Nominal diameter is what mean?

Nominal diameter ( 公称直径) , also known as the average diameter ( 平均外径) 。 Refer to the standard diameter after standardization, as a midpoint between inner and outer diameter, expressed in DN, the unit mm. For example, the average diameter of 1200 mm container marked the nominal diameter of 48 '). For design and manufacture, and maintenance convenience, nominal diameter is artificially provisions of a standard, is also called nominal diameter, pipe ( Or pipes) The specifications of the 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 with 5, 108 * 102 * 5 several kinds, such as 108 for the outer diameter of the pipe, said the pipe wall thickness, therefore, the inner diameter of the steel pipe for ( 108 - 5 - 5) = 98 mm, but it is not completely equal to the difference between the steel pipe diameter double wall thickness reduction, in other words, nominal diameter is close to the inner diameter, but is not equal to the inner diameter of a pipe diameter of the specifications of the name, so want to use nominal diameter in the design drawings, the purpose is to according to the nominal diameter can be determined pipes, pipe fittings, valves, flange, gasket, such as structure size and connection size, nominal diameter using sign DN, said said if used in the design drawing diameter, also should make the pipe specification table, suggests that some pipes of nominal diameter, wall thickness.