The purpose with a pipe is the transport of a fluid like water, oil or similar, and the most import property is the capacity or the inside diameter.
Pipe rated to ANSI B 36.10 ASME (both welded and seamless) follow that the inside diameter (ID) of a NPS 2 inches pipe with
- Schedule 40 is 2.067"
- Schedule 80 is 1.939"
The IDs are close to 2" and the nominal diameter relates to this inside diameter. Outside diameter (OD) are 2-3/8" for both schedules 40 and 80 (most commonly manufactured schedules in industry)
Since the outside diameter of a single nominal pipe size is kept constant the inside diameter of a pipe will depend on the "schedule" or the thickness of the pipe. The schedule and the actual thickness of a pipe varies with the size of the pipe.
It is common to identify pipes in inches by using NPS or "Nominal Pipe Size". The metric equivalent is called DN or "diametre nominel". The metric designations conform to International Standards Organization (ISO) usage and apply to all plumbing, natural gas, heating oil, and miscellaneous piping used in buildings. The use of NPS does not conform to American Standard pipe designations where the term NPS means "National Pipe Thread Straight".
Nominal Bore (NB) may be specified under British standards classifications along with schedule (wall thickness). The tolerances are looser to pipes compared with tubes and they are often less expensive to produce.
The nominal dimensions of tubes are based on the outside diameter. If we look at Copper Tubes - ASTM B88 the outside diameter of a 2" pipe is 2.125", relatively close to 2".
The inside diameter of a tube will depend on the thickness of the tube. The thickness is often specified as a gauge. If we look at Copper Tubes - ASTM B88 the wall thickness of 0.083"of a 2" pipe is gauge 14.
The tolerances are higher with tubes compared to pipes and tubes are often more expensive to produce than pipes.