Process & Mechanical
Pipe Wall Thickness (ASME B31.3)
Minimum required wall thickness for straight pipe under internal pressure, per ASME B31.3 Para. 304.1.2. Valid while t < D/6 and P/(S·E) ≤ 0.385.
Standard method — stated scope limits apply
Design Basis
S from ASME B31.3 Table A-1 for your material/temperature. Y = 0.4 for ferritic/austenitic steel below the creep range (<482°C); above that, Y comes from Table 304.1.1.
Allowances
Enter pressure, diameter and material data.
About This Calculation
A pressure pipe's minimum required wall thickness, per ASME B31.3 (process piping), is set by the hoop stress the internal pressure creates in the pipe wall — thin-wall pressure vessel theory relates that stress directly to pressure, pipe diameter, and wall thickness, with the material's allowable stress (from its ASME-listed properties at design temperature) setting the limit. Getting this wrong in the unsafe direction risks a genuine pressure-boundary failure; getting it wrong in the conservative direction just costs more in pipe and fittings than necessary.
Frequently Asked Questions
Why does the calculation need a corrosion allowance on top of the pressure-only thickness?
The pressure calculation gives the MINIMUM thickness needed to safely contain the design pressure at the moment of installation — corrosion allowance adds extra material specifically so the pipe still meets that minimum requirement after a design-life’s worth of expected internal or external corrosion has consumed some wall thickness.
Why does allowable stress depend on temperature, not just the material grade?
Metal strength genuinely decreases at elevated temperature (and some materials also derate at very low temperature) — ASME B31.3’s allowable stress tables give a different, temperature-specific value for the same material grade, since the pipe must be checked against its actual design temperature, not just room-temperature material properties.
Is a schedule number (Sch 40, Sch 80) the same as a specific wall thickness?
No — schedule is a dimensionless designation that corresponds to a DIFFERENT actual wall thickness depending on nominal pipe size, since larger pipes need proportionally different wall thickness for the same schedule number to maintain a consistent approximate pressure rating across sizes.
Does this calculation account for pipe bends or only straight pipe?
This is the straight-pipe (longitudinal) hoop-stress calculation — pipe bends see additional stress from the bending process itself (thinning on the extrados, thickening on the intrados) and are often specified with additional wall thickness margin or checked separately per the piping code’s bend-specific requirements.