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What is the difference between Inch, DN, PN and Class? 2020/09/27 16:18:46

Because in the piping and process industry, most advanced and pioneering technologies are developed by companies originated in US, the piping nomenclature has been heavily influenced by imperial system. In this system, pounds per square inch(psi) is used to refer to pressure while nominal pipe size(NPS) is used to refer to size of pipe. However there is also another very important nomenclature system: metric system, which was established by the International Standards Organization(ISO). The basic unit to describe length or size is meter in metric system with an extension ofmillimeter and centimeter, etc. It also refers to the pipe diameters as nominal diameter(DN) and pressure ratings asnominal pressure(PN). Pressures are noted in kilopascal(kPa) or bar.

Nominal Pipe Size(NPS) & Nominal Diameter(DN) Comparison Table

Nominal Pipe Size(NPS)Nominal Diameter(DN)
inchesmillimeters
1/4"6
1/2"15
3/4"20
1"25
1-1/4"32
1-1/2"40
2"50
2-1/2"65
3"80
4"100
5"125
6"150
8"200
10"250
12"300
14"350
16"400
18"450
20"500
24"600


ASME Pressure Class & ISO Nominal Pressure

ASME Pressure ClassNominal Pressure(PN)
psibar
15016
30040
40064
600100
900160
1500250
2500400
4500700

 *Note: PN is an approximation to the corresponding ASME pressure class, and should not be used as an exact correlation between the two standards. PN correlate to EN/DIN/BS pressure-temperature rating standards, which may vary significantly from ASME pressure-temperature ratings.

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