Minimum shell thickness for internal pressure, with joint efficiency and corrosion allowance — a first estimate before the code calculation.
Double-welded butt joint: 1.00 full radiography, 0.85 spot, 0.70 none (ASME Table UW-12).
Not a design or code calculation. Pressure vessels and piping must be designed by a qualified engineer to the applicable code (such as ASME Section VIII or B31.3, in the edition adopted where they are installed) and inspected as that code requires.
Example Result
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Internal pressure tries to split a cylindrical shell along its length, so the hoop stress sets the wall thickness. The ASME Section VIII, Division 1 formula t = P R ÷ (S E − 0.6 P) turns that into a minimum thickness from the design pressure P, the inside radius R, the material's allowable stress S at design temperature and the efficiency E of the longitudinal weld; the corrosion allowance is added on top. Enter those values to get the required thickness, a check of the longitudinal (circumferential-seam) formula and, at high pressures, the thick-wall result. A pipe mode applies the ASME B31.3 formula to outside diameter and adds mill tolerance. The examples below show how joint efficiency and corrosion allowance move the answer and where the thin-wall formula stops applying.
Choose the vessel shell or the pipe mode, and SI (MPa, mm) or Imperial (psi, in).
Enter the design pressure and the inside radius (shell) or outside diameter (pipe).
Enter the allowable stress at the design temperature from the code's material tables.
Enter the joint efficiency (shell: 1.00 full radiography, 0.85 spot, 0.70 none for a double-welded butt joint) and the corrosion allowance.
Press Calculate. The result shows which formula was used; outside the formulas' range no result is shown.
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Hoop stress (longitudinal seams), ASME Section VIII Division 1 UG-27: t = P R ÷ (S E − 0.6 P), while t ≤ R/2 and P ≤ 0.385 S E. Longitudinal stress (circumferential seams): t = P R ÷ (2 S E + 0.4 P). Above those limits the thick-shell equation of Mandatory Appendix 1-2 applies: t = R (√Z − 1), Z = (S E + P) ÷ (S E − P). When P reaches S E no thickness satisfies the equation, and the calculator shows no result. R is the inside radius in the corroded condition — the nominal radius plus the corrosion allowance — because the code checks the vessel at the end of its life; the corrosion allowance is then added to the thickness.
| Symbol | Meaning |
|---|---|
| P | Internal design pressure (gauge) |
| R | Inside radius + corrosion allowance |
| S | Maximum allowable stress at the design temperature |
| E | Joint efficiency (Table UW-12) |
11.5 MPa (about 218 psi), inside radius 300 mm, S = 130 MPa, seamless or fully radiographed (E = 1.0), corrosion allowance 3 mm.
Corroded R = 303 mm; limit 0.385 S E = 50.1 MPa; 1.5 × 303 ÷ (130 − 0.9)
Hoop 3.52 mm · longitudinal 1.74 mm · required 6.52 mm with corrosion
The plate is then ordered at the next standard thickness that still meets 6.52 mm after any plate under-tolerance.
2Thick shell: 100 MPa, R = 100 mm, S = 130 MPa, E = 1.0.
P > 0.385 S E = 50 MPa → Z = 230 ÷ 30
176.9 mm (Appendix 1-2)
The thin-shell formula would give 142.9 mm — not conservative at this pressure. Below the limit the two agree closely: at 30 MPa and S = 150 MPa the UG-27 formula's 22.7 mm stands.
3US shell: 250 psi, 30 in inside diameter (R = 15 in), S = 20,000 psi, spot radiography (E = 0.85), 1/8 in corrosion allowance.
250 × 15.125 ÷ (17,000 − 150)
0.224 in + 0.125 = 0.349 in — a 3/8 in plate
2 MPa, inside radius 500 mm, S = 138 MPa, corrosion allowance 3 mm, double-welded butt joint:
| Radiography of the longitudinal seam | E (Table UW-12) | Required thickness |
|---|---|---|
| Full | 1.00 | 10.35 mm |
| Spot | 0.85 | 11.66 mm |
| None | 0.70 | 13.55 mm |
Paying for radiography is often cheaper than 30% more steel.
Process piping is specified by outside diameter and ordered with a mill under-tolerance, typically 12.5% of the nominal wall. ASME B31.3 uses t = P D ÷ [2 (S E W + P Y)], ordered wall ≥ (t + c) ÷ 0.875, with W the weld strength reduction factor (1.0 below the creep range), Y = 0.4 for ferritic and austenitic steels at moderate temperature, and c the corrosion and thread allowance; the formula applies while t < D/6. For 10 MPa in a 114.3 mm (4 in NPS) seamless pipe at S = 138 MPa: t = 4.03 mm, plus 1.5 mm = 5.53 mm, ÷ 0.875 = 6.31 mm minimum nominal wall. The simple Barlow formula P D ÷ 2 S gives 4.14 mm because it ignores the Y term.
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