Definition of nominal pipe size
The Nominal Pipe Size (NPS) is a North American standard pipe size that is applied to standard pipe sizes at high or low temperatures and pressures. The size of the pipe is characterized by two dimensionless numbers: the nominal pipe size (NPS) measured in inches and a nominal number (schedule or Sched. Or Sch.). NPS is often incorrectly referred to as the National Pipe Size, due to confusion with a national pipe thread (NPT). There are also other standard pipe sizes, see also pipe (material) – sizes. The name of the corresponding European pipe size standard with NPS is DN (diamètre nominal or nominal diameter), in which the pipe size is measured in millimeters. The abbreviation NB (nominal bore) can also be used as an alternative to NPS.
Based on NPS and nominal pipe name, outside diameter OD and wall thickness can be interpolated from the following tables, according to ASME B36.10M and B36.19M standards. For example, the NPS 14 Sch 40 has an outer diameter of 14 inches and is 0.437 inches thick. However, the NPS values and the outside diameter (OD) are often not equal, which can cause a lot of confusion.
With pipes with NPS from ⅛ to 12 inches, the NPS and OD values are always different. For example, the OD of the NPS 12 tube is 12.75 inches. To find the actual OD for each NPS value, we have to look up the table below. (Note that with tubing, the NPS value is always OD.)
For pipes with NPS over 14 inches, the NPS and OD values are always equal. In other words, the NPS 14 tube has a true OD of 14 inches.
The reason for the difference with NPS pipes from ⅛ to 12 inches is that these NPS values were originally set to be equal to the inside diameter ID, based on the pipe thickness standard at that time. However, as the list of wall thicknesses increases, the inner diameter changes and the NPS gives only a value that is not directly related to the inside and outside diameters.
When given the NPS values, with each fixed outside diameter and pipe thickness increasing gradually according to the ratings in the table. When given the rated value, the outside diameter increases gradually with the NPS index while the thickness is constant or increasing. Using the tables in ASME B31.3, we can show that the withstand pressure of the pipe decreases proportionally with the increase in NPS while the nominal index remains constant.
Both polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) tubes are fabricated to NPS dimensions.
Common Mistakes
Realistic when you operate in the field of piping systems. Occasionally you will come across but reading like
DN25 then
Many of you confuse DN15 for pipe Ø15, which is completely wrong
In this case, I see in many of you who just graduated, went to work and cut the wind, read the drawings DN15 => Ø15, but then measured it was Ø 21.34 mm, asking me what’s wrong.
DN15 => equivalent to Outer Diameter Ø 21.34 mm