Shell Course Design for Atmospheric Storage Tanks
Standard: 8 ft (96" plate width). Range: 4-10 ft.
Water = 1.0, Crude oil = 0.82-0.95, Brine = 1.05-1.20
Default = tank height (worst case). May be less for overflow protection.
Typical: 1/16" (0.0625). Sour service: 1/8" (0.125).
Auto-filled from material selection. May override manually.
0.85 = single-welded butt (typical), 1.0 = double-welded butt with RT
Typically 1.0 (water). Test fill to top of shell.
Understand API 650 design basis, 1-Foot vs VDP methods, material selection, wind and seismic design, and construction practices
The 1-Foot Method (API 650 Section 5.6.3.1) calculates shell thickness at a point one foot above the bottom of each shell course. This simplified approach uses the formula td = 2.6 D(H-1)G / (Sd E) + CA, where D is tank diameter, H is design liquid level above the calculation point, G is specific gravity, Sd is allowable stress, E is joint efficiency, and CA is corrosion allowance. It is the most commonly used method for tanks up to 200 feet in diameter.
The Variable-Design-Point Method (API 650 Section 5.6.3.3) calculates shell thickness at a variable point that depends on plate geometry rather than a fixed 1-foot location. It typically yields thinner upper courses compared to the 1-Foot Method, resulting in material savings on large tanks. The 1-Foot Method is simpler and more conservative; the VDP method requires iterative calculation but is more economical for large-diameter tanks.
API 650 Table 5.6.1.1 specifies minimum shell plate thicknesses based on tank diameter: 3/16 inch (4.8 mm) for tanks up to 50 ft diameter, 1/4 inch (6.4 mm) for 50-120 ft diameter, and 5/16 inch (7.9 mm) for tanks over 120 ft diameter. These minimums apply regardless of the calculated design or hydrostatic test thickness.
Common API 650 shell materials include ASTM A36 (Sd = 23,200 psi), A283 Grade C (Sd = 21,300 psi), A516 Grade 60 (Sd = 22,700 psi), A516 Grade 70 (Sd = 25,400 psi), and A573 Grade 70 (Sd = 25,400 psi). A516-70 is the most widely used for new construction due to its good strength, weldability, and notch toughness.