usually finished by machining to produce a spiral round-bottomed groove, or a V shaped spiral or concentric groove, termed as serrated finish. Smooth finish is generally made to order. Serrated finish is used with asbestos and other gaskets. The regular smooth finish is used with gaskets made from harder materials and with spiral wound gaskets.
Flange Finishes : The term finish refers to the type of surface texture produced by machining the flange face which contracts the gasket. Two principal finish types are serrated and smooth.
Weld Neck Flange, Regular and Long : Regular welding neck flanges are used with butt-welding fittings. Long welding neck flanges are primarily used for vessel and equipment nozzles, rarely for pipe. These are suitable where extreme temperatures, shear, impact and vibratory stresses apply. Regularity of bore is maintained in this.
Slip On Flange : It is properly used to flange pipe. Slip-on flanges can be used with long-tangent elbows, reducers, and swages. The internal weld is slightly more subject to corrosion than butt-weld. The flange has poor resistance to shock and vibration. It introduces irregularity in the bore. It is cheaper to buy than weld neck flange, but costlier to assemble. It is easier to align than welding neck flange. Calculated strengths under internal pressure are about one third that of corresponding welding neck flange. Pipe or fitting is set back from the face of flange a distance equal to wall thickness.
Reducing Flange : It is suitable for changing size, but should not be used if abrupt transition would create turbulence, as at pump connections. Available to order in weld neck type and from stock in slip-on types.
Expander Flange : It is similar to weld neck but increases pipe size to first or second larger size. It is alternative to using reducer and welding neck flange. Useful for connecting to valves, compressors, and pumps. Pressure ratings and dimensions are in accord with ANSI B16.5.
Lap Joint Flange : It is economical if costly pipe such as stainless steel is used, as the flange can be carbon steel and only the lap joint stub end need be of line material. A stub end must be used in a lap joint, and the cost of two items must be considered. Useful where alignment of bolt holes is difficult, such as with spools to be attached to flanged nozzles of vessels.
Blind Flange : It is used to blind off a flange, generally at the end of pipe. This provides an easy to open end for further running of the pipe.
Bolt Holes in Flanges are equally spaced. Specifying the number of holes, diameter of bolt circle and hole size sets the bolting configuration. Number of bolts and size of bolts depends on rating of the flange.
Gaskets : These are used to make a fluid-resistant seal between two surfaces. The common gasket patterns for pipe flanges are full-face and ring type, for use with flat-faced and raised face flanges respectively. Widely used materials for gaskets are compressed asbestos (2 and 5 mm thick) and asbestos filled metal. The filled metal gasket is especially useful if maintenance requires repeated uncoupling of flanges, as gasket separates clean and is often reused.
Bolts for flanges : Two types of bolting is available, stud bolt using two nuts and the machine bolt using one nut. Stud bolts have largely displaced regular bolts for bolting flanged piping joints, as stud bolt can be removed easily, confusion with other bolts at site is avoided, the odd sized can be readily made from round stock.
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