Pipe Working Pressure
Maximum working pressure of a thin-wall cylindrical pipe via Barlow's formula: P = 2·t·S/D.
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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.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.