The diameter of the pipe can be divided into outer diameter, inner diameter and nominal diameter. The outer diameter of a tube made of seamless steel tubes is generally used. The outer diameter and wall thickness are appended, and the letter D is used to represent it.

For example, the outer diameter of 108 seamless steel tube, the wall thickness of 5MM, plastic pipe is also expressed with outer diameter, expressed in D108*5. Such as De63 other such as reinforced concrete pipe, cast iron pipe, galvanized steel pipe design drawings commonly used nominal diameter to express, and so on using DN representation. Nominal diameter is to design, manufacture and dimension also nominal diameter, repair convenient artificial standard. Specification of pipe (or pipe fittings). For example: nominal diameter is 100MM seamless steel pipe 102*5 nominal diameter and its inner diameter, outer diameter are not equal. 108 for pipe outer diameter, 108*5 and so on several kinds. 5 represents the wall thickness of the pipe

Therefore, the inner diameter of the steel pipe is (108*5 steel tube knowledge Daquan, also can say, -5 = 98MM, but not equal to two times the wall thickness difference of the steel pipe diameter. Nominal diameter is close to but not equal to the diameter of a pipe diameter name, in the inner diameter. The design drawings should be used in order to determine the nominal diameter, pipe, pipe fittings, valves, flanges, gaskets and other structural dimensions and connecting diameter. Nominal diameter is expressed by symbol DN. If the outer diameter is expressed in the design drawing, the nominal diameter of a certain pipe shall be marked with a standard comparison table. Wall thickness. The selection of specification series for pressure piping and its components is first considered in the specification of piping series.