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( Pipe (material))
A pipe is a tube or hollow cylinder used to convey materials or as a structural component. The terms pipe and tube are almost interchangeable. A pipe is generally specified by the internal diameter (ID) whereas a tube is usually defined by the outside diameter (OD) but may be specified by any combination of dimensions (OD, ID, wall thickness). A tube is often made to custom sizes and may often have more specific sizes and tolerances than pipe. Also, the term tubing can be applied to non-cylindrical shapes (i.e. square tubing). The term tube is more widely used in the United States, whereas pipe is more common elsewhere in the world. Both pipe and tube imply a level of rigidity and permanence, whereas a hose is usually portable and flexible. Pipe may be specified by standard pipe size designations, such as nominal pipe size (in the United States), or by nominal, outside, or inside diameter and wall thickness. Many industrial and government standards exist for the production of pipe and tubing. There are three processes for metallic pipe manufacture. Seamless pipe is formed by drawing a solid billet over a piercing rod to create the hollow shell. Seamless pipe provides the most reliable pressure retaining characteristics, and is often more easily available than welded pipe. Welded pipe is formed by rolling plate and welding the seam. The weld flash can be removed from the outside or inside surfaces using a scarfing blade. The weld zone can also be heat treated, so the seam is less visible. Welded pipe often has tighter dimensional tolerances than seamless, and can be cheaper if manufactured in the same quantities. Cast pipe is no longer very common, but still exists. Pipe is sometimes cast in a centrifuge. Tubing, either metal or plastic, is generally extruded.
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