1. Demystifying NPS, DN, and Pipe Schedule Numbers
In industrial piping engineering, pipe size is designated by NPS (Nominal Pipe Size - inches) in North America or DN (Diameter Nominal - mm) in Europe and ISO systems. Crucially, NPS does not equal exact outer diameter for sizes below NPS 14.
The term Schedule (Sch) was introduced by ASME B36.10 to standardise wall thicknesses. The schedule number is approximately equal to:
Schedule ≈ 1000 × (Internal Service Pressure P / Allowable Stress S)
2. Dimension Comparison Matrix: NPS 1" to NPS 10" Pipes
| NPS (Inches) | Outside Diameter OD (mm) | Sch 10S Wall (mm) | Sch 40 / STD Wall (mm) | Sch 80 / XS Wall (mm) | Sch 160 Wall (mm) |
|---|---|---|---|---|---|
| 1" (DN 25) | 33.4 mm | 2.77 mm | 3.38 mm (STD) | 4.55 mm (XS) | 6.35 mm |
| 2" (DN 50) | 60.3 mm | 2.77 mm | 3.91 mm (STD) | 5.54 mm (XS) | 8.74 mm |
| 3" (DN 80) | 88.9 mm | 3.05 mm | 5.49 mm (STD) | 7.62 mm (XS) | 11.13 mm |
| 4" (DN 100) | 114.3 mm | 3.05 mm | 6.02 mm (STD) | 8.56 mm (XS) | 13.49 mm |
| 6" (DN 150) | 168.3 mm | 3.40 mm | 7.11 mm (STD) | 10.97 mm (XS) | 14.27 mm |
| 8" (DN 200) | 219.1 mm | 3.76 mm | 8.18 mm (STD) | 12.70 mm (XS) | 22.23 mm |
| 10" (DN 250) | 273.1 mm | 4.19 mm | 9.27 mm / (STD=9.53) | 12.70 mm (XS) | 28.58 mm |
3. Calculating Pressure Rating Using Barlow's Formula
To determine the internal burst pressure or maximum allowable design pressure for seamless or welded stainless steel pipe, mechanical engineers utilize Barlow's Formula:
P = (2 × S × t) / D
- P: Internal working pressure (PSI or MPa)
- S: Allowable material stress value (PSI or MPa) under ASTM A312 SS 316L (approx. 16,700 PSI at room temp)
- t: Nominal wall thickness minus mill manufacturing tolerance (typically -12.5% under ASTM A312)
- D: Outside Diameter of pipe (inches or mm)
4. Difference Between ASME B36.10M (Carbon Steel) and ASME B36.19M (Stainless Steel)
While ASME B36.10M dictates heavy carbon steel pipe wall thicknesses up to Schedule 160 and XXS, stainless steel lines operating in corrosive environments often call for thinner wall schedules (5S, 10S) to reduce raw material weight and cost while preserving structural pressure resistance.