How Do You Gauge Your (Nominal Bore) Pipework?

By Rob Ellis | 31st May, 2016 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.

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