Pipe vs. Tube: Deciphering the NPS, Schedule, and Dimensional Standards

1. The “Nominal” vs. “Actual” Confusion In the international steel trade, the terms “Pipe” and “Tube” are often used interchangeably by laypeople, but they represent two distinct engineering standards.

• Pipe: Sized by NPS (Nominal Pipe Size) and Schedule (SCH). The NPS refers to the approximate inside diameter (ID) for smaller sizes and the outside diameter (OD) for sizes 14″ and above. The wall thickness is not a fixed dimension but a “Schedule” rating.
• Tube: Sized by the Actual Outside Diameter (OD) and wall thickness, usually measured in inches or millimeters (e.g., BWG).

2. Manufacturing Methods

• Seamless (SMLS): Produced by piercing a solid billet. No longitudinal weld, making it the gold standard for high-pressure containment.
• Welded (ERW, LSAW, SSAW): Formed from steel plate or coil. Modern welding techniques (like DTC’s LSAW) now achieve high Joint Efficiency Factors (E=1.0), making them suitable for large-diameter infrastructure.

3. Typical Standards

• Carbon Steel: ASTM A106 (Seamless), ASTM A53 (Welded), API 5L (Line Pipe).
• Stainless Steel: ASTM A312 (Pipe), ASTM A213/A269 (Tube).