DN and PN explained: nominal size and nominal pressure for valves
Every valve enquiry contains DN and PN. They are often confused with the pipe bore and the working pressure, which leads to wrong orders. Here is a short explanation without formulas.
TAZ Engineering Team · · 6 min
DN: nominal size
DN is a dimensionless number roughly equal to the bore in millimetres: DN 100, DN 150, DN 300. It identifies the size series in which pipes, flanges and valves match. The actual bore of a DN 100 pipe depends on wall thickness. In Soviet documents the same parameter appears as Du. The US system uses NPS in inches: NPS 4 corresponds to DN 100 and NPS 12 to DN 300.
PN: nominal pressure
PN is the nominal pressure in bar at the reference temperature: PN 16 means 16 bar, about 1.6 MPa. Older Russian documents write Ru 1.6 MPa or Ru 16 kgf/cm². PN is a strength class of the product, not the pressure in your system.
Working and test pressure
Allowable working pressure depends on material and temperature. Steel and iron lose strength when hot, so a PN 16 valve at 300 °C is allowed less pressure. Pressure-temperature tables are given in the standards and the datasheet. The shell test pressure is usually 1.5 times the nominal pressure.
PN and ASME classes
US practice uses pressure classes such as Class 150, 300 and 600. They do not convert one-to-one into PN because they are based on different material groups and temperature tables. Class 150 is often compared with PN 16–20 and Class 300 with PN 40–50, but a project must compare allowable pressure at the operating temperature.
What to put in the RFQ
State DN, PN, working pressure and temperature, and the flange standard. For replacements, quote the designation stamped on the old valve. TAZ engineers will check that the chosen PN suits the service conditions.
Checklist
- DN
- PN
- working pressure
- working temperature
- flange standard
- designation of the valve being replaced