DN and PN: Key values that determine function and safety
Two abbreviations crop up time and again in industrial piping systems: DN and PN. Short and technical, yet crucial for the function, safety and service life of a system. Nominal diameter (DN) and nominal pressure (PN) are characteristic features standardised according to geometric step increments in order to define compatible parts such as flange connections. Understanding and correctly combining them helps to avoid mistakes in design. Read more about the definition and design here and get a handy guide for everyday operations.
How big is the component? Or DN for size and flow
The nominal diameter (DN) describes the standardised size of pipes and fittings. It is based on the approximate internal diameter and is specified in millimetres. This characteristic value indicates how much space is available for the medium to flow and is a key parameter for the dimensioning of a piping system.
Since the tools used to manufacture pipes are specified by their outer diameter, the inner diameter varies depending on the wall thickness. However, for rough dimensioning, it is usually sufficient to use the nominal diameter as the calculation value for the inner diameter.
| How big is the component? Or DN for size and flow | |||
|---|---|---|---|
| DN (nominal diameter) | Outer diameter of pipe (mm) | Inner diameter approx. (mm) | Typical external thread size (BSP) |
| DN 10 | 17,2 | ≈10 | 3/8" |
| DN 15 | 21,3 | ≈15 | 1/2" |
| DN 20 | 26,9 | ≈20 | 3/4" |
| DN 25 | 33,7 | ≈25 | 1" |
| DN 32 | 42,4 | ≈32 | 1 1/4" |
| DN 40 | 48,3 | ≈40 | 1 1/2" |
| DN 50 | 60,3 | ≈50 | 2" |
| DN 65 | 76,1 | ≈65 | 2 1/2" |
| DN 80 | 88,9 | ≈80 | 3" |
| DN 100 | 114,3 | ≈100 | 4" |
How much pressure is permissible? Or PN for pressure and safety
The nominal pressure (PN) indicates the maximum operating pressure that a component can safely withstand. It is specified in bar and refers to a reference temperature, usually 20 degrees Celsius. The permissible pressure depends on the material and the temperature. If the medium temperature rises, the permissible operating pressure decreases.
A fitting with PN 16, for example, is designed for 16 bar. However, whether this design is sufficient for a specific application can only be determined in conjunction with the medium and temperature.
DN and PN – only really meaningful when used together
Only together do DN and PN provide a complete picture. A typical example from industry: a DN 150 pipe transports hot water at 12 bar and 90 degrees Celsius. Technically, a DN 150 fitting with PN 16 may be sufficient, but depending on the safety requirements, a version with PN 25 may be more appropriate.
Important: The same DN does not automatically mean the same pressure rating. A PN that is too low can cause leaks, malfunctions or damage to the system.
Marking directly on the component
Industrial fittings are clearly labelled to ensure clarity and safety during operation. Information such as DN 100 / PN 40 / EN 1092-1 allows plant operators and fitters to see at a glance whether a component fits geometrically, can withstand the intended operating pressure and complies with the relevant standard. This marking prevents dangerous mix-ups during installation and maintenance. EN 1092-1 is the European standard for flanges. It defines the exact connection dimensions, such as the bolt circle diameter, the number and size of the bolt holes and the flange thickness. The marking guarantees that the valve fits exactly onto a mating flange conforming to the same standard.
Understanding DN and PN correctly – the basics for safe and efficient piping systems
This is exactly where TRI-MATIC comes in – with technical understanding, practical experience and an eye for the overall system. Contact us for a consultation:
+41 41 780 22 22
info@tri-matic.ch
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