In general, the diameter of the pipe can be divided into outer diameter, inner diameter and nominal diameter. Pipe for seamless steel tube outer diameter of the pipe by the letter D, followed by the size of the additional outer diameter and wall thickness, such as the diameter of 108 seamless steel tube, wall thickness is 5 mm, with 5 D108 * said, plastic pipe also expressed in diameter, such as De63, other such as reinforced concrete pipe, cast iron pipe, galvanized steel pipe with DN, said nominal diameter commonly used in the design drawings, nominal diameter is convenient to design and manufacture, and maintenance of artificially provisions of a standard, also relatively nominal diameter, is of the specifications of the pipe (or pipes) name. Nominal diameter of the pipe and the inner diameter, outside diameter are not equal, for example: nominal diameter is 100 mm seamless steel tube by 108 * 102 * 5 5 several kinds, such as 108 for pipe diameter, 5