The Slip On type flanges are attached by two fillet welds, inside as well as outside the flange. The calculated strength from a Slip On flange under internal pressure is of the order of two-thirds that of Welding Neck flanges, and their life under fatigue is about one-third that of the latter. Normally, these flanges are of forged construction and are provided with hub. Sometimes, these flanges are fabricated from plates and are not provided with the hub.The disadvantage of the flange is that a combination of flange and elbow or flange and tee is not possible because named fittings have not a straight end that complete slid in the Slip On flange.
Welding Neck Flanges are easy to recognize as the long tapered hub, that goes gradually over to the wall thickness from a pipe or fitting. The long tapered hub provides an important reinforcement for use in several applications involving high pressure, sub-zero and / or elevated temperatures. The smooth transition from flange thickness to pipe or fitting wall thickness effected by the taper is extremely beneficial, under conditions of repeated bending, caused by line expansion or other variable forces.These flanges are bored to match the inside diameter of the mating pipe or fitting so there will be no restriction of product flow. This prevents turbulence at the joint and reduces erosion. They also provide excellent stress distribution through the tapered hub.The Weld neck flanges are attached by butt-welding to the pipes. These are used mainly for critical services where all the weld joints need radiographic inspection. While specifying these flanges, the thickness of the welding end also should be specified along with flange specification.
Blind Flanges are manufactured without a bore and used to blank off the ends of piping, Valves and pressure vessel openings.From the standpoint of internal pressure and bolt loading, blind flanges, particularly in the larger sizes, are the most highly stressed flange types.However, most of these stresses are bending types near the center, and since there is no standard inside diameter, these flanges are suitable for higher pressure temperature applications.