So, at the IPS time only three walltickness were in use. In March 1927, the American Standards Association surveyed industry and created a system that designated wall thicknesses based on smaller steps between sizes. The designation known as nominal pipe size replaced iron pipe size, and the term schedule (SCH) was invented to specify the nominal wall thickness of pipe. By adding schedule numbers to the IPS standards, today we know a range 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 pipe size when followed by the specific size designation number without an inch symbol. For example, 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 pipe has outside diameter greater than the size designator. For NPS 14 and larger, the NPS is equal to 14inch.
For a given NPS, the outside diameter stays constant and the wall thickness increases with larger schedule number. The inside diameter will depend upon the pipe wall thickness specified by the schedule number.
Stainless Steel Pipe dimensions determined by ASME B36.19 covering the outside diameter and the Schedule wall thickness. Note that stainless wall thicknesses to ASME B36.19 all have an “S” suffix. Sizes without an “S” suffix are to ASME B36.10 which is intended for carbon steel pipes.
The International Standards Organization (ISO) also employs a system with a dimensionless designator.
Diameter nominal (DN) is used in the metric unit system. It indicates standard pipe size when followed by the specific size designation number without a millimeter symbol. For example, DN 80 is the equivalent designation of NPS 3. Below a table with equivalents for NPS and DN pipe sizes.
Below you will find an example of the true inside diameters of a 1 inch pipe.
- NPS 1-SCH 40 = O.D.33,4 mm – WT 3,38 mm – I.D. 26,64 mm
- NPS 1-SCH 80 = O.D.33,4 mm – WT. 4,55 mm – I.D. 24,30 mm
- NPS 1-SCH 160 = O.D.33,4 mm – WT 6,35 mm – I.D. 20,70 mm
Such as above defined, no inside diameter corresponds to the truth 1″ (25,4 mm).
The inside diameter is determined by the wall thickness (WT).