FieldEngineersKitEngineering Reference & ASME Code Basis
1. Core Formula & Variable Definitions
API Standard 650 — Welded Tanks for Oil Storage, 13th Edition — Section 5.6.3 Calculation by 1-Foot Method · API Standard 650 — Section 5.6.1.1 / Table 5.2a & 5.2b Minimum Shell Thickness · API Standard 650 — Section 5.6.4 Variable-Design-Point Method (out of scope — diameter screen)
Design thickness (SI): td = 4.9·D·(H−0.3)·G / (Sd·E) + CA
Hydrotest thickness (SI): tt = 4.9·D·(H−0.3) / St + CA
USC: replace 4.9→2.6 and (H−0.3)→(H−1) with D, H in ft and stresses in psi
Governing plate: trequired = max(td, tt, tmin,API), tnom = CeilToCommercialPlate(trequired)
API Standard 650 — Section 5.6.3 1-Foot Method · Table 5.2a / 5.2b minimums
H is the height from the bottom of the course under consideration to the design liquid level. Course-by-course screening uses successive H reductions by course height. Shell mass uses π·D·h_course·t_nom·ρ with ρ = 7850 kg/m³. Screening only — confirm with the project API 650 edition.
- t_d (Design shell thickness) — Product design thickness including CA (mm / in).
- t_t (Hydrostatic test thickness) — Hydrotest condition thickness including CA (mm / in).
- D, H (Diameter / liquid height) — Nominal inside diameter and liquid height above course bottom (m / ft).
- G, E (Specific gravity / joint efficiency) — Design specific gravity (water = 1.0) and weld joint efficiency (0.70–1.0).
- S_d, S_t (Allowable stresses) — Product design and hydrotest allowables from API 650 Table 5.2a/5.2b for the selected plate.
- t_min, t_nom (Code floor / commercial plate) — Table 5.2a/b minimum by diameter; t_nom is next mill plate ≥ t_required.
2. Screening Rules Matching This Calculator
API 650 §5.6.3 1-foot equations, Table 5.2a/5.2b minimum shell thickness by diameter, and commercial plate ceil. Variable-Design-Point, wind girders, and seismic buckling are out of scope.
Bottom course hero ≈ 12 mm · 61.7 t with t_d ≈ 11.00 mm and t_t ≈ 10.42 mm.
Larger tanks require Variable-Design-Point Method (§5.6.4) for final design.
D < 15 m → 5 mm; 15–36 m → 6 mm; 36–60 m → 8 mm; D > 60 m → 10 mm.
No intermediate wind girders, annular plates, or appendix seismic checks.
Quick Reference Lookup Table
| Quantity | Value | Unit | Notes |
|---|---|---|---|
| Bottom t_d | 11.00 | mm | 4.9 D (H−0.3) G /(S_d E) + CA |
| Bottom t_t | 10.42 | mm | 4.9 D (H−0.3)/S_t + CA |
| API 650 t_min | 6 | mm | 15 ≤ D < 36 m |
| Bottom t_nom | 12 | mm | Ceil commercial plate |
| Courses | 6 | — | 15 / 2.5 |
| Shell mass | 61.7 | t | π D h t_nom ρ · Σ courses |
| 40 m · 18 m · A516-70 · G=1 · CA=3 · E=1 | t_d 22.0 · t_nom 22 | mm | S_d = 183 MPa |
| 60 ft · 48 ft · A36 · CA=1/16" | t_d 0.378 · t_nom 0.500 | in | Course height 8 ft |
| 100 ft · 50 ft · A283-C · CA=1/8" | t_d 0.766 · t_nom 0.875 | in | D ≈ 30.5 m < 60 m screen |
| 1-foot diameter limit | 60 m / 200 ft | — | Above → §5.6.4 VDP |
Hero reports bottom-course commercial plate and total shell mass. Default duty asserts t_d ≈ 11.00 mm, t_t ≈ 10.42 mm, t_nom = 12 mm, mass ≈ 61.7 t. Spec LLM heroes that omit course mass or mis-state t_t are discarded.
3. Plate Metallurgy & Allowable Stress Limits
API 650 Table 5.2a/5.2b screening allowables for common carbon and HSLA tank plate. Confirm group, thickness category, and design metal temperature against the project edition.
| Material Group | Temperature Range | Allowable Stress / Limit | Engineering Notes |
|---|---|---|---|
| ASTM A283 Grade C | Ambient design metal ≤ ~90 °C screening | S_d = 137 MPa · S_t = 154 MPa | USC: 20,000 / 22,500 psi. Lower-cost CS plate. |
| ASTM A36 | Ambient design metal ≤ ~90 °C screening | S_d = 160 MPa · S_t = 171 MPa | USC: 23,200 / 24,900 psi. Default calculator material. |
| ASTM A516 Grade 70 | Ambient design metal ≤ ~90 °C screening | S_d = 183 MPa · S_t = 196 MPa | USC: 26,600 / 28,500 psi. Pressure-vessel quality CS. |
| ASTM A537 Class 1 | Ambient design metal ≤ ~90 °C screening | S_d = 200 MPa · S_t = 214 MPa | USC: 29,000 / 31,100 psi. HSLA quenched & tempered plate. |
Code Applicability & Safety Boundaries
- Not a substitute for Variable-Design-Point (§5.6.4) when D > 60 m
- Allowables are ambient screening — check elevated-temperature tables when required
- Joint efficiency E must match the radiographic / examination plan
4. Step-by-Step Worked Example
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Worked Example — 20 m × 15 m A36 Tank
Size bottom and upper shell courses for a 20 m diameter tank with 15 m design liquid height, 2.5 m courses, G = 0.85, ASTM A36, E = 0.85, CA = 2 mm using the API 650 1-foot method.
Bottom-course design thickness
Hydrotest thickness
Code floor and commercial plate
Course stack and shell mass
5. How to calculate API 650 tank shell thickness (1-foot method)
- 1
Enter tank diameter, liquid height, and course height
Use inside diameter D and design liquid height H. Course height sets how many shell rings are stacked.
- 2
Select plate material, joint efficiency, G, and CA
Choose A283-C / A36 / A516-70 / A537-1, set E from the RT plan, specific gravity, and corrosion allowance.
- 3
Review bottom-course t_nom and shell mass
Hero shows commercial bottom-course thickness and total shell weight. Summary lists t_d, t_t, and t_min.
- 4
Check course MTO table and diameter screen
Use the course step chart/table for plate take-off. If D > 60 m, apply Variable-Design-Point instead of 1-foot.
6. Frequently Asked Questions & Technical References
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Section 5.6.3 calculates each shell course using liquid height H measured from the bottom of that course, with the (H − 1 ft) or (H − 0.3 m) offset built into the code equations.
When D > 60 m (200 ft), API 650 requires the Variable-Design-Point Method (§5.6.4) for final shell design. This calculator still shows a screening thickness but raises a diameter-limit warning.
t_required = max(t_d, t_t, t_min). If calculated thicknesses fall below the diameter-based floor, the course is raised to t_min before commercial plate selection.
No. Mass is a screening take-off from π·D·h·t_nom·ρ. Fabrication scrap, weld overlay, and wind-girder steel are not included.