DN, PN and Schedule: which figure applies to what

Anyone ordering pipes, flanges and fittings keeps running into three abbreviations: DN, PN and Schedule. They are what makes components from different manufacturers fit together – and they are the three figures without which no supplier can quote reliably. Here is what they mean, and where the usual misunderstandings lie.

DN – the nominal size

DN stands for “Diamètre Nominal”, the nominal size. It is a dimensionless designation for matching components – pipe, flange, bend, valve and gasket of a line all carry the same DN and therefore fit together.

The most common error: DN 100 does not mean the inside diameter is exactly 100 mm. Nominal size is an ordering number, not a measurement. What is fixed instead is the outside diameter – and that is what you need to state as soon as the pipe itself is concerned.

Outside diameters to EN 10220 (series 1). The inch figures are the customary equivalents, not an exact conversion.

PN – the pressure rating

PN stands for “Pression Nominale”, the nominal pressure rating. PN 16 means the component is rated for 16 bar at reference temperature. Common ratings in plant engineering are PN 6, 10, 16, 25 and 40; in high-pressure and power plant applications PN 63, 100, 160, 250, 320 and 400 follow.

Critical and often overlooked: as temperature rises, the permissible working pressure falls. A PN 40 flange holds its 40 bar at 20 °C, considerably less at 300 °C. How steep the drop is depends on the material – creep-resistant grades such as 16Mo3 or 13CrMo4-5 retain their capability longer than unalloyed steel. The governing figures are in the pressure-temperature tables of EN 1092-1.

PN and Class – why they are not the same

In the American system, Class to ASME B16.5 takes the place of PN. The two can be roughly mapped onto each other but are not interchangeable: bolt circle, number of bolts and flange thickness differ. A Class 150 flange does not mate with a PN 16 counterflange.

For design orientation only. The actual connecting dimensions are in our flange dimension tables, the designs under flange types. The figure is the pressure in bar, not the PN series: a Class 150 flange is rated at roughly 20 bar and does not mate with any PN flange.

Schedule – the wall thickness

While DN and PN come from European standardisation, Schedule (Sch.) comes from the American system – ASME B36.10M for steel pipe, B36.19M for stainless. The Schedule number describes the wall thickness at a given outside diameter. The higher the number, the thicker the wall.

Important: Schedule is not a fixed millimetre figure. Sch. 40 means a different wall thickness at DN 50 than at DN 200.

Wall thicknesses to ASME B36.10M. Stainless pipe follows B36.19M with series 5S, 10S, 40S and 80S, where wall thicknesses are partly lower.

How the figures work together

In practice they are combined. A designation such as “DN 100 / PN 16” describes a line of roughly 100 mm nominal size for 16 bar nominal pressure. Where the pipe itself is concerned, wall thickness is added – either as a Schedule or as an actual figure in millimetres, for example “114.3 × 6.3 mm”.

In European pipework, the notation outside diameter × wall thickness is the least ambiguous form. It leaves no room for interpretation and is preferable to “DN 100, Sch. 40” once it comes to ordering.

What belongs in an enquiry

So that a supplier can quote without coming back with questions, five details are needed:

• Material – for example P235GH, 16Mo3 or 1.4404. An overview is under materials.
• Dimensions – outside diameter × wall thickness, or DN with Schedule
• Pressure rating and standard – PN or Class, plus the underlying product standard
• Quantity and length – mill length, fixed length or fitting piece
• Certificate – mill certificate 2.2 or inspection certificate 3.1 or 3.2 to EN 10204

If you are unsure about sizing, the piping calculator offers tools for material selection, nominal size and flange connections. Further reading: flanges to EN 1092-1 and inspection certificate 3.1 or 2.2.