In order to make the connection size of pipes and fittings consistent, the nominal diameter (also called nominal caliber and nominal diameter) is adopted. For example, welded steel pipes can be divided into thin-walled steel pipes, ordinary steel pipes and thickened steel pipes according to their thickness. Each nominal diameter corresponds to an outer diameter, and its inner diameter value varies with thickness.

Nominal diameter can be expressed in metric millimeters or British inches. Pipe materials such as water and gas transmission steel pipes (galvanized steel pipes or non-galvanized steel pipes), cast iron pipes, steel-plastic composite pipes and polyvinyl chloride (PVC) pipes shall be marked with the nominal diameter "DN".

Different marking methods of pipelines

1, water gas conveying steel pipe (galvanized or not), cast iron pipe and other pipes, the pipe diameter should be expressed as nominal diameter DN (such as DN 15, dn50);

2, seamless steel pipe, welded steel pipe (straight seam or spiral seam), copper pipe, stainless steel pipe and other pipes, the pipe diameter should be expressed by external diameter D× wall thickness (such as D 108×4, D 159×4.5, etc.). );

3, reinforced concrete (or concrete) pipes, clay pipes, acid-resistant ceramic pipes, cylinder tile pipes and other pipes, the pipe diameter should be expressed by the inner diameter d (such as d230, d380, etc.). );

4, plastic pipes, pipe diameter should be expressed according to the product standard method;

5. When the nominal diameter DN is used to represent the pipe diameter in the design, there should be a comparison table between the nominal diameter DN and the corresponding product specifications.