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A weld flange is a device used to attach two pieces of pipe in a bolt-together fashion. Unlike a flat flange that is, as the name implies, a flat, steel flange that can be welded to a pipe, the weld flange has a tapered, cone-like apparatus machined into the pipe side of the flange. This taper begins wide on the flange and gradually decreases in size until it matches the diameter of the pipe it is designed to fit. Used on both hot and cold high-pressure pipelines, the taper in the weld flange reduces turbulence at the pipe joint and adds strength to withstand the constant flexing of the joint due to temperature and pressure changes.
The most common weld used to attach a weld flange to a pipe is known as a butt weld, or a weld flange-weld. To complete this weld, the welder uses a grinder to make a taper at both the end of the pipe and the end of the taper on the weld flange. When positioned together, the two tapers create a V shape, which is then filled in with weld. This type of weld provides the greatest amount of penetration to both the pipe and the flange, therefore giving the most strength to the joint or connection. This type of weld is also a good choice when the weld needs to be X-rayed.
The center hole on the weld flange is bored to be the same diameter as the inside of the pipe it is intended to be fitted onto. This creates a joint with the least amount of flow restriction possible when creating a joint in a pipeline. The taper of the weld flange is also representative of the weight of the pipe it is being fitted to with a lighter-weight pipe requiring a larger bore and a heavier weight pipe requiring a smaller bore. This is done to maintain a match inside of the flange connection and reduce turbulence of the flowing liquid.
Applications where the weld-type flange is required can range from severe, cold climates to dangerously hot environments and high-pressure applications as well. Commonly providing increased strength as compared to a flat flange in the same or comparable installation, the added material in the taper of the weld flanges provide superior protection from cracking and breaking due to bending and pressure. The smooth transition of liquid caused by the lack of turbulence inside of the connection reduces the amount of routine pipeline maintenance that must be performed.