The terms “DN” and “PN” are frequently found in the piping engineering literature. These are very important terms related to piping system design. In this article, we will discuss the meaning and significance of DN and PN.
What is DN in Pipes?
“DN” is an abbreviated form of Diameter Nominal (DN). It is a dimensionless number that denotes the pipe size in the metric unit system. The DN pipe size designator is developed by the International Standards Organization or ISO.
The DN value does not provide any physical pipe dimensions but is used as a reference number that indicates the size of a pipe or fitting. The pipe sizes are designated by DN followed by a number to indicate standard pipe size. For example, DN50 indicates a 2″ NPS pipe; Similarly, DN80 indicates a 3″ NPS pipe. The pipe manufacturers print the DN number on the pipe surface which makes the identification easier.
DN vs NPS
NPS or Nominal pipe size is also a dimensionless pipe size designator. A specific pipe size is indicated by NPS followed by a number. For example, NPS 2 indicates a pipe having an outside diameter of 2.375 inches. The pipe outside diameter is more than its NPS designator for NPS 12 and smaller pipes. However, NPS 14 and larger pipes have the same pipe OD in inches as the NPS designator. This means an NPS 24 pipe has an OD of 24 inches.
So, the main difference between DN and NPS is that DN indicates the approximate pipe dimensions in mm whereas the NPS indicates the pipe dimension in inches. For pipes larger than NPS 3.5, the DN number is calculated by multiplying the NPS size designation number by 25. For example, NPS 4 is the same as DN 100.
Table 1 below provides typical co-relations between NPS and DN pipe size designations.
What is the PN Rating Number?
Pipes are usually classified based on the pressure-temperature rating system used for flange classification. The PN number system is a similar pressure rating designator. “PN” stands for the French term Pression Nominal which means pressure nominal. The PN number is indicated by the term PN followed by a designation number. For example, PN 10, PN 50, etc. The value that follows the term PN indicates the approximate pressure rating in bars. Note that the PN rating does not provide a proportional relationship between different PN numbers.
PN basically represents the virtual pressure that a pipe or tube can withstand. The piping components are designed based on the reference PN number value. The PN rating number is most widely used for Plastic pipes, HDPE, PVC, and PE pipes. PN is generally used in the European standards BS, DIN, EN, ISO, and so on. Different PN rating indicates different pipe thickness for the same pipe DN.
ISO 7268 standard defines PN as follows:
PN is an alphanumeric designation used for reference purposes, comprising the letters PN followed by a dimensionless number relating to component pressure/temperature capability, taking into account component material mechanical properties and component dimensional characteristics.
What is PN in HDPE Pipe?
The PN abbreviation for pressure nominal in Polyethylene pipes indicates the maximum pressure that the pipe can withstand without failure at a specified temperature range. For example, PN 16 pipe means the pipe can withstand a pressure of 16 bar without failure. Similarly, PN 20 pipe means the pipe is designed to withstand a pressure of 20 bar at the maximum design temperature.
For PE/HDPE pipes the PN rating pressure usually denotes the pressure capability at a temperature of 20°C. PN values are generally mentioned in the Pipe Diameter, Thickness, and Weight chart for HDPE/PE pipes. With an increase in PN number the pipe thickness increases.
Pipe Class Rating vs. PN Numbers
Piping Class Ratings are based on the ASME B16.5 or ASME B16.47 standard whereas The PN rating is based on ISO 7005 standard. Table 2 below provides a cross-reference for the PN rating and ASME Class rating.
|Pipe Pressure Nominal (PN)||20||50||68||110||150||260||420|