Enter the shell geometry, modulus and the external design pressure (1.013 bar = full vacuum), then press Calculate . Shortening the unsupported length L with stiffening rings is the most effective way to raise the collapse pressure.
How vacuum collapse works Uniform external pressure ovalises the shell (n = 2 lobe) L long → collapses rings → L₁ resists Collapse depends on t/Do and the unsupported length L — not on strength alone. Adding stiffening rings shortens L and is the cheapest way to raise the collapse pressure.
Pcr(WT) = 2.6·E·(t/Do)^2.5 / [L/Do − 0.45√(t/Do)]
Engineering tips Length matters: collapse pressure scales roughly with 1/L until the long-tube limit — add stiffeners to shorten L.Inelastic buckling: when the collapse stress exceeds yield the elastic formula over-predicts — switch to the UG-28 chart.Out-of-roundness: real shells collapse below the theoretical value; UG-80 tolerances and the factor of 3 cover this.Recommended values Full vacuum: Design external pressure = 1.013 bar (atmospheric); add margin for jacket/steam-out cases.Safety factor: Design typically to ≥ 3 on elastic collapse (as applied here).Stiffening rings: When L/Do is large the tube is unstable — add rings to shorten the effective length.Out-of-roundness: Keep ovality ≤ 1% of Do; initial imperfection sharply lowers real collapse pressure.Limitations Elastic method: Timoshenko elastic collapse — not the full ASME Sec VIII UG-28/geometric-chart procedure.Not included: Ring detailing/spacing design, nozzle openings, and combined external + axial/bending loads.About the standards ASME BPVC VIII-1: Rules for Construction of Pressure Vessels.