Welding Neck Flanges (Weld neck Flanges) are distinguished from others by their long tapered hub and gentle transition of thickness in the region of the butt weld joining them to the pipe. The long tapered hub provides an important reinforcement of the flange from the standpoint of strength and resistance to dishing.

The smooth transition from flange thickness to pipe wall thickness is extremely beneficial under conditions of repeated bending, caused by line expansion/contraction or other forces, and produces a strength equivalent to that of a butt welded joint between pipes. This type of flange is preferred for every severe service condition, whether this results from high pressure, from sub-zero or high temperatures or from extreme loading conditions.

This type of flange is recommended for handling explosive, flammable or costly liquids, where the loss of tightness or local failure must be minimized.

Slip-On Flanges continue to be the preferred flanges by most installing contractors because of their lower first cost, the reduced accuracy required in cutting the pipe to length, and the greater ease of alignment of the piping assembly. Unfortunately, their final installed cost is likely not much less (if any) than that of welding neck flanges. Their calculated strength under internal pressure is approximately 2/3 that of the weld neck flange and their life under fatigue is about 1/3 that of weld neck flanges. For these reasons, slip-on flanges are limited to line sizes 0.5” to 2.5” in the Class 1500 ANSI standard.