Allowable stress, joint efficiency, and corrosion allowance above are all directly editable — override the database defaults whenever you have project-specific values. Design margin is implicit in the ASME allowable stress (already includes a safety factor of ≈3.5 on tensile).
ASME VIII Div.1 Formulas & Reference
| Component | Required thickness | Code |
|---|---|---|
| Cylindrical Shell | t = P·R / (S·E − 0.6P) | UG-27 |
| Ellipsoidal (2:1) | t = P·D / (2S·E − 0.2P) | UG-32 |
| Torispherical | t = 0.885·P·L / (S·E − 0.1P) | UG-32 |
| Hemispherical | t = P·L / (2S·E − 0.2P) | UG-32 |
| Conical | t = P·D / (2·cosα·(S·E − 0.6P)) | UG-32(g) |
P = design pressure, R = inside radius, D = inside diameter, L = inside crown/sphere radius, S = allowable stress at design temperature, E = joint efficiency, α = half-apex angle. All dimensions are taken in the corroded condition, and the corrosion allowance is added to the required thickness to get the nominal thickness. MAWP is the same formula solved for P using the available thickness.
- Cylindrical shell: thin-wall formula valid only while P ≤ 0.385·S·E. Above that, use the thick-wall Appendix 1-2 equations.
- Ellipsoidal / torispherical: minimum thickness ratio t/L ≥ 0.002.
- Hemispherical: thin-shell formula valid while t ≤ 0.356·L or P ≤ 0.665·S·E.
- Conical: half-apex angle α ≤ 30°; beyond that a toriconical transition or reinforcement is required.
What This Means
The required thickness is the thinnest the wall is allowed to get and still hold the design pressure at temperature. The final thickness adds your corrosion allowance on top, so the vessel still passes at the end of its life when some metal has been eaten away. MAWP is the flip side: given the steel you actually have, the highest pressure it may legally see.
Why It Matters
This is the calculation that keeps a pressure vessel from becoming a bomb. Under-thickness, the wrong joint efficiency, or a forgotten corrosion allowance all eat directly into the safety margin between operating pressure and burst. The MAWP you compute here is what sets the relief valve and the nameplate — get it wrong and the vessel is either over-protected and useless or under-protected and dangerous.
Common Design Mistakes
- Using outside diameter instead of the inside diameter the code formulas call for — it under-predicts thickness.
- Ignoring the corrosion allowance, or adding it before the formula instead of after.
- Using a joint efficiency higher than the actual radiography level justifies.
- Mixing unit systems — feeding psi into an MPa formula is a classic and costly slip.
Field Notes
Order plate to the next standard thickness above the calculated final value, not the exact number — mills roll in steps and you want some margin for forming thinning on the heads. Formed heads thin at the knuckle, so fabricators often order them a gauge heavier than the shell. And remember the shell and each head can be different thicknesses; the vessel's MAWP is governed by whichever component is weakest, so check them all.
Inside, in the corroded condition. UG-27 and UG-32 are written in inside radius/diameter. Using OD under-predicts the required thickness and is a classic unsafe error.
Maximum Allowable Working Pressure — the highest pressure the weakest part can hold at design temperature in the corroded state. It sets the relief valve and the nameplate rating.
Per UW-12: 1.0 for full radiography of a double-welded butt joint, 0.85 for spot radiography, 0.70 for no radiography. Match it to the examination you will actually perform.
No. It covers UG-27/UG-32 internal-pressure thickness only. A complete design also addresses external pressure, nozzles and reinforcement, flanges, supports, and hydrotest — confirm with the full code and a qualified engineer.
Pressure Vessel Thickness Guide
3 topics • ASME VIII Div.1 internal pressureSizing a pressure vessel wall is one of the most consequential arithmetic problems in process engineering. The formulas in ASME Section VIII Division 1 look simple — a pressure, a radius, an allowable stress — but every term hides a decision: which diameter, what temperature, how much corrosion, what joint efficiency. Get the inputs right and the result is a wall that holds for decades; get one wrong and the margin that keeps the vessel safe quietly disappears.
This guide walks through how the thin-wall membrane formulas come about, how the shell and the five head types differ, and how corrosion allowance and MAWP turn a single thickness number into a full picture of the vessel's working limits.