DN, NPS, PN and ASME Class Are Not Direct Equivalents
DN and NPS identify nominal size systems. PN and ASME Class identify pressure-rating systems. None of them, by itself, completely defines a valve connection or allowable service condition.
Use DN with the applicable metric standard and NPS with the applicable inch-based standard. Use PN or ASME Class together with the governing design and flange standards, material and temperature. Do not approve a substitution from a simple conversion chart.
DN and NPS describe nominal size
DN is a dimensionless nominal size designation commonly used with metric standards. NPS is the nominal pipe size system used in many inch-based standards. A nominal designation is not a direct measurement of bore or outside diameter.
PN and ASME Class describe rating systems
PN is a nominal pressure designation used within relevant standards. ASME pressure classes such as Class 150 or Class 300 belong to another rating framework. The allowable pressure depends on the applicable standard, material group and temperature.
Why shortcut conversion tables cause trouble
A table may place PN16 near Class 150 for rough commercial discussion, but “near” is not engineering equivalence. Flange dimensions, drilling, facing, material ratings and temperature limits still require comparison under the governing standards.
What to put in an RFQ
- Nominal size: DN or NPS, clearly labelled.
- Pressure designation: PN or ASME Class.
- Valve and flange standards, including facing where relevant.
- Body and pressure-retaining material.
- Design and maximum operating temperature.
- Required face-to-face standard and end connection.
When the existing valve only shows a nameplate
Send a clear nameplate photo, connection measurements and photographs of both ends. Keep the existing pipeline standard separate from assumptions about the replacement valve.