When you work with metals on a regular basis, sometimes you just get used to using certain terms or measurements without thinking anything further about them. One of these is the NPS, which is nominal pipe size, set by the USA for pipes used at high and low pressures and temperatures.
It works by having two sets of numbers, one to describe the diameter of the pipe and the other to show the thickness of the wall of the pipe, otherwise known as the schedule. These numbers, because this is an American system, are based in good old inches but there is a European version that uses the metric system which is known as DN. DN works nicely because no matter which common language you use, it still makes sense! The French know it is diamètre nominal, the English speakers call it nominal diameter and the German have Durchmesser nach Norm.
As you might expect, NPS came about because the requirements of the previous system simply weren’t up to code. Until the late 1920’s, there were only a few wall thicknesses in use, known as standard (STD), extra strong (XS) and double extra strong (XXS), which followed the iron pipe size that was in use at the time. The problem arose when the sizes weren’t actually useful for common uses. By 1927, the American Standards Authority created a committee that, having spoken to the industry, made the NPS where the schedule for wall thicknesses was more useful.
Originally, one of the aims of the new system was to allow certification to be granted to certain schedule sizes to act as a pressure rating, but this was never followed up because the numbers deviated greatly from the schedules in common use. It’s also interesting to note that the old 1920’s naming system of standard, extra and double extra strong are still in common usage.
The numbers of the NPS system appear fairly straightforward, with one number referring to the diameter and the other the wall thickness. The diameter is measured under the heading OD, as it’s the outside diameter of the pipe, inclusive of the wall thickness. NPS 14 Sch 40, as an example, measures 14” across and the schedule thickness is 0.437”. There are some peculiarities on the pipe sizes, but they’re all stored on tables which are easily available, but these have arisen because the OD was originally intended to be the internal diameter. This was quickly replaced, however, as the wall thicknesses evolved over time and the NPS wasn’t as strictly tied to the ID or OD.
The NPS has been an incredibly useful way of measuring and quantifying pipe sizes as technology has evolved. By keeping a series of standardised, set measurements it’s allowed those creating machinery or processes to not only select a gauge that’s suitable for purpose, but know that no matter where it’s ordered, made, processed and fitted, the final machinery will be reliable and safe to use.