Settlement Evaluation per API 653 & API 650 Appendix B
Total weight of shell, roof, and appurtenances
Weight of stored product at maximum fill level
Allowable bearing: 1500-3000 psf (clay), 3000-6000 psf (sand), 6000+ (rock)
Enter settlement readings at 8 equally-spaced points (every 45°) around the tank shell perimeter, starting from North (Point 1) and proceeding clockwise. Enter elevation or settlement values in inches. Positive values = downward settlement.
Understand tank settlement types, API 653 evaluation criteria, foundation design, and remediation methods
API 653 Section 12.3 provides settlement evaluation criteria. The maximum allowable differential settlement between adjacent measurement points is typically limited to 1 inch per 10 feet of arc length for rigid bottom tanks and 2 inches per 10 feet for flexible bottoms. Planar tilt is limited to 1/100 to 1/200 of the tank diameter depending on diameter, and uniform settlement is generally acceptable up to 6-12 inches if uniform and the connected piping can accommodate it.
The out-of-plane settlement index measures the deviation of each measurement point from the best-fit tilted plane of the tank bottom. After removing the planar tilt component from the settlement data, the remaining deviations represent true localized distortion. The out-of-plane index is the maximum deviation divided by the arc length between adjacent measurement points, expressed in inches per 10 feet.
The three common tank foundation types are ringwall (concrete ring supporting the shell with compacted fill center), mat/slab (full concrete pad under the entire tank bottom), and gravel pad (compacted crushed stone ring or full pad). Ringwall foundations are most common for large tanks, slab foundations for heavy or critical-service tanks, and gravel pads for small field tanks or where soil conditions are favorable.
API 653 recommends settlement monitoring during hydrostatic test (before, during fill, at full, and after drainage), then annually for the first 5 years after construction, and every 5 years thereafter for tanks on stable foundations. Tanks showing active settlement should be monitored quarterly or semi-annually. Settlement rate exceeding 1 inch per year typically warrants engineering investigation and more frequent monitoring.