I feel as if the terms “pipes” and “tubes” are often used synonymously. Are the terms really interchangeable or do they both refer to separate parts for different applications? From what I’ve noticed, in the industrial setting at least, there are a few major differences when referring to tubes than referring to pipes. Basically the main difference between the two is tubes are typically used as structural supports whereas pipes are vessels used to transfer fluids and gases. All other differences revolve around this point.
Pipes are typically more ridged than tubes and are produced to have heavier wall thicknesses. The specifications of pipes used in pipe fabrication shops are based on nominal dimensions, which bear little resemblance to the actual dimensions of the pipe. Since pipes are mainly used to transfer fluids and gases, pipe measurements are based on the inner diameter of the pipe. Outside diameter is practically irrelevant because it is more imperative to know how much room fluid and gases will have when flowing through the pipe. On the other hand, tube measurements are based on outside diameter. There is not much need to consider the inner diameter because no fluids are flowing through the tube. Tubes are generally used to support structures and frames.
Most other differences between pipes and tubes reference the process by which they were made. High strength welded tubes are called structural tubes and seamless tubes that are as-welded and drawn over a mandrel are called mechanical tubes. The standard pipe carries liquid or gases and goes through either several seamless or electric welding processes. Stainless tubing and piping go through several seamless and welding processes as well, but require resistances to corrosive materials.