Out Diameter(OD) VS Nominal Diameter(NB)

The nominal diameter (diameter) is also called 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 the metric mm. A metric unit such as a DN100 tube has an inner diameter of more than 100 mm if it encounters a thin-walled tube, but it has an inner diameter of less than 100 mm when it is encountered with a thick-walled tube. But take 100mm when calculating. Also called DN100.

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

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

Generally, the diameter of the pipe can be divided into an outer diameter, an inner diameter, and a nominal diameter.

The outer diameter and the inner diameter are better understood. The outer diameter of the pipe is the carbon seamless steel pipe and the metal pipe is represented by the letter D, and then the outer diameter and the wall thickness are added, for example, the seamless steel pipe with an outer diameter of 108, and the wall thickness is 5MM, indicated by D108*5, the plastic tube is also represented by the outer diameter, such as De63. Others such as reinforced concrete pipe, cast iron pipe, galvanized steel pipe, etc. are represented by DN. 

Generally, the nominal diameter is used in the design drawings. The nominal diameter is a standard that is artificially specified for the convenience of designing and manufacturing, and also a nominal diameter, is the specification name of the pipe (or pipe fitting). The nominal diameter of the pipe is not equal to the inner diameter and outer diameter. 

For example, the seamless steel pipe with a nominal diameter of 100 mm is 102*5, 108*5, etc. 108 is the outer diameter of the pipe, and 5 is the wall thickness of the pipe. Therefore, the inner diameter of the steel pipe is (108-5-5)=98MM, but it is not completely equal to the difference between the outer diameter of the steel pipe and the double wall thickness. It can also be said that the nominal diameter is close to the inner diameter, but not equal to the inner diameter. The name of a pipe diameter specification, in the design drawings, the nominal diameter is used. The purpose is to determine the structural dimensions and connection dimensions of pipes, fittings, valves, flanges, gaskets, etc. according to the nominal diameter. The nominal diameter is the symbol DN. It is stated that if the outer diameter is used in the design drawing, a pipe specification comparison table should also be made to indicate the nominal diameter and wall thickness of a certain pipe.