1. A nominal diameter DN is the approximate inner diameter, on behalf of the inner diameter of the pipe, but not absolutely the same number of inner diameter, the reason is that the pipeline using different pressures so the pipe wall thickness is different. High pressure wall thickness inside diameter is small. Conversely, the low pressure of the thin-walled inner diameter is larger. General pipe production in order to unify the naming of the tube is the outer diameter of the pipe to do the provisions, so the pressure is not the same time wall thickness to make adjustments.
Example: DN200 tube outer diameter is 219 mm, designers choose different wall thicknesses according to the requirements of the use of pressure, then the inner diameter of the selected tube may be 200 mm, high pressure may also be 195 mm. But are called DN200 pipe.
2.The nominal diameter (nominal diameter), also known as the average outside diameter (mean outside diameter).
This is due to the metal tube of the tube is very thin, tube outside diameter and tube inside diameter difference, so take the tube outside diameter and tube inside diameter of the average as the tube diameter name.
Nominal diameter is metric mm as the basis, said DN (metric unit)
For example, DN100 pipe, if you meet the thin-walled pipe, it will be greater than 100mm, but meet the thick-walled pipe it will be less than 100mm, but the calculation of 100mm. also called DN100
Because the unit has the metric system (mm) and the imperial system (inch) distinction, so there are the following methods of designation.
a.in metric system (mm) as the basis, said DN (metric unit)
b.inch (inch) as the basis, called NB (inch unit)
c.DN (nominal diameter) / NB (nominal bore)
3. The relationship between OD and DN, NB is as follows：
| DN(mm) | NB(inch) | OD(mm) |
| 15 | 1/2″ | 21.3 |
| 20 | 3/4″ | 26.7 |
| 25 | 1″ | 33.4 |
| 32 | 1-1/4″ | 42.2 |
| 40 | 1-1/2″ | 48.3 |
| 50 | 2″ | 60.3 |
| 65 | 2-1/2″ | 73 |
| 80 | 3″ | 88.9 |
| 100 | 4″ | 114.3 |
| 125 | 5″ | 139.8 |
| 150 | 6″ | 168.3 |
| 200 | 8″ | 219.1 |
The above data is based on ASTM A53 for reference only. Please note that the outside diameter may vary slightly according to different specifications.