Pipe Working Pressure

Pipe working pressure: Barlow's formula

Barlow's formula P = 2·σ_perm·t/D gives you a quick estimate of how much internal pressure a thin-wall pipe can take, using the allowable stress σ_perm, the wall thickness t and the outside diameter D. How high σ_perm goes depends on what the pipe is made of. HDPE PE100 sits around 8–10 MPa at 20 °C, while ASTM A106 carbon steel runs up to 110–150 MPa. Convention is to keep a safety factor of roughly 4 against bursting, so a pipe is rated at about a quarter of its ultimate strength. Take a steel pipe with t = 3 mm, D = 60 mm and σ_perm = 150 MPa: that works out to P = 2·150·3/60 = 15 MPa of working pressure. Keep in mind the formula assumes a thin wall (D/t > 20) and an even circumferential, or hoop, stress.

Applications

Sizing oil and gas pipelines, domestic water supply, fire-fighting networks, compressed-air lines, industrial steam piping, irrigation systems. The codes you'll bump into here include NBR 12266 (water supply), NBR 15280 (gas), ASME B31.1/B31.3 (power and process piping) and API 5L (line pipe).

FAQ

Why divide by D and not by the inside diameter? Using the outside diameter is a deliberately conservative shortcut. It comes out a little lower than the exact thick-wall (Lamé) solution, which puts you on the safe side.

Does this apply to thick-walled pipes? No. Once D/t drops below 20, switch to the Lamé equations, which bring radial stress into the picture as well.

How do I pick σ_perm? Start from the material's ultimate tensile strength and divide it by the safety factor in your code: usually 3–4 for steel, and 1.25 over the hydrostatic design basis for plastics.