Question 1

What is the difference between pipe and tube sizing?

Accepted Answer

Pipe is sized by NPS (nominal bore) with standardized OD per ASME B36.10M — the OD does not match the NPS for sizes under 14". Tube is sized by actual outside diameter and wall thickness (e.g., 25.4 mm OD × 1.65 mm wall). Pipe uses schedules; tube uses gauge numbers or actual wall thickness. Pipe standards focus on pressure containment; tube standards focus on dimensional precision.

Question 2

How do I determine the maximum allowable pressure for a pipe?

Accepted Answer

Use the Barlow formula: P = 2St/OD, where S is allowable stress (from ASME B31.3 Table A-1 at design temperature), t is wall thickness minus corrosion allowance and manufacturing tolerance (12.5% for seamless), and OD is outside diameter. For example, NPS 4 Sch 40 carbon steel at 200°C: P = 2 × 115 × (6.02 × 0.875) / 114.3 ≈ 10.6 MPa.