What is the difference between slip on flange and weld neck flange?

The Weld Neck Flange is easily recognizable by the hub that gradually tapers from the bolted connection to the pipe. At the Point of Weld on the Weld Neck it is supplied with a specific bore to mate with the attaching pipe or fitting. This connection is made with a full penetration butt weld, this prevents turbulence at the joint, reduces erosion and restriction to product flow. The Weld Neck Flange offers increased strength under higher pressures, subzero or elevated temperatures that cause line expansion or retraction or other variable forces. 

The Slip On Flange is connected to the pipe by two welds, one on the back side of the Flange and one on the inside of the Flange. The inside weld of the Flange to the pipe is recessed a minimum of the wall thickness plus 3 millimeters or 3/16” of an inch. This is to avoid damage to the gasket seating surface or face. 

Although the initial cost of a Slip On Flange is slightly less than a Weld Neck Flange, you have two welds one on the back side and one on the inside of the Flange and the added inspection of the second weld doubling weld and inspection cost. The Slip On Flange is designed to be welded to pipe then a fitting as fittings are designed for butt weld connections. The calculated strength of a Slip On Flange under internal pressure is about two thirds that of a Weld Neck Flange, and their life under fatigue is about one third that of a Weld Neck Flange.