It is called nominal diameter, also called mean outside diameter. This is due to the fact that the pipe wall of the metal pipe is very thin, and the outer diameter of the pipe is almost the same as the inner diameter of the pipe, so the average value of the outer diameter of the pipe and the inner diameter of the pipe is taken as the pipe diameter. Because the unit is distinguished between metric (mm) and imperial (inch), there are the following addressing methods. 1. Based on the metric system (mm), called DN (metric unit) 2. Based on the inch system, called NB (inch unit) 3. DN (nominal diameter) NB (nominal bore) OD (outside diameter) 4 [Example] Galvanized steel pipe DN50, sch 20 Galvanized steel pipe NB2”, sch 20 5. The relationship between outer diameter 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 ———–73.0 80————– 3 ———–88.9 100————– 4 ————114.3 125——– —— 5 ————139.8 150————– 6 ————168.3 200 ————— 8————219.1 The above data is for reference only according to ASTM A53. The outer diameter will be slightly different due to different specifications, please note
Nominal diameter, also known as average outer diameter. Since the wall of the metal pipe is very thin, the outer diameter is almost the same as the inner diameter, so the average value of the pipe is taken as the pipe diameter. The nominal diameter is based on the metric system mm, called DN (metric unit). For example, DN100 pipes, if they encounter thin-walled pipes, their inner diameter will be greater than 100mm, but if they encounter thick-walled pipes, their inner diameter will be less than 100mm. However, 100mm is used in the calculation. Also called DN100
The relationship between DN and NB is like this: DN/25=DB, for example, DN50 is DB2, that is to say 2″, 1″=25.4mm (not 33.3mm), that is, DN25=OD25.4mm.
Link to this article：The difference between the nominal diameter and outer diameter of pipe fittings
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