what is nominal pipe size
what is nominal pipe size
The abbreviation of Nominal Pipe Size (NPS) is designated by the American Society of Mechanical Engineers (ASTM) and is widely used in North America (the United States, Canada, etc.). NPA is not a specific inner or outer diameter, but a standardized “size” system, the core expression of which is to use “inches” to represent the “nominal size” of the pipe.
The difference between NPS and DN
DN stands for Diametre Nominal, which is also a type of nominal diameter. NPS is used in most North American regions, but Diametre Nominal is more frequently used in other regions outside North America (Europe, Asia, Africa, etc.).
NPS, DN, SCH OD And Outer Diameter Correspondence Table
If you need to change DN to NPS, or NPS to DN, or you want to determine NPS/DN based on outer diameter/SCH40/SCH80, or you want to determine outer diameter/SCH40/SCH80 based on NPS/DN, please refer to the common size correspondence table below.
|
NPS (inches) |
DN (mm) |
Actual outer diameter (OD, mm) |
Schedule 40 wall thickness (mm) |
Schedule 80 wall thickness (mm) |
|
1/2 |
DN15 |
21.3 |
2.77 |
3.73 |
|
3/4 |
DN20 |
26.7 |
2.87 |
3.91 |
|
1 |
DN25 |
33.7 |
3.38 |
4.55 |
|
1 1/4 |
DN32 |
42.4 |
3.56 |
4.83 |
|
1 1/2 |
DN40 |
48.3 |
3.68 |
5.08 |
|
2 |
DN50 |
60.3 |
3.91 |
5.54 |
|
3 |
DN80 |
88.9 |
4.19 |
6.35 |
|
4 |
DN100 |
114.3 |
4.57 |
7.11 |
|
6 |
DN150 |
168.3 |
5.54 |
8.74 |
|
8 |
DN200 |
219.1 |
6.35 |
Relationship between nominal pipe diameter and actual size
- For small-sized pipes with NPS ≤ 12, the nominal diameter is close to the actual inner diameter. (For example, the inner diameter of NPS 1 pipes is about 26.9 mm, which is close to 1 inch = 25.4 mm)
- For large-sized pipes with NPS > 12, the nominal diameter is close to the actual outer diameter (for example, the outer diameter of NPS 14 pipes is 355.6 mm, which is approximately equal to 14 inches = 355.6 mm)
The purpose is to adapt to pipe accessories such as flanges and threads to ensure that pipes with different wall thicknesses can use the same pipe fittings. For example, for NPS 1 pipes, no matter how thick the wall thickness is, the inner diameter of the pipe is sacrificed, and the outer diameter of the pipe remains unchanged, so the same NPS 1 pipe fittings can be used for connection.
FAQs：
1.What is the relationship between wall thickness grade (Schedule) and NPS?
For the same NPS, the larger the SCH, the thicker the wall and the smaller the inner diameter.
2.Why does the actual outer diameter of a small-sized NPS not match the numerical value?
The size of small-diameter pipes is based on the approximate inner diameter of the early IPS system. Although the outer diameter is standardized later, the NPS number is retained.
