Tank bottom edge settlement calculator (Bew / Be)
The allowable edge settlement at the shell for a settled area of radius R — Bew, Be or the angled-weld Bα — and whether your measured settlement is within it.
For estimating and checking. The inspector of record is responsible for every number that goes into a report. Verify against the current edition of API 653 — this page was checked against API 653, 5th Edition, Addendum 2 (May 2020). Not a substitute for the standard or for engineering judgment.
Edge settlement is the bottom dropping away from the shell. API 653 Annex B gives an allowable amount of it, B, that depends on the tank diameter, on how far in from the shell the dip reaches (the radius R of the settled area), and on how the bottom plates are welded where it happens. This page reads the allowable from the code figures, compares it with the settlement you measured, and tells you whether the edge weld needs NDE (a visual check plus magnetic particle or liquid penetrant).
- D = 100 ft sits on the 100 ft curve.
- R = 3 ft sits on the R = 3 ft station.
- Bew (Figure B.11) = 3.47 in
- Be (Figure B.12) = 4.20 in
- B ÷ allowable = 2.50 ÷ 3.47 = 72 % → within
- NDE when B ≥ 0.75 × allowable (2.60 in) and B > 2 in: no
| R (ft) | Bew (in) | Be (in) |
|---|---|---|
| 1.5 | 2.00 | 2.00 |
| 2 | 2.70 | 2.98 |
| 2.5 | 3.09 | 3.75 |
| 3 | 3.47 | 4.20 |
| 3.5 | 3.83 | 4.64 |
| 4 | 4.18 | 5.07 |
| 4.5 | 4.52 | 5.49 |
| 5 | 4.84 | 5.89 |
| 5.5 | 5.15 | 6.28 |
| 6 | 5.44 | 6.66 |
The formula
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
D | Tank diameter | ft | Tank record |
R | Radius of the settled area, shell to breakover point | ft | Elevation survey |
B | Measured edge settlement from the original bottom profile | in | Elevation survey / Figure B.7 |
Bew | Allowable, lap weld parallel to the shell (±20°) | in | Figure B.11 |
Be | Allowable, no weld, butt weld or perpendicular lap | in | Figure B.12 |
α | Angle of the lap weld to a tank radius | ° | Figure B.13 |
Worked example
Using the pre-filled example above (US customary units):
- D = 100 ft sits on the 100 ft curve.
- R = 3 ft sits on the R = 3 ft station.
- Bew (Figure B.11) = 3.47 in
- Be (Figure B.12) = 4.20 in
- B ÷ allowable = 2.50 ÷ 3.47 = 72 % → within
- NDE when B ≥ 0.75 × allowable (2.60 in) and B > 2 in: no
- Bew — lap weld parallel to the shell: 3.47 in
- Be — no weld, butt weld or perpendicular lap: 4.20 in
- Allowable edge settlement (Bew): 3.47 in
- B as a share of the allowable: 72 %
- Verdict: Within the allowable
- NDE (VT + MT/PT): Not required
Where the inputs come from
- Diameter comes from the tank record. R is the distance from the shell out to the breakover point, where the bottom stops dropping and meets the original profile — read it off your elevation survey around the dip.
- B is measured down from where the bottom originally sat at the shell, not from a level line or an arbitrary benchmark.
- If your rod reads in feet and tenths, multiply the feet by twelve to get inches (inches divided by twelve gives feet). A reading of two and a half tenths of a foot is a very different number from two and a half inches.
- Weld type is the bottom lap weld in the settled area. A lap weld running parallel to the shell, within a small angle of it, uses Bew. No weld, a butt weld, or a lap weld perpendicular to the shell uses Be. Anything in between is an angle, and Bα blends the two by the angle.
Built by TankForge — API 653 inspection software
TankForge keeps the edge-settlement areas with the bottom survey, applies the same figures and the same NDE trigger as this page to every area, and carries the governing area into the bottom evaluation in the report.
Related
Frequently asked
What is R, exactly?
R is the radius of the settled area measured from the shell to the breakover point — the place where the bottom stops dropping and rejoins the original profile. A shallow dip that reaches far into the tank has a large R, and a large R allows a larger B.
Which weld type should I pick if I am not sure?
Pick the parallel lap weld. It gives the smaller allowable, so it is the conservative choice. Only move to the other options when you have confirmed the weld layout in the settled area.
Why do the numbers sometimes jump between diameters or radii?
The page interpolates between the published points of the figures, and the curves in the figures genuinely change slope. A step in a row of the table is how the published figure behaves, not a rounding error.
What happens when my tank or radius is outside the figures?
Outside the published diameter range the nearest curve is used, a radius beyond the largest station uses that station's curve, and a very small radius gives the floor value. The page tells you whenever one of these applies.
Does a result inside the allowable mean there is nothing to do?
It means the settlement is inside the Annex B allowable. If it reaches the NDE share of the allowable, the page flags that the edge weld needs visual and magnetic particle or liquid penetrant examination, even though the settlement is acceptable.
Where to find labeled drawings
- API 650 Annex H (internal floating roofs) and Annex C (external floating roofs): figures of the roof and its fittings.
- API 653 Annex C checklists: the inspection names for most of these parts (C.1.4 roofs, C.1.5 roof accessories, C.2.8–C.2.9 floating roofs and seals).
- EPA AP-42, Chapter 7.1 (Organic Liquid Storage Tanks): cutaway diagrams of guide poles, deck fittings, rim seals and vents, each part named. EPA AP-42 Chapter 7
- Vendor catalogs for close-up photos: Enardo, Protectoseal, Groth, Varec, Shand & Jurs (vents, hatches, gauges); HMT, Matrix (floating roofs, seals).
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tankforge.us/guide/calc/edge-settlement — TankForge