What is the nominal diameter？

Generally speaking, the diameter of the pipe can be divided into outer diameter (De), inner diameter (D), nominal diameter (DN).
Below to give you a distinction between these “De, D, DN” difference.

DN is the nominal diameter of the pipe

Note: This is neither the outside diameter nor the inside diameter; should be related to the early development of pipeline engineering and imperial units; usually used to describe galvanized steel pipe, which corresponds to the imperial units as follows:

4-part pipe: 4/8 inch: DN15;
6-minute pipe: 6/8 inch: DN20;
1 inch pipe: 1 inch: DN25;
Inch two pipe: 1 and 1/4 inches: DN32;
Half-inch pipe: 1 and 1/2 inches: DN40;
Two inch pipe: 2 inches: DN50;
Three-inch pipe: 3 inches: DN80 (many places are also labeled as DN75);
Four-inch pipe: 4 inches: DN100;
Water, gas transmission steel pipe (galvanized steel pipe or non-galvanized steel pipe), cast iron pipe, steel-plastic composite pipe and polyvinyl chloride (PVC) pipe and other pipe materials, should be marked with nominal diameter “DN” (such as DN15, DN20).

 

De mainly refers to the outer diameter of the pipe
General use of De labeling, need to be labeled into the form of outer diameter X wall thickness;

Mainly used to describe: seamless steel pipe, PVC and other plastic pipes, and other pipes that require a clear wall thickness.
Take galvanized welded steel pipe as an example, with DN, De two labeling methods are as follows:
DN20 De25×2.5mm
DN25 De32×3mm
DN32 De40×4mm
DN40 De50×4mm

......

 

D generally refers to the inner diameter of the pipe, d indicates the inner diameter of the concrete pipe, and Φ indicates the diameter of an ordinary circle

Φ can also indicate the outer diameter of the pipe, but then it should be multiplied by the wall thickness.
For example, Φ25×3 means a pipe with an outside diameter of 25mm and a wall thickness of 3mm.
Seamless steel pipe or non-ferrous metal pipe, should be marked “outer diameter × wall thickness”.
For example: Φ107×4, where Φ can be omitted.
China, ISO and Japan's part of the steel pipe labeling using wall thickness dimensions to indicate the wall thickness of steel pipe series. For this type of steel pipe, the method of expression for the pipe outside diameter × wall thickness. For example: Φ60.5×3.8

De, DN, d, ф of the respective range of expression!
De-- PPR, PE pipe, polypropylene pipe OD
DN -- polyethylene (PVC) pipe, cast iron pipe, steel-plastic composite pipe, galvanized steel pipe nominal diameter
d -- concrete pipe nominal diameter
ф -- seamless steel pipe nominal diameter