Nominal pipe sizes for tubes and flanges can be confusing. First, NPS defines standard sizes. More specifically, NPS defines a pipe’s outer diameter and flange inner diameter. This standardization is important because of pipe and flange interchangeability from various manufacturers. NPS is the single most important parameter of a pipe or a flange. By specifying the nominal pipe size which is a dimensionless number, most other actual dimensions of the flange follow from the standard. For example, ASME B16.5 Class 150, Size 8 instantly defines the flange outer diameter, thickness, and the mounting bolt pattern.
Nominal Pipe Size is a dimensionless number that indirectly relates to the pipe or flange physical size. The importance of having NPS in the industry is two-fold: it makes products interchangeable, and in a conveniently simple number NPS presents multiple actual dimensions of a flange.
Difference between NPS and DN
DN stands for Diameter Nominal loosely spelled from the French “Diamètre Nominal”. DN is the European equivalent of American NPS. In other words, for every NPS number there is a DN number; for example NPS 3/4″ is equivalent to DN20. This means that a pipe having NPS 3/4″ can also be defined as DN20, where 3/4 (0.75 inches) approximates the nearest standard size in millimeters which is DN20 since there is no DN19.05. Conversion from NPS to DN is straightforward, where the number in inches is converted to millimeters and the DN is the nearest standard size up or down. NPS 1/8 is DN6 because there is no DN less than DN6, NPS 1/4 is DN8 because DN6 is taken. The larger sizes make more sense where NPS 3/8 is DN10, with 9.525mm rounded up to 10mm which is standard DN10. And so on. There are plenty of NPT to DN conversion tables available from pipe and flange suppliers.
Nominal Pipe Size vs. Nominal Flange Size
NPS and nominal flange size refer to the same dimensionless number. In other words nominal pipe size and nominal flange size is the same number, except NPS defines the pipe outside diameter, while nominal flange size defines the bore diameter of a flange. Think of a pipe and a flange being welded together. They will have the same nominal pipe size. Of note, blank flanges are also governed by the nominal flange sizes, where a bore can actually be machined in.
Standards for Nominal Pipe Sizes
ASME B36.10M governs the dimensions of welded and seamless wrought steel pipes. This is for pipes made of carbon steel. The standard standardizes pipe outer diameters and also wall thickness. A typical pipe designation will have a NPS and a Schedule defining pipe wall thickness, therefore pipe internal or bore diameter.
ASME B36.19 is similar to ASME B36.10M except it standardizes the dimensions of stainless steel pipes.
ASTM A53/A53M governs fabrication methods, material properties and testing protocols for seamless and welded steel and hot-dipped galvanized pipes.
ISO 6708 is the European equivalent of the ASME B36.10M and ASME B36.19 standards defining the DN of components in a pipeline system.
ASME B16.5 is for flanges, not to be confused with pipes and tubes. ASME B16.5 governs dimensions, pressure classes and shapes of flanges.
Shapes and sizes of vacuum flanges are not governed by any of the ASME standards. Typically vacuum flanges are measured in millimeter units, they fall under ISO dimensioning system. Vacuum flanges nominal sizes are, as in NPS in ASME flanges denote bore diameters, not the outer diameters, except for Conflat flanges, where a DN size indicates CF flange O.D.