Tube and Pipe appears to be same in term its visual structure. However, both have different characteristics based on which the application change. Both of them have- Outer Dimension (OD), Inner Dimension (ID) and the thickness of the wall (wt).
“For any hollow cylinder, there are three important dimensions - the outside diameter (OD), the inside diameter (ID) and the wall thickness (wt). Since these three are related by a simple equation : OD = ID + 2 * wt (wall thickness). One can completely specify a piece of pipe/tube (plate 5A&B) by supplying any two of these numbers.
Tubing is more frequently used in structures so the OD is the important number. Strength depends on the wall thickness. So tubing is specified by the OD and the wt very logical and simple to measure. The ID is simply whatever falls out of the equation above. Pipe is normally used to convey gases or fluids so the internal cross-sectional area (defined by the id) is important. It's therefore not surprising that pipe is specified by the id.”
(Ref.http://www.schsm.org/html/pipe_and_tube.html ; Aug 14, 2012)
