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Pipe and Tube

June 28, 2019

What’s a tube?

The tube can be defined as a hollow product of round or any other cross section having a continuous periphery. It is also called as “tubing”. Round tubes are most often seen in piping and pipeline engineering. Various tubes or tubing are endorsed by relative ASTM and ASME standard specifications. The dimensions and tolerances are also specified within the specifications.

What’s a pipe?

The pipe can be defined as a tube with a round cross section conforming to the dimensional requirements for nominal pipe size (NPS) as tabulated in ASME B36.10M and ASME B36.19M. For special pipe having diameter not listed in the above-mentioned standards, the nominal diameter corresponds to the outside diameter.

Differences between pipe and tube

Both pipe and tube can be made from a myriad of ferrous and non-ferrous metals. From the definition, it seems that pipe is kind of a particular tube and the two resembles each other. However, there are some significant differences from the view of engineering:

  • The cross section of a pipe must be exactly round or circular, while the cross section of a tube may be round, oval, rectangular, and square, etc.
  • Generally, a pipe shall be furnished in straight length, while the tube may be furnished in many different forms such as straight length, coiled, bent, serpentine, finned, etc.
  • A pipe shall be designated by nominal pipe size (NPS) or nominal diameter (DN) and its nominal wall thickness which are specified by the standard specification of ASME B36.10M and ASME B36.19M. The NPS and DN designations are correlated and interchangeable. Pipes of NPS 12 and smaller have outside diameters numerically larger than their corresponding sizes. Pipes of NPS 14 and larger have outside diameters numerically equal to the corresponding sizes. The nominal wall thickness is designated as schedule (SCH) which is expressed in numbers and letters (5, 5S, 10, 10S, 20, 20S, 30, STD, 40, 40S, 60, XS, 80, 80S, 100, 120, 140, 160, XXS). In practice, NPS 2, 2-1/2, 3, 4, 6, … are also called as 2″, 2-1/2″, 3″, 4″, 6″, … Thus, a typical pipe of NPS 4, SCH40 may also be stated as 4″ SCH40 or DN100 SCH40. Different from the pipe, the size of a round tube/ tubing shall be specified with respect to any two, but not all three, of the following: outside diameter, inside diameter, and wall thickness. The outside diameters of tubes are numerically identical to the size number for all sizes. Thus, a round tube may be specified as O.D=25.4 mm, W.T = 1.27 mm; O.D = 1″, W.T = 0.050″; or O.D=1″, I.D=0.90″.

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