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A weld neck flange is a type of pipe flange that incorporates a cast-in neck that is welded to the pipe. Unlike a plain flange that is slipped over the pipe prior to being welded into place, a weld neck flange has a spout-like neck that is beveled to match up to the pipe. Once the pipe is beveled to match the weld neck flange, a perfect V shape is created in which a butt weld is placed to connect the two components.
The benefit of a weld neck flange is that the welded connection does not interfere with the flow of liquid through the flange. By placing the weld joint at the end of the funnel-like neck of the flange, the pipe is much easier to align to the flange, and a stronger weld is typically possible with this type of flange. The flange's neck is also perfectly beveled to demonstrate the type of bevel that the welder must place on the pipe before making the connection with the flange.
One of the benefits of the weld neck flange is the ability to withstand extreme pressures. The flange is used in applications with high pressure due, in part, to the ability of the neck to transfer stress away from the flange and into the pipe. Stress transferred to the pipe can run the length of the pipe to be dissipated, however, stress at the flange can only be dissipated at the flange. The strength of the weld joint with this type of flange comes from the welder's ability to create a perfect joint by dressing the pipe to match the flange neck. When properly dressed, the pipe bevel matches the neck bevel, creating the best condition for a butt weld to properly penetrate both components.
Unlike a socket flange, the welder has the ability to visually confirm the proper alignment of the flange neck and the pipe. This makes it possible to make any required fine fitting with a grinder to create a perfect fit. The weld neck flange is typically applied to the end of a long run of high-pressure pipe to absorb any vibration or movement in the pipeline without creating damage. This is also the flange used at the connection of a large shutoff valve. The strength of the flange is used to combat the pressure applied to a stuck valve and prevents any pipeline problems caused by inferior joint and unions.
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