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. The connection with the pipe is done with 2 fillet welds, as well at the outside as also at the inside of the flange. The X measure on the image, are approximately: Wall thickness of pipe + 3 mm. This space is necessary, to do not damage the flange face, during the welding process. A disadvantage of the flange is, that principle always firstly a pipe must be welded and then just a fitting. 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.