The nominal diameter (diameter) is also known as the average outer diameter. This is because the tube from the metal tube is very thin, and the outer diameter of the tube is almost the same as the inner diameter of the tube. Therefore, the average diameter of the tube and the inner diameter of the tube are taken as the diameter.


The nominal diameter is based on metric mm. If a thin-walled pipe is encountered, the inner diameter of a metric unit such as DN100 pipe exceeds 100 mm, but when a thick-walled pipe is encountered, its inner diameter is less than 100 mm. But the calculation needs 100 mm. Also known as DN100.


If it is based on inches, it is called NB (inch unit).


The relationship between DN and NB is as follows: DN/25 = DB, for example, DN50 is DB2, that is, 2 ", 1" = 25.4mm.


Usually, the diameter of pipe can be divided into outer diameter, inner diameter and nominal diameter.
Outer and inner diameters are better understood. The outer diameter of the pipe is carbon seamless steel pipe, and the metal pipe is denoted by the letter D. Then the outer diameter and wall thickness are added, such as seamless steel pipe with an outer diameter of 108, and the wall thickness is 5MM. The plastic pipe is also denoted by the outer diameter, such as De63. Others such as reinforced concrete pipe, cast iron pipe, galvanized steel pipe, etc. are expressed in DN. Normally, nominal diameters are used in design drawings. Nominal diameter is an artificial standard for convenience of design and manufacture. Nominal diameter is the specification name of pipeline (or pipe fittings). The nominal diameter of the pipeline is not equal to the inner diameter and the outer diameter. For example, the nominal diameter of 100 mm seamless steel pipe is 102 x 5, 108 x 5, etc. 108 is the outer diameter of the pipe, 5 is the wall thickness of the pipe. Therefore, the inner diameter of the steel tube is (108-5-5) = 98MM, but it is not exactly equal to the difference between the outer diameter of the steel tube and the thickness of the double wall. It can also be said that the nominal diameter is close to the inner diameter, but not equal to the inner diameter. In the design drawing, the pipe diameter specification name, nominal diameter is used. The purpose is to determine the structural and connecting dimensions of pipes, fittings, valves, flanges, gaskets, etc. according to the nominal diameter. The nominal diameter is the symbol DN. It is said that if the outer diameter is used in the design drawings, a pipe specification comparison table should also be made to indicate the nominal diameter and wall thickness of a pipe.