Pipe vs Tube: What's the Difference?
Pipe and tube are both hollow cylindrical products, but they differ in how they are sized, ordered, and used. The distinction is not about shape; it is about dimensional conventions and engineering purpose.
Pipe is sized by Nominal Pipe Size (NPS) and schedule. The NPS does not equal the actual OD (except for NPS 14 and above). Tube is sized by its actual outside diameter (OD) and wall thickness (often in BWG or mm).
Key Differences
| Feature | Pipe | Tube |
|---|---|---|
| Sizing | NPS + schedule (e.g., NPS 4, SCH 40) | Actual OD x wall (e.g., 25.4 x 1.65 mm) |
| OD reference | Nominal (NPS ≠ actual OD below NPS 14) | Actual measured OD |
| ID | Varies with schedule (same NPS, different ID) | Calculated from OD minus 2x wall |
| Dimensional standard | ASME B36.10, B36.19 | ASTM A269, A213, BWG/SWG |
| Wall tolerance | ±12.5% (seamless) | ±10% or tighter |
| OD tolerance | ±1% or per ASTM | ±0.5% or tighter |
| Purpose | Fluid/gas transport, pressure containment | Heat transfer, instrumentation, structural |
| Connections | Flanges, welds, threads | Compression fittings, expansion joints |
| Ordering unit | Length (meters or feet) | Length or weight |
Sizing Example
An NPS 4 SCH 40 pipe has:
- OD: 114.3 mm (4.500 in); not 4 inches
- Wall: 6.02 mm (0.237 in)
- ID: 102.3 mm (4.026 in)
A 4-inch OD tube with 1.65 mm wall has:
- OD: 101.6 mm (4.000 in); actual 4 inches
- Wall: 1.65 mm (0.065 in)
- ID: 98.3 mm (3.870 in)
Common Tube Applications
- Heat exchanger tubes: ASTM A179 (carbon steel), ASTM A213 (alloy/stainless)
- Instrumentation tubing: Stainless steel per ASTM A269, connected with compression fittings
- Boiler tubes: ASTM A192, A210
- Hydraulic tubing: ASTM A519 (seamless mechanical tubing)
- Condenser tubes: Copper-nickel, titanium, or stainless steel
Common Pipe Applications
- Process piping in refineries and petrochemical plants
- Oil and gas pipelines (API 5L)
- Steam and water distribution
- Fire protection systems
- Structural applications (when using ASTM A500/A53)
For a detailed comparison including tubing gauges and size charts, see the full article on pipe vs tube vs tubing.
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