Flange markings often combine two short designations: DN and PN. Knowing the DN and PN meaning helps teams communicate nominal size and pressure class, but these markings are only part of a complete specification. They do not, by themselves, give every physical dimension or prove that two flanges will mate.
This guide explains what does DN stand for, what PN means on a flange, how the two designations work together, and what to check before ordering. Use the applicable standard, its edition and the project drawings to confirm dimensions and pressure-temperature suitability.
What does DN stand for?
DN stands for nominal diameter (from the French diamètre nominal). It is a standardized size designation used for piping components such as pipes, valves and flanges. DN is a reference label, not a promise that the measured inside diameter or outside diameter is exactly that number in millimeters.
For example, DN50 identifies a nominal pipe size within a standard system. The real pipe outside diameter is controlled by the relevant pipe dimensional standard, while the inside diameter also depends on wall thickness. A DN flange therefore needs to be specified with its flange standard and pressure designation, not matched by the DN label alone.
What does PN mean on a flange?
PN means nominal pressure (often expressed as Pressure Nominal in product literature). It is a pressure-class designation used with a specified standard and material group. PN16, for instance, identifies the PN16 class; it should not be treated as a universal guarantee that the flange can operate at exactly 16 bar at every temperature.
The allowable pressure depends on the applicable standard, material, temperature and component design. As temperature rises, material strength and allowable pressure may change. Always use the pressure-temperature tables and design rules in the governing standard. Do not convert PN directly into an ASME Class designation using a simple numerical formula.
DN vs PN: what each marking tells you
| Marking | Meaning | It helps identify | It does not specify by itself |
|---|---|---|---|
| DN | Nominal diameter designation. | Nominal size family for the pipe and component. | Exact inside diameter, outside diameter, wall thickness or flange drilling. |
| PN | Nominal pressure class designation within a defined standard system. | Pressure class framework used to select components. | A universal allowable pressure independent of material and temperature. |
| EN / flange standard | Dimensional and technical requirements for the component. | Face, drilling, dimensions, materials and other requirements in scope. | Compatibility with a different standard unless the details are checked. |
| Material grade | Specified material and properties. | Strength, temperature limits and service compatibility. | Whether the complete joint is suitable without design review. |
How DN and PN work together
A marking such as DN100 PN16 combines a nominal size designation with a pressure class. The flange standard, type, face, material and dimensions still need to be known. In practical procurement, include the standard and its edition, for example the applicable EN flange standard, then identify DN, PN, facing, material, flange type and any project-specific requirements.
This is why PN and DN meaning should be read as two separate pieces of information: DN helps define the nominal size; PN identifies the nominal pressure class within a standard framework. Neither marking replaces the full data sheet.
| Order description field | Example information to provide | Why it matters |
|---|---|---|
| Dimensional standard | Applicable EN flange standard and edition. | Defines the dimensional and technical basis. |
| Nominal size | DN designation. | Connects the flange to the intended pipe size family. |
| Pressure class | PN designation. | Identifies the required class for verification against conditions. |
| Flange type and facing | For example, weld neck or blind; RF, FF or other specified face. | Determines the connection geometry and sealing arrangement. |
| Material and service | Grade, fluid, design temperature and design pressure. | Enables strength and compatibility checks. |
| Joint components | Gasket, bolts, nuts and assembly requirements. | The complete bolted joint must seal and carry loads together. |
Are DN and PN flanges automatically interchangeable?
No. Two flanges with the same DN and PN labels may still differ if they are made to different standards, editions, flange types or facing arrangements. Bolt-circle diameter, number and size of bolt holes, outside diameter, thickness and sealing face must match the mating component. A metric DN designation should not be assumed to equal an inch-based NPS designation in every dimensional detail.
Before joining flanges from different suppliers or standard systems, compare approved drawings and dimensional tables. Verify the complete mating interface, gasket style, bolt set, materials and pressure-temperature basis. If any item differs, obtain engineering approval before fabrication or installation.
Common misunderstandings
- “DN is the exact bore measurement.” DN is nominal; actual pipe dimensions are set by the relevant dimensional standard and wall thickness.
- “PN16 always means a maximum of 16 bar.” PN is a class designation; allowable pressure depends on standard, material and temperature.
- “Matching DN and PN means the flanges fit.” The flange standard, type, face and drilling must also match.
- “PN and ASME Class are direct equivalents.” They belong to different rating systems; use the approved standard data rather than a simple conversion.
Frequently asked questions
What does DN stand for in flange specifications?
It stands for nominal diameter, a standardized size designation. It is not necessarily the exact measured bore or outside diameter.
What is the meaning of PN on a flange?
PN is the nominal pressure class designation used within a defined standard system. The allowable pressure at a given temperature must be checked against the applicable standard and material.
What does “meaning: flange” refer to in a marking?
If a flange is marked with DN and PN, those markings describe its nominal size and pressure class. The flange standard and product details explain its dimensions, type, material and facing.
Final takeaway
For flange markings, DN identifies nominal size and PN identifies a nominal pressure class under a defined standard system. Read them together with the standard, material, temperature, facing and flange type. A complete specification prevents dimensional mismatches and supports a correct pressure-temperature review. Songhai can assist with flange drawings, material details and dimensional verification for industrial piping orders.