What does nominal diameter mean?
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What does the nominal diameter represent? The nominal diameter, such as DN100, indicates that the outer diameter of the steel pipe is 114 mm. What about the inner diameter of the pipe? I’ve always been confused about this issue
Generally, the diameter of a pipe can be divided into outer diameter, inner diameter, and nominal diameter. For pipes that are seamless steel tubes, the outer diameter is denoted by the letter D, followed by the size of the outer diameter and the wall thickness. For example, a seamless steel tube with an outer diameter of 108 mm and a wall thickness of 5 MM is indicated as D108*5. Plastic pipes are also specified using their outer diameter, such as De63. Pipes made of reinforced concrete, cast iron, galvanized steel, etc., are denoted by DN. In design drawings, the nominal diameter is generally used for representation. The nominal diameter is a standard established artificially for the convenience of design, manufacturing, and maintenance; it is also known as the nominal bore, and it serves as the specification name for pipes (or pipe fittings). The nominal diameter of a pipe is not equal to its inner or outer diameter. For example, for a seamless steel pipe with a nominal diameter of 100 MM, there are several variants such as 102*5 and 108*5; here, 108 represents the outer diameter of the pipe, while 5 indicates the wall thickness. Thus, the inner diameter of this pipe is (108 – 5 – 5) = 98 MM. It is not exactly equal to the outer diameter minus twice the wall thickness. In other words, the nominal diameter is a specification name for the pipe diameter that is close to, but not exactly equal to, the inner diameter. Nominal diameters are used in design drawings so that the structural and connection dimensions of pipes, fittings, valves, flanges, gaskets, etc., can be determined based on them. The nominal diameter is denoted by the symbol DN. If the outer diameter is used in the design drawings, a table showing the specifications of the pipes, including their nominal diameter and wall thickness, should also be provided.
The nominal diameter is slightly larger than the inner diameter. It’s in many manuals. The corresponding relationship between nominal diameter, inner diameter, and outer diameter