Nominal Diameter Interpretation of Pipe Fittings

by | Jul 11, 2022 | News

The nominal diameter is the size of all the piping accessories in the piping system. It is a convenient round integer for reference, and has no strict relationship with the machining size. The letter “DN” represents for the nominal diameter and followes a numerical symbol.

In pipework, pipe sizes are referred to as Nominal Pipe Size – NPS, or “Nominal Pipe Size”. The metric equivalent is called DN or “Nominal Diameter”. The metric designation conforms to the use of the International Standards Organisation (ISO) for all piping, gas, heating oil and miscellaneous piping used in buildings.The use of NPS does not conform to the American Standard Pipe Names, where the term NPS stands for “National Straight Pipe Thread”.

What is the nominal diameter?

The mean outside diameter is also meaning the nominal diameter. This is the edge from the metal pipe wall is very thin. The outer diameter and the inner diameter of the tube is almost the same. So take the average of the outside diameter and inside diameter as a pipe diameter. DN is the common diameter of various pipes and pipe accessories. It is same for the pipe and pipe accessories connect with each other, with interchangeability. It is not the actual sense of the outside diameter or inside diameter of pipe, although its value with the pipe diameter is closer or equal. In order to make the pipe and pipe fitting uniform, we will use nominal diameter.
For example, welded steel pipe can be divided into thin-walled steel pipe, ordinary steel pipe and thickened steel pipe. Its value is not the outside diameter, nor the inside diameter, but a nominal size of approximate ordinary steel pipe inside diameter. Each nominal size corresponds to an outer, and the inner value varies with the thickness. It uses metric mm or in inch to express. Nominal diameters also use for pipe accessories, meaning the same as the seam pipe.

Symbol Interpretation

Generally speaking, the diameter of the tube can be divided into outside diameter, inside diameter, nominal diameter. The letter D represents the outer diameter of the seamless steel pipe. Followed by the size and wall thickness of the additional outer diameter. For example, the outside diameter of 108 seamless steel pipe, wall thickness of 5MM, with D108 * 5 said, plastic pipe also with outside diameter, such as De63, other such as reinforced concrete pipe, cast iron pipe, galvanized steel pipe using DN said. It is generally to express in the design drawings. It is useful to design, manufacture and maintenance of convenient artificial provisions of a standard. Also more than the nominal size, is the pipe (or pipe) specification name.

The nominal size of the pipe and its internal diameter, outside diameter are not equal. For example: the nominal size of 100MM seamless steel pipe 102 * 5, 108 * 5 and several other kinds, 108 for the pipe outside diameter. Therefore, the inner diameter of the steel tube is (108-5-5) = 98mm. But it is not exactly equal to the difference between the outer diameter of steel tube minus two times the wall thickness.

In other words, we can say the nominal size is close to the inside size. But is not equal to the diameter of a pipe diameter specification name. In the design drawings use a nominal size, the purpose is to determine the size of the pipe, pipe fittings, valves, flanges, gaskets and other structural dimensions and connecting dimensions. In the design drawing, a table of pipe specifications will use to indicate a pipe’s nominal diameter and wall thickness.

Size Conversion:

Converted to:
1 inch = 2.54cm = 25.4mm
1 foot = 12 inches = 30.48cm = 304.8mm

What is “Nominal Pipe Size”?

You may have seen the acronym NPS before on our website or similar, it means “Nominal Pipe Size” and it is the North American standard for measuring pipe sizes for a wide range of pressures and temperatures.The NPS standard is widely accepted in the USA but there are a few things that must be followed correctly in order to find the correct nominal pipe size.

The reason for this is simple – NPS indicates the diameter of the outside wall of the pipe, but only for sizes larger than NPS 14. For smaller pipe sizes from NPS 1/8 to NPS 12, this number is not directly related to the outside diameter of the pipe. To be more precise, NPS 12 pipe has an outside diameter of 12.75 inches, while NPS 14 pipe has an outside diameter of 14 inches.

nominal pipe size image

ISO 6708 – Pipework components – DN (Nominal Pipe Size NPS)

ISO 6708 – Pipework components – DN (Nominal Pipe Size NPS)
Nominal Pipe Size – NPSOutside Diameter (mm)
DN
(mm)
InchISO 6708
Pipework components
DIN EN 10220
Seamless Steel Pipes
DIN EN 10255
Threaded Tube
ASME
10 3/8 17.2
15 1/2 21.3 20.0 21.3 21.3
20 3/4 26.9 25.0 26.9 26.7
25 1 33.7 30.0 33.7 33.4
32 1 1/4 42.4 38.0 42.4 42.2
40 1 1/2 48.3 44.5 48.3 48.3
50 2 60.3 57.0 60.3 60.3
2 1/2 73.0 73.0
65 76.1 76.1 76.1
80 3 88.9 88.9 88.9 88.9
3 1/2 101.6 101.6
100 4 114.3 108 114.3 114.3
125 139.7 133 139.7
5 141.3 141.3
150 6 168.3 159 168.3 168.3
200 8 219.1 216 219.1 219.1
250 10 273.0 267 273.0 273.0
300 12 323.9 318 323.9 323.8
350 14 355.6 368 355.6 355.6
400 16 406.4 419 406.4 406.4
450 18 457 470 457 457
500 20 508 521 508 508
600 24 610 622 610 610
700 28 711 720 711 711
800 32 813 820 813 813
900 36 914 920 914 914
1000 40 1016 1020 1016 1016
1200 48 1220 1219 1219
  • NPS – Nominal Pipe Size – references inside diameter of pipe
  • IPS – Iron Pipe Size – initially a system established to designate the pipe size representing the approximate inside diameter of the pipe
  • DIPS – Ductile Iron Pipe Size – references inside diameter of pipe

Which dimensions are worth using, metric or imperial?

Finally, we will return to the frequently asked question of whether to use metric or imperial dimensions. Let’s face it, providing the diameter of a pipe in inches, talking about a half-inch or ¾-inch pipe, results only from a long-standing habit rather than convenience.

In Europe, however, the metric system applies, so you need to start using it, and you can easily find tables online to help you convert the diameter of a pipe from inch to metric or vice versa, if you still don’t want to change your habits.

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