What is DN (Dimension Nominell) & Why Does DN32 Break the Trend?

[TSN79]

A lot of the time I see dimensions of pipes and fittings (mostly steel) written as "DNxx" where xx is a number I've seen from 15 to several hundered, but usually pretty round figures such as 20, 25, 40, 100, 150 and so on. Strangely, between 25 and 40 comes 32 which suddenly totally breaks with the trend.

I wonder what these dimensions are all about. Steel pipes don't usually have these dimensions, they are often much more spesific like 76.1, 114,3 mm etc.

Is DN an approximate dimension used for valves and such? Is it short for Dimension Nominell or something...? And what is up with that DN32 breaking with the "round figures" ??

 

[Q_Goest]

Is DN an approximate dimension used for valves and such? Is it short for Dimension Nominell or something...?

Yes, that's exactly right. Even in Europe, it seems the same pipe dimensions are used. It stands forhttp://www.engineeringtoolbox.com/nps-nominal-pipe-sizes-d_45.html"

And what is up with that DN32 breaking with the "round figures" ??

That's just inch and a quarter pipe in millimeters. Kinda weird I suppose, but it seems the world has standardized on this given set of dimensions for pipe.

Note that the dimension is strictly a nominal one. For example, there is nothing you can measure which is exactly 1" on a 1 inch (DN25) pipe. The OD is 1.315", and depending on the schedule wall thickness, the ID is not particularly close to 1" either.