Welding Neck Flanges:

The welding neck flange is normally referred to as the "high hub" Flange. It is designed

to transfer stresses to the pipe, thereby reducing high stress concentrations

at the base of the Flange. The welding neck flange is the best designed butt-welded

flange of those currently available because of its inherent structural

value. It is expensive because of the design.

 

Threaded (Screwed) Flanges:

The threaded flange is similar to the slip-on flange, but the bore is threaded. Its

chief merit is that it can be assembled without welding, explaining its use in

low pressure services at ordinary atmospheric temperatures and in highly explosive

areas where welding create a hazard.

 

Slip-On Flanges:

The slip-on flange has a low hub because the pipe slips into the flange prior to

welding. It is welded both inside and out to provide sufficient strength and prevent

leakage. Slip-on flanges are all bored slightly larger that the O.D. of the

matching pipe. They are preferred over welding neck flanges by many users due to

their lower initial cost, but final installation cost is probably not much less

than that of the welding neck flange because of the additional welding involved.