Nominal pipe size?

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Nominal Pipe Size (NPS) is a standardized measurement used to describe pipe size in North America, while other regions use the metric nominal diameter (DN) measurement system. This system of measurements is closely, but not exactly, correlated with actual pipe size, and the system used to describe pipe measurements can get quite arcane. For example, a lower nominal pipe size reflects the approximate inside diameter, while higher numbers are based on the outside diameter.

In addition to the nominal size of the pipe, the wall thickness is also subject to standardized measurement guidelines. The “schedule” of a pipe refers to the thickness of the walls, which can have an impact on the inside diameter. Identical nominal sizes of pipes may have different internal diameters due to being at different schedules. Both dimensions are needed to determine which hose is appropriate for an application.

Nominal tube size information can be printed on a tube or added to a sticker mounted on the tube to provide information to consumers. Contractors can look up the pipe they need from a hardware supply or order directly through a supplier. Variations between the actual size and nominal size of the pipe must be accounted for in the design, as incorrectly sized pipe can cause significant problems with pumping and other piping tasks.

Pipes made from any material can be measured and sized using this standard. Manufacturers use rigorous quality controls to ensure their tubing stays within tolerances. Small errors in the piping can become a problem on a construction site, where errors could make it impossible to create a secure connection or could create leaks and other problems. In the event that the nominal size information of the tube is inaccurate, it is usually possible to take the tube back for return or replacement, as the product is defective.

Conversion tables are available to assist people in converting between nominal pipe size and nominal diameter. Some charts simply provide one-to-one between these two systems while others may note the actual diameter of the pipe, while also indicating whether it is the outside or inside diameter. Schedule measurements can also be converted to benefit contractors who need to know pipe wall thickness for various project needs. Such charts can be posted at hardware stores for convenience, and can also be easily found online, especially on plumbing websites with information about different types of piping.

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