Understanding Steel Pipe Sizes and Schedules
Steel pipe selection requires understanding nominal pipe size (NPS), diameter nominal (DN), and schedule numbers that determine wall thickness and pressure ratings. This guide provides essential reference information for pipe selection.
Nominal Pipe Size (NPS) and Diameter Nominal (DN)
NPS is the North American standard for pipe sizing, representing the approximate internal diameter. DN is the ISO metric equivalent for international projects. The relationship between NPS and DN provides global compatibility.
| NPS (inches) | DN | OD (mm) | Typical OD (inches) |
|---|---|---|---|
| 1/8 | 6 | 10.3 | 0.405 |
| 1/4 | 8 | 13.7 | 0.540 |
| 1/2 | 15 | 21.3 | 0.840 |
| 3/4 | 20 | 26.7 | 1.050 |
| 1 | 25 | 33.4 | 1.315 |
| 1-1/2 | 40 | 48.3 | 1.900 |
| 2 | 50 | 60.3 | 2.375 |
| 4 | 100 | 114.3 | 4.500 |
| 6 | 150 | 168.3 | 6.625 |
| 8 | 200 | 219.1 | 8.625 |
| 10 | 250 | 273.1 | 10.750 |
| 12 | 300 | 323.9 | 12.750 |
Understanding Pipe Schedules
Schedule numbers (Sch) define wall thickness and are standardized by ASME B36.10M. Higher schedule numbers indicate thicker walls and higher pressure ratings. Common schedules include:
| Schedule | Characteristics | Common Applications |
|---|---|---|
| Sch 10 | Thin wall, lightweight | Low-pressure instrumentation |
| Sch 40 | Standard wall | General service, most common |
| Sch 80 | Extra strong | Higher pressure, mechanical strength |
| Sch 160 | Double extra strong | Severe service conditions |
| XXS | Double extra strong (different OD) | Very high pressure |
Wall Thickness by Schedule
For NPS 2 (OD 60.3mm):
| Schedule | Wall Thickness (mm) | ID (mm) |
|---|---|---|
| Sch 10 | 2.1 | 56.1 |
| Sch 40 | 3.9 | 52.5 |
| Sch 80 | 5.5 | 49.3 |
| Sch 160 | 8.7 | 42.9 |
Pressure-Temperature Ratings
Pressure ratings decrease as temperature increases. For carbon steel ASTM A106 Gr B at elevated temperatures:
- 38°C (100°F): 100% of rated pressure
- 204°C (400°F): 83% of rated pressure
- 315°C (600°F): 60% of rated pressure
- 427°C (800°F): 37% of rated pressure
Material Grades for Different Services
Carbon Steel (ASTM A106, A53): General service to 425°C. A53 Grade B most common for process piping.
Stainless Steel (ASTM A312): Corrosive service, cryogenic to 870°C. 304L/316L preferred for welding.
Alloy Steel (ASTM A335): High-temperature service to 595°C. P11, P22 common for refinery applications.
Selection Considerations
Pressure Requirements: Calculate design pressure including surge and relief scenarios. Select schedule providing adequate margin above operating pressure.
Temperature: Match material grade to maximum operating temperature with consideration for thermal cycling.
Corrosion Allowance: Add wall thickness for corrosive services to ensure adequate remaining thickness at design life.
Flow Requirements: Verify selected pipe ID provides adequate flow capacity without excessive velocity.
Conclusion
Proper pipe selection balances pressure/temperature ratings, material compatibility, and cost considerations. Reference applicable codes (ASME B31.1, B31.3, B31.4) for specific requirements.
Xi’an Coremetal Steel Co., Ltd. supplies carbon steel, stainless steel, and alloy steel pipe in various schedules. Contact Tracy at tracy@coremetalsteel.com or +86 18291910632 for pipe inquiries.