Minimum shell thickness for a thin-wall cylindrical pressure vessel per ASME Section VIII Division 1, UG-27 — the real code equation, not the further-simplified textbook thin-wall approximation.
Standard method — stated scope limits apply
Design Conditions
Enter design conditions.
About This Calculation
A pressure vessel's minimum shell wall thickness, per ASME Section VIII Division 1, follows thin-wall pressure vessel theory similarly to piping — internal pressure creates hoop stress in the cylindrical shell (and a different stress state in formed heads), with the vessel's material allowable stress at design temperature setting the safe limit. Vessels additionally require joint efficiency factors reflecting the quality and inspection level of welded seams, since a weld that hasn't been fully radiographed is treated as inherently less reliable than the base metal it joins.
Frequently Asked Questions
Why does joint efficiency reduce allowable stress rather than just add a safety factor?
Joint efficiency directly represents how much of the base material’s strength a specific weld quality/inspection level can be trusted to deliver — a fully radiographed weld can be assigned close to 100% joint efficiency (full base-metal strength), while a spot-radiographed or unradiographed weld gets a lower efficiency factor, genuinely reflecting a higher uncertainty in that weld’s actual strength, not an arbitrary conservatism.
Why is a vessel head’s required thickness different from the cylindrical shell’s?
A formed head (hemispherical, ellipsoidal, torispherical) is in a genuinely different stress state than a cylindrical shell under the same internal pressure — a hemispherical head, for instance, needs roughly half the thickness of the cylindrical shell it’s attached to for the same pressure and material, since its geometry distributes stress differently.
Does this calculation include external pressure (vacuum) design?
No — internal pressure design (hoop stress limiting minimum thickness) and external pressure/vacuum design (buckling/collapse limiting minimum thickness, per ASME Section VIII Division 1 UG-28) are governed by entirely different failure modes and require separate calculations; a vessel subject to vacuum conditions needs both checks, not just the internal-pressure one.
Why add corrosion allowance to the calculated minimum thickness?
The pressure calculation gives the thinnest wall that safely contains design pressure AT THE MOMENT of fabrication — corrosion allowance adds extra material specifically so the vessel still meets that minimum requirement after its design life’s worth of expected internal or external corrosion has consumed part of the wall thickness.