Pressure Vessel Wall Thickness Calculator
Calculate the minimum required wall thickness for a cylindrical pressure vessel shell under internal pressure, per the thin-wall design formula with a corrosion allowance. Essential for ASME-code vessel design and safety verification.
About this calculator
The required wall thickness t of a cylindrical pressure vessel is given by t = (P·R) / (S·E − 0.6·P) + C_a, which is derived from the ASME Boiler and Pressure Vessel Code (Section VIII, Division 1). Here P is the internal pressure, R is the internal radius (half the internal diameter), S is the allowable stress of the shell material, E is the weld joint efficiency, and C_a is the corrosion allowance added to account for material loss over the vessel's service life. The denominator term (S·E − 0.6·P) effectively reduces the allowable stress to account for the biaxial hoop-to-longitudinal stress ratio. Hoop stress is the governing failure mode for thin-walled cylinders: σ_hoop = P·R / t. Joint efficiency E ranges from 1.0 for fully radiographed welds to 0.7 for spot-examined welds, directly impacting the required thickness.
How to use
A vessel has internal pressure P = 1.5 MPa, internal diameter = 600 mm (R = 300 mm), allowable stress S = 138 MPa, joint efficiency E = 1.0, and corrosion allowance C_a = 3 mm. t = (P·D) / (2·S·E − 1.2·P) + C_a = (1.5 × 600) / (2 × 138 × 1.0 − 1.2 × 1.5) + 3. Numerator: 900. Denominator: 276 − 1.8 = 274.2. t = 900 / 274.2 + 3 ≈ 3.28 + 3 = 6.28 mm. Round up to the next standard plate thickness, e.g., 8 mm.
Frequently asked questions
What is the ASME formula for minimum wall thickness of a cylindrical pressure vessel?
ASME Section VIII, Division 1 specifies t = (P·R) / (S·E − 0.6·P) + C_a for the shell thickness, where P is design pressure, R is inside radius, S is the maximum allowable stress for the material at design temperature, E is the weld joint efficiency, and C_a is the corrosion allowance. The 0.6·P term is an empirical correction for the longitudinal stress component. The calculated thickness is a minimum; the actual plate selected must be equal to or greater than this value after deducting any mill undertolerance.
How does weld joint efficiency affect the required wall thickness of a pressure vessel?
Joint efficiency E directly reduces the effective allowable stress of the shell — a lower E means a thicker wall is required. Fully radiographed butt welds earn E = 1.0, spot-examined welds earn E = 0.85, and welds with no radiographic examination earn E = 0.70. For example, dropping from E = 1.0 to E = 0.70 for the same design conditions can increase the required wall thickness by over 40%. Radiographic inspection adds cost but allows thinner, lighter vessels.
Why is corrosion allowance added to pressure vessel wall thickness calculations?
Pressure vessel shells lose material over time due to corrosion, erosion, or chemical attack from the process fluid. The corrosion allowance is an extra thickness — typically 1.5 mm to 6 mm depending on the service — added on top of the structurally required thickness to ensure the vessel remains safe throughout its design life. Without it, a vessel that meets stress requirements on day one could fall below minimum thickness after years of service. The allowance is determined by the corrosion rate of the material in the specific process environment, multiplied by the intended service life.