Valve and Piping Nomenclature: DN/PN vs NPS/Class

There are two piping nomenclature systems: imperial system and metric system. In imperial system, we use pounds per square inch(psi) to refer to pressure or nominal pipe size(NPS) to refer to piping sizes. In metric system which is based on international system of unit, generally pressure can be noted in kilopascal(kPa)(or bar) whilst pipe diameter are noted in nominal diameter(DN). PN is an approximation to the corresponding ASME pressure class and it correlates to DIN pressure-temperature rating standards, which may vary significantly from ASME pressure-temperature ratings.

1. Nominal Pipe Size & Nominal Diameter