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How It Works
Barlow's formula relates a steel pipe's pressure rating to its allowable stress, wall thickness, and outside diameter. For smooth wall steel pipe, the allowable stress is typically 50% of the yield strength. This calculator lets you solve for any one of the four variables when the other three are known.
Example Problem
A steel pipe has S = 17,500 psi, wall thickness 0.375 in, and outside diameter 12.75 in.
- PR = 2 × 17,500 × 0.375 / 12.75
- PR = 13,125 / 12.75 = 1,029 psi
Frequently Asked Questions
Why is allowable stress 50% of yield strength?
The 50% factor provides a safety margin against variations in material properties, wall thickness tolerances, and corrosion over the pipe's service life. Some codes use different percentages for specific applications.
What are typical steel pipe yield strengths?
Common grades include A53 Grade B at 35,000 psi, A106 Grade B at 35,000 psi, and API 5L X52 at 52,000 psi. The allowable stress at 50% would be 17,500 psi for A53/A106.
How does corrosion affect pressure rating?
Corrosion reduces wall thickness over time, directly lowering the pressure rating. Engineers add a corrosion allowance (typically 0.05–0.125 in) to the required thickness for uncoated steel pipe.
Related Calculators
- Aluminum Pipe Design Calculator — Barlow's formula for aluminum pipe.
- Ductile Iron Pipe Calculator — wall thickness and internal pressure.
- Thrust Block Calculator — size thrust blocks for pipe bends.
- Stress & Strain Calculator — analyze yield stress and deformation in steel materials.
- Pressure Unit Converter — convert pipe pressure between psi, kPa, and bar.
National Resources Conservation Service. National Engineering Handbook. 1995. USDA.