Definition and Details of Pipe - 

NominalPipeSize

Nominal Pipe Size

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high orlow pressures and temperatures. The name NPS is based on the earlier "Iron Pipe Size" (IPS)system.That IPS system was established to designate the pipe size. The size represented theapproximate inside diameter of the pipe in inches. An IPS 6" pipe is one whose insidediameter is approximately 6 inches. Users started to call the pipe as 2inch, 4inch, 6inch pipeand so on. To begin, each pipe size was produced to have one thickness, which later wastermed as standard (STD) or standard weight (STD.WT.). The outside diameter of the pipewas standardized.As the industrial requirements handling higher pressure fluids, pipes were manufactured withthicker walls, which has become known as an extra strong (XS) or extra heavy (XH). Thehigher pressure requirements increased further, with thicker wall pipes. Accordingly, pipeswere made with double extra strong (XXS) or double extra heavy (XXH) walls, while thestandardized outside diameters are unchanged. Note that on this website only terms XS andXXS are used.

Pipe Schedule

So, at the IPS time only three walltickness were in use. In March 1927, the AmericanStandards Association surveyed industry and created a system that designated wallthicknesses based on smaller steps between sizes. The designation known as nominal pipesize replaced iron pipe size, and the term schedule (SCH) was invented to specify the nominalwall thickness of pipe. By adding schedule numbers to the IPS standards, today we know arange of wall thicknesses, namely:SCH 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, STD, XS and XXS.Nominal pipe size (NPS) is a dimensionless designator of pipe size. It indicates standard pipesize when followed by the specific size designation number without an inch symbol. Forexample, NPS 6 indicates a pipe whose outside diameter is 168.3 mm.The NPS is very loosely related to the inside diameter in inches, and NPS 12 and smaller pipehas outside diameter greater than the size designator. For NPS 14 and larger, the NPS isequal to 14inch.