Pipe ends and flanges are prepared for butt welding according to pipe-wall thickness. For walls 3/4 inch thick or less, the walls are beveled to an included angle of 70° and a 3/16 inch gap is left between them. The welder makes a root pass, a fill pass (or passes), and a capping pass, often varying the filler material between passes. For larger thickness, the pipe is tapered to a similar angle but only partway up the wall. In addition, a small relief angle is ground on the inside wall, serving as the location for a backing ring. Socket welds are generally used for thinner-walled pipes. Welding procedures are spelled out by an engineer in Weld Procedure Specifications and the welder making the weld will be certified for the specific process. Pipes sometimes must be preheated prior to welding and heat-treated after to relieve heat stress.
Steel weld joined pipe to plate by arc welding process.
A slip-on flange is welded front (shown) and back.
Screw-on flanges are sometimes reinforced with a similar front weld.
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The necessity of proper pipe-end preparation and the need for careful fit-up prior to joining butt-welded pe fitting makes the use of socket-weld fittings appealing. No bevel is required for socket-weld fittings and the socket itself serves to align the pipe. About the only special requirement is that the pipe must be backed out of the fitting slightly to allow for expansion during the weld.
Prefabrication of pipeline sections, called “spools,” is often done indoors where automation can be applied to the fabrication process. Pipes joints can be rolled on slow turning machines to bring the work to the welder. Robot welders can be used. Techniques such as submerged-arc welding can be applied for productivity gains.
There are non-welded pipe fittings or no weld pipe connectors available as alternatives to the traditionally welded piping systems. Using a combination of swaged mechanical fittings along with the cold bending of pipe or tubing, this solution eliminates the stresses to the pipeline from the welding operation, reduces costs, and can provide for a modular system that is easier to disassemble or modify as needed.
Plastic pipe, and HDPE pipe, in particular, can be joined by heat welding, sometimes referred to as electrofusion welding. Pipes can be butt-welded or socket-welded. This is a fairly common practice for large-diameter HDPE pipeline installations. A range of specialized equipment is available for producing these welds.
pipe connection types PE pipe welding for connecting the water pipe in the plant.
A welding machine for joining sections of HDPE pipe thermally.
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Generally, plastic pipe and pipe fitting applications need to consider pressure derating concerning the pipe or fitting size and the operating temperature. For PVC and CPVC materials, manufacturers will recommend a reduction in the pressure rating for temperatures above 73 degrees F. And, for a given operating temperature, pressures need to be further reduced as the diameter of the pipe or fitting increases. Furthermore, the use of certain fittings, such as flanges, unions, or valves, may have pressure ratings that are less than that of a straight pipe of the same dimension.
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