Generally speaking, the diameter of stainless steel seamless pipes can be divided into outer diameter, inner diameter, and nominal diameter. The outer diameter of a seamless steel pipe is indicated by the letter D, and the size and wall thickness of the outer diameter is added later.

For example, a seamless steel pipe with an outer diameter of 108, with a wall thickness of 5MM, is represented by D108*5. Others, such as reinforced concrete pipes, cast iron pipes, galvanized steel pipes, etc., are represented by DN. In the design drawings, the nominal diameter is generally used. The nominal diameter is a standard artificially specified for the convenience of design, manufacture and maintenance. Diameter is the specification name of the pipe or fitting.

The nominal diameter of the pipe is not equal to its inner diameter and outer diameter. For example, seamless steel pipes with a nominal diameter of 100MM have 102*5, 108*5, etc., 108 is the outer diameter of the pipe, and 5 represents 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 minus twice the wall thickness.

It can also be said that the nominal diameter is the specification name of a pipe diameter close to the inner diameter but not equal to the inner diameter. Therefore, the nominal diameter is used in the design drawings. 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 represented by the symbol DN. If the outer diameter is used in the design drawings, a pipe specification comparison table should also be made to indicate the nominal diameter and wall thickness of a certain pipe.In order to make the connecting dimensions of pipes and fittings uniform, DN is used to indicate their nominal diameters.

Chemical plants are used to transport fluids of general pressure such as water, gas, air, oil, and heating steam. The pipelines often use electric welded steel pipes, called seamed pipes. Seam pipes can be divided into thin-walled steel pipes, ordinary steel pipes and thickened steel pipes according to their thickness.

Its nominal diameter is neither the outer diameter nor the inner diameter, but a nominal size similar to the inner diameter of ordinary steel pipes. Each nominal diameter corresponds to an outer diameter, and the value of the inner diameter varies with the thickness. Nominal diameter can be expressed in metric system mm or in the imperial system in.

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