For newcomers to the piping industry, pipe sizing can be confusing.
There are various reasons for this, with history probably being the most significant. We still use many aspects of a system that was in place long ago. Since then, technology has evolved and the industry has changed, so what made perfect sense back then might seem a bit awkward today.
However, with a little work, piping sizing isn’t too difficult to understand. Here’s a brief introduction.
The North American system for designating the outside diameter (OD) of a pipe is nominal pipe size, or NPS. Its European equivalent is Nominal Diameter, or DN. The primary advantage of classifying pipes in this way, by their outside diameter, is that it allows pipes of the same size to be fit together even if they have differing wall thicknesses.
The tricky part is this: the NPS value of a pipe doesn’t always equal its OD. Intuitively, you would think that a pipe with a NPS value of ⅛ would have a OD of ⅛ inches. However, it actually has a diameter of .405 inches. The same is true for all NPS values from ⅛ -12; they don’t match up with their OD.
This is because the NPS for these values was originally designed to refer to the inside diameter of a pipe based on wall thicknesses that were standard at the time.
However, for NPS of 14 and above, the NPS value does match up to OD.
Standard Pipe Schedule
In addition to NPS, the other factor to consider when analyzing pipe sizes is pipe schedule.
Standard pipe schedule refers to the wall thickness of a pipe. The greater the schedule, the thicker the wall. It should be noted, however, that a pipe’s schedule doesn’t equal its wall thickness in inches, and that a given schedule may refer to different thicknesses for pipes of different NPS values. For example, a pipe with an NPS of 1/8 and a schedule of 30 has a wall thickness of .057 inches, while a pipe with an NPS of 1/4 and a schedule of 30 has a wall thickness of .073 inches.
ASME and ANSI Standardization
As you can tell, pipe sizing can be a bit tricky. However, you don’t need to remember it all. Sizing is tightly regulated by organizations like the ASME and ANSI. Plus, in practice, you’ll simply refer to standardized tables that list OD, schedules, wall thicknesses and NPS values.
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