Understanding DN and PN Numbers for Pipes and Class Ratings

In my 20+ years of active piping engineering on global oil, gas, and power projects, I have seen brilliant young engineers make costly mistakes simply because they did not understand how metric and imperial piping systems interface. I still remember a major refinery expansion in Rotterdam where a junior engineer assumed an ASME Class 150 flange would bolt directly to an EN PN 16 flange because “the sizes looked about the same.” The resulting mismatch halted a critical hydrotest, costing the project thousands of dollars in hot-work modifications.

Understanding the exact relationship between DN, PN, and ASME Class ratings is not just academic; it is a fundamental field requirement. Let me walk you through the exact engineering principles, calculations, and conversion matrices that will keep your designs safe, compliant, and perfectly aligned on-site.

Understanding DN and PN Numbers for Pipes in Industrial Systems

To design robust piping systems, we must look at the standards that govern these designations. The metric system relies heavily on ISO 6708 for Nominal Diameter (DN) and ISO 7268 for Pressure Nominal (PN). In contrast, the imperial system used predominantly in North America relies on NPS (Nominal Pipe Size) and Class ratings governed by ASME B16.5 and ASME B36.10M.

The Mechanics of DN (Nominal Diameter)

DN is an alphanumeric designation for size which is common to all components in a piping system other than components designated by outside diameters or by thread size. It is a convenient round number for reference purposes. For example, when we specify a DN 150 pipe, the actual outside diameter (OD) of the pipe is 168.3 mm. The internal diameter (ID) will vary depending on the wall thickness (schedule) of the pipe.

This is a critical point: the DN number is neither the exact internal diameter nor the exact external diameter, but rather a standardized reference size that ensures all valves, flanges, and fittings of the same DN will physically align and connect.

The Mechanics of PN (Pressure Nominal)

PN is a rating system designated by the letters PN followed by a dimensionless number, which indicates the maximum allowable pressure of a piping component at a reference temperature of 20 degrees Celsius. For instance, a PN 16 rating indicates that the component is designed to operate safely at a maximum pressure of 16 bar (approximately 1.6 MPa or 232 psi) when the operating temperature is 20 degrees Celsius.

Pressure-Temperature Derating Calculation

To calculate the actual maximum allowable working pressure (P_T) at an elevated temperature (T), we use the derating factor derived from the material’s allowable stress values:

Where:
P_T = Maximum allowable working pressure at design temperature T (bar)
P_PN = Nominal pressure rating (PN value in bar at 20°C)
S_T = Allowable stress of the material at design temperature T (MPa)
S_RT = Allowable stress of the material at room temperature 20°C (MPa)

ASME Class Rating vs. PN Numbers

One of the most common points of confusion in international engineering offices is comparing ASME Class ratings (Class 150, 300, 600, 900, 1500, 2500) with European PN ratings. ASME ratings are based on pressure-temperature limits defined in ASME B16.5, which vary significantly depending on the material group (e.g., Group 1.1 Carbon Steel, Group 2.2 Stainless Steel).

Because of these material-specific curves, there is no single, constant multiplier to convert Class to PN. However, for general comparison and selection of equivalent standard classes, engineers use standardized approximations. For example, Class 150 is generally paired with PN 20, and Class 300 is paired with PN 50.

NPS to DN and Pipe Outside Diameter (OD) Mapping

The table below provides the standard conversion between Nominal Pipe Size (NPS) in inches, Nominal Diameter (DN) in millimeters, and the actual physical outside diameter (OD) according to ASME B36.10M and ISO 6708.

ASME Class Rating to PN Rating Comparison

The following table maps the standard ASME Class ratings to their closest equivalent PN ratings as defined in international standards like EN 1092-1 and ISO 7005-1.

Technical Mapping & Specifications Matrix

Site Verification Checklist for DN and PN Numbers for Pipes

Before releasing any piping system for hydrotesting or commissioning, field engineers must physically verify that the installed components match the design drawings (P&IDs and Isometrics). Use this checklist on-site to prevent catastrophic mismatches.

Field Case Study: Real-World Application

My recommendation for all international projects is clear: always map out your piping interfaces during the FEED (Front-End Engineering Design) phase. Never let the field construction crew resolve standard mismatches on-site without engineering approval.

Here are the most common technical questions I encounter regarding DN and PN numbers for pipes and how they interface with imperial standards.