This is a step-by-step, illustrative walkthrough of the [ELI-REVIEW: ASME Section VIII, Division 1] shell and head thickness calculation PRELIM runs, for a representative [ELI-REVIEW: 48 in.] outside-diameter ammonia accumulator. Every number below is a plausible, rounded placeholder value, marked [ELI-REVIEW], for the site owner to check or replace before this page is published — it is not a validated engineering calculation and must not be used for an actual project. To run the same calculation on your own vessel data, use PRELIM directly.
1. Vessel inputs
| Parameter | Assumed value |
|---|---|
| Diameter basis | Outside diameter (OD) |
| Outside diameter | [ELI-REVIEW: 48 in.] |
| Shell tangent-to-tangent length | [ELI-REVIEW: 120 in.] |
| Orientation | Horizontal |
| Design pressure | [ELI-REVIEW: 250 psig] |
| Head type | [ELI-REVIEW: 2:1 ellipsoidal] |
| Material | [ELI-REVIEW: SA-516-70 carbon steel] |
| Allowable stress, S | [ELI-REVIEW: approximately 20,000 psi at design temperature] |
| Longitudinal joint efficiency, E | [ELI-REVIEW: 0.85 (spot radiography)] |
| Head joint efficiency | [ELI-REVIEW: unwelded (no-seam) head] |
| Corrosion allowance | [ELI-REVIEW: 1/8 in. (0.125 in.)] |
2. Required shell thickness ([ELI-REVIEW: UG-27], OD basis)
Using the relationship described in the [ELI-REVIEW: UG-27] shell thickness formula, rearranged for an outside-diameter basis: [ELI-REVIEW: denominator = (S × E) − (0.6 × P) = (20,000 × 0.85) − (0.6 × 250) = 17,000 − 150 = 16,850; required thickness before rounding = P × (Do ÷ 2) ÷ (denominator + P), plus corrosion allowance = 250 × 24 ÷ 17,100 + 0.125 ≈ 0.351 + 0.125 ≈ 0.476 in.]. Rounding up to the next standard plate gauge gives a nominal shell thickness of [ELI-REVIEW: 0.5 in. (1/2 in.)].
3. Required head thickness (OD basis)
With an unwelded, no-seam head and a shell seam at [ELI-REVIEW: E = 0.85 or higher], the head’s own joint efficiency is taken as [ELI-REVIEW: 1.0]. For a [ELI-REVIEW: 2:1 ellipsoidal] head on an outside-diameter basis: [ELI-REVIEW: denominator = (2 × S × E_head) + (1.8 × P) = (2 × 20,000 × 1.0) + (1.8 × 250) = 40,000 + 450 = 40,450; required thickness before rounding = P × Do ÷ denominator, plus corrosion allowance = 250 × 48 ÷ 40,450 + 0.125 ≈ 0.297 + 0.125 ≈ 0.422 in.]. Rounding up to the next standard plate gauge also gives [ELI-REVIEW: 0.5 in. (1/2 in.)] for the head, which happens to match the shell in this example.
4. Nominal thickness and as-built inside diameter
The governing nominal thickness is the larger of the shell and head results, so both shell and head are set to [ELI-REVIEW: 0.5 in.] nominal plate in this example. The as-built inside diameter follows directly from the outside diameter and nominal shell thickness: [ELI-REVIEW: 48 in. − (2 × 0.5 in.) = 47 in. inside diameter].
5. MAWP back-check, corroded condition
Back-calculating MAWP on the corroded thickness (nominal minus corrosion allowance) checks that the as-built vessel meets or exceeds the 250 psig design pressure — see MAWP vs. design pressure. For this example: [ELI-REVIEW: corroded thickness = 0.5 − 0.125 = 0.375 in.; shell MAWP ≈ 267 psig; head MAWP ≈ 317 psig; the lower of the two, the shell result, governs at roughly 267 psig], which clears the 250 psig design pressure as required.
| Item | Value |
|---|---|
| Governing nominal thickness | [ELI-REVIEW: 0.5 in.] |
| As-built inside diameter | [ELI-REVIEW: 47 in.] |
| Shell MAWP (corroded) | [ELI-REVIEW: ≈267 psig] |
| Head MAWP (corroded) | [ELI-REVIEW: ≈317 psig] |
| Governing MAWP | [ELI-REVIEW: ≈267 psig (shell governs)] |
6. What this walkthrough leaves out
PRELIM also runs an external-pressure check, an MDMT impact-test screen, a mandatory full-radiography-by-thickness check, and vessel volume/weight and plate-cutting layout — all left out of this page to keep the thickness calculation itself easy to follow. Those additional checks can change the governing thickness or joint efficiency for a real design and are exactly why the full tool, not this walkthrough, is the right place to size an actual vessel.
Run it
Run this exact thickness sequence, plus the MDMT, external-pressure, and radiography checks left out above, against your own vessel data in PRELIM, free.
Open PRELIM →FAQ
Is this worked example a validated thickness calculation?
No. Every value on this page is a plausible, rounded placeholder for illustration, marked for the site owner's review, and none of it has been checked against a real project's data or the current code edition. It must never be used for an actual vessel.
Why did the head and shell come out to the same nominal thickness?
In this example the two independent calculations happened to round up to the same standard plate gauge. That is not guaranteed in general; the head and shell required-thickness values are computed independently and can land on different stock gauges.
Why is corrosion allowance added after rounding to a stock gauge, not before?
PRELIM rounds the pressure-required thickness up to a stock plate gauge first, then adds corrosion allowance to that nominal figure — see corrosion allowance basics for why the two are kept separate rather than combined into a single rounding step.
What would change if this vessel used full radiography instead of spot?
A higher joint efficiency ([ELI-REVIEW: up to 1.0 for full radiography]) would reduce the required thickness for the same design pressure, at the cost of the additional radiography itself. PRELIM reports what a full-RT re-design would allow as part of its results.