The DN system of conversions simplifies these measurements by ignoring the demical point and rounding down to the nearest zero or five. Using the above example, 1 inch equals 33.40 milimetres but rendered as DN25; 8 inch equals 219.1 milimeters but rendered as DN200; while, a 14 inch flange will be termed as DN350.
DN To Inches Chart
| Diameter Nominal DN (mm) | Nominal Pipe Size (inches) | Outside diameter (OD) inches (mm) |
|---|---|---|
| 6 | 1⁄8 | 0.405 in (10.29 mm) |
| 8 | 1⁄4 | 0.540 in (13.72 mm) |
| 10 | ⅜ | 0.675 in (17.15 mm) |
| 15 | ½ | 0.840 in (21.34 mm) |
| 20 | ¾ | 1.050 in (26.67 mm) |
| 25 | 1 | 1.315 in (33.40 mm) |
| 32 | 1 1⁄4 | 1.660 in (42.16 mm) |
| 40 | 1 ½ | 1.900 in (48.26 mm) |
| 50 | 2 | 2.375 in (60.33 mm) |
| 65 | 2 ½ | 2.875 in (73.02 mm) |
| 80 | 3 | 3.500 in (88.90 mm) |
| 100 | 4 | 4.500 in (114.30 mm) |
| 150 | 6 | 6.625 in (168.27 mm) |
| 200 | 8 | 8.625 in (219.08 mm) |
| 250 | 10 | 10.75 in (273.05 mm |
| DN | NPS | DN | NPS | DN | NPS |
| (Diameter Nominal) | (Nominal Pipe Size) | (Diameter Nominal) | (Nominal Pipe Size) | (Diameter Nominal) | (Nominal Pipe Size) |
| Milimeters | Inches | Milimeters | Inches | Milimeters | Inches |
| 100 | 4 | 700 | 28 | 1800 | 72 |
| 115 | 4 1/2 | 800 | 32 | 1900 | 76 |
| 125 | 5 | 900 | 36 | 2000 | 80 |
| 150 | 6 | 1000 | 40 | 2200 | 88 |
| 200 | 8 | 1050 | 42 | 2400 | 96 |
| 250 | 10 | 1100 | 44 | 2600 | 104 |
| 300 | 12 | 1200 | 48 | 2800 | 112 |
| 350 | 14 | 1300 | 52 | 3000 | 120 |
| 400 | 16 | 1400 | 56 | 3200 | 128 |
| 450 | 18 | 1500 | 60 | 3400 | 134 |
| 500 | 20 | 1600 | 64 | 3600 | 142 |
| 600 | 24 | 1700 | 68 | 3800 | 150 |
Diameters in DN or in NPS
From within the industrial market we find these two systems to size the diameters for pipes, tubes, valve connections and other devices:
- Nominal pipe size (NPS) , defined by the American standard ANSI / ASME B36.19 is based on inches .
- Nominal diameter (DN) , defined by the European standard ISO 6708 is based on millimeters. Diameter Nominale, is a non-precise conversion of Imperial Units to Metric Units.
We know that to determine the conversion operation from one value to another, we should only perform a simple simple mathematical formula (the value of a inch is 25.4 millimeters and that of 1 millimeter 0.0393701 inches); however we could not apply it to determine these values because each system uses a commercial measure that does not correspond with the real value of the outside diameter of the pipe but with the inside of it. We observe a difference of 0.4 millimeters in the value DN 25 while in larger sizes the difference is even greater , for example: 120 inches is actually 3048 millimeters, 48 millimeters more than the DN 3000 value.
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