Weld Neck Flanges are distinguished from other flange types by their long tapered hub and gentle transition of thickness in the region of the butt weld that joins them to pipe or fitting. A weld-neck flange is attached to a pipe or a fitting with a single full penetration, “V” bevel weld. The long tapered hub provides an important reinforcement of the flange proper from the standpoint of strength and resistance to dishing.

The smooth transition from the flange thickness to the pipe wall thickness by the taper is extremely beneficial under conditions of repeated bending caused by line expansion or other variable forces and produces an endurance strength of welding neck flanged assemblies equivalent to that of a butt-welded joint. This type of flange is preferred for severe service conditions, whether loading conditions are substantially constant or fluctuate between wide limits.

The weld neck flange is used in each of the seven flange ratings and has the advantage of requiring only one weld to attach it to the adjacent pipe or fitting.

The key dimension for a weld neck flange is the length through the hub from the beveled end to the contact face of the flange. This “length” includes the bevel, the tapered hub, and the thickness of the plate part of the flange and the raised face. To obtain the correct dimension you must look at a correctly constructed flange dimension chart or a flange manufacturers catalog. Electronic piping design software will normally already have the correct dimension built-in.

It is important to understand and remember that the (1/16″) raised face on Class 150 raised face and on Class 300 raised face flanges are normally included in the length dimension. However, the (1/4″) raised face is not included in the chart or catalog length dimension for the Class 400 and higher pressure rated flanges. The raised face dimension for Class 400 flanges (and up) normally must be added to the chart or catalog length to arrive at the true total length of these higher-pressure flanges.