Pipe and tube terms are interchangeable. They are both produced with hollow cylinder to move gases and/or liquids as well as other non-fluid products. They are widely used in a wide range of industries but the most stringent requirements are in the Oil and Gas industry.
With that said, pipe is often specified by a nominal diameter and a schedule that defines the thickness, and tube is usually specified by the outside diameter and wall thickness but could be specified by outside diameter or inside diameter and wall thickness. For example, an 1” pipe is not actually measured 1” in either outside or inside diameter, whereas 1” tube is specified by actual inside diameter or outside diameter, and wall thickness.
However, a tube has tighter engineering requirements than a pipe. When industry specifies a tube, it is often referred to as a small tube while pipe is often referred to as larger and more solid, rigid.
In addition, the term “pipe” is widely used in the world as a hollow cylinder; the term "tube" is often differentiated in the United States as a smaller diameter cylinder.
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