liminfoPressure Vessel Calculator
Free web tool: Pressure Vessel Calculator
Details
ASME Sec VIII Div 1 Formulas
About Pressure Vessel Calculator
The Pressure Vessel Wall Thickness Calculator computes the minimum required shell thickness and head thickness for cylindrical pressure vessels using the ASME Boiler and Pressure Vessel Code, Section VIII Division 1 formulas. It calculates two stress directions simultaneously: circumferential (hoop) stress, which governs the longitudinal joint and uses t = PR / (SE − 0.6P), and longitudinal stress, which governs the circumferential joint and uses t = PR / (2SE + 0.4P). The governing (larger) value is reported as the required shell thickness before and after adding the corrosion allowance.
Process engineers, mechanical engineers, and inspection professionals use this tool during preliminary vessel design to determine nominal plate thickness before detailed finite-element analysis. Inputs include internal design pressure in MPa, inner diameter in mm, material allowable stress S in MPa, weld joint efficiency E (typically 0.7–1.0 per ASME UW-12), and a corrosion allowance in mm. The tool also calculates the Maximum Allowable Working Pressure (MAWP) based on the resulting shell thickness, providing a quick sanity check.
Four head geometries are supported: 2:1 Semi-Ellipsoidal (the most common for moderate pressure), Hemispherical (the thinnest and most efficient), ASME Flanged and Dished Torispherical (standard for lower pressures), and Flat Head (the thickest, used for manways and handhole covers). Each uses a separate ASME code formula. The tool also reports the D/t ratio and flags when the thin-wall assumption is no longer valid (D/t ≤ 10), indicating that thick-wall (Lamé) equations should be considered.
Key Features
- Computes shell thickness using both ASME circumferential and longitudinal stress formulas simultaneously
- Identifies the governing stress direction (hoop vs. longitudinal) and reports it explicitly
- Calculates head thickness for four ASME geometries: 2:1 ellipsoidal, hemispherical, torispherical (F&D), and flat head
- Adds user-specified corrosion allowance to both shell and head thickness
- Reports MAWP (Maximum Allowable Working Pressure) for the computed shell thickness
- Displays D/t ratio and warns when thin-wall assumption is invalid (D/t ≤ 10)
- Shows the governing ASME Sec VIII Div 1 formulas inline for full transparency
- 100% client-side — no server calls, works on any device with a browser
Frequently Asked Questions
What code does this calculator follow?
The calculator implements the formulas from ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Division 1. The circumferential stress formula is t = PR/(SE − 0.6P) and the longitudinal stress formula is t = PR/(2SE + 0.4P), where P is design pressure, R is inner radius, S is allowable stress, and E is joint efficiency.
What is joint efficiency E and what value should I use?
Joint efficiency E accounts for the welding quality and degree of radiographic examination. Per ASME UW-12: E = 1.0 for full radiography (RT-1), E = 0.85 for spot radiography (RT-2), and E = 0.70 for no radiography (RT-3). Using a lower E value results in a thicker (more conservative) required wall.
Why does circumferential stress govern most cylindrical vessels?
Hoop (circumferential) stress is twice the longitudinal stress in a thin-walled cylinder under the same pressure. The circumferential stress formula denominator (SE − 0.6P) is also smaller than the longitudinal denominator (2SE + 0.4P), making the circumferential thickness requirement larger. Therefore, circumferential stress typically governs shell design.
Which head type is the most material-efficient?
The hemispherical head is the most efficient, requiring roughly half the thickness of the shell for the same pressure rating. The 2:1 semi-ellipsoidal head is close to the shell thickness and is commonly chosen for a balance of cost and efficiency. The torispherical (F&D) head is slightly thicker and is popular for low-pressure storage tanks. The flat head is the least efficient and is mainly used for small-diameter openings like manways.
What is MAWP and how is it calculated?
MAWP (Maximum Allowable Working Pressure) is the maximum pressure at which the vessel is permitted to operate, calculated from the actual selected thickness rather than the minimum required thickness. The calculator computes MAWP as the lesser of the circumferential and longitudinal MAWP values based on the shell thickness including corrosion allowance.
When should I use thick-wall equations instead?
The thin-wall (membrane) approximation is valid when D/t > 10. When D/t ≤ 10, the wall thickness is a significant fraction of the radius, and the stress distribution across the wall becomes non-uniform. In that case, Lamé's thick-wall cylinder equations should be used. The tool explicitly flags this condition with a warning.
What units does the calculator use?
Pressure is in MPa, dimensions (diameter, corrosion allowance) are in mm, and allowable stress is in MPa. These are standard SI-based units used in most international pressure vessel codes. If your data is in different units, convert first: 1 MPa = 145.038 psi, 1 in = 25.4 mm.
Can I use this for external pressure design?
No. This tool is for internal pressure design only. External pressure design (vacuum vessels, jacketed vessels) requires a different set of ASME code rules involving elastic buckling analysis (UG-28) and geometric parameters (L/Do, Do/t). A dedicated external pressure tool should be used for those cases.