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　　Because the seamless steel pipe is round, when we say the specification of the seamless steel pipe, we all distinguish the specification of the seamless steel pipe by its diameter.

　　The outer diameter of seamless steel pipe is indicated by letter D. here we should pay attention to that the actual outer diameter data of seamless steel pipe is not equal to the nominal diameter. For example, the outer diameter data of DN200 seamless steel pipe is 219mm, and the outer diameter of DN300 seamless steel pipe is 325mm.

　　The diameter of ASTM A53 4" Sch10s Seamless Steel Pipe is divided into nominal diameter, outer diameter and inner diameter. The nominal diameter is also called nominal diameter. It is represented by the letter DN. For example, the nominal diameter of seamlesssteel pipe with a nominal diameter of 200 is DN200 seamless steel pipe. Generally, the nominal diameter will appear in the design drawing, so as to determine the structural dimension and joint dimension of pipe, pipe fitting, valve, flange, gasket, etc. according to the nominal diameter, The nominal diameter is the specification title of the pipe diameter close to the inner diameter of the seamless steel pipe, but not equal to the inner diameter. In terms of the outer diameter, it is not difficult to guess that the outer diameter of the seamless steel pipe is the maximum distance from the outermost end of the seamless steel pipe diameter to the diagonal outer end of the other side.

　　

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