Tank Design and Construction
The design chain you must be able to reproduce
API 653 governs existing tanks; the original design basis comes from API 650 (or the older API 12 series). You still need the shell formula, because every minimum-thickness and remaining-life calculation starts there.
Shell (1-foot method, US units): t = 2.6 x D x (H - 1) x G / (S x E); D and H in feet, t in inches, S in psi. H is measured from the bottom of the course being checked to the top of the shell, and the (H - 1) moves the design point to one foot above the course's lower seam. The metric form uses 4.9 instead of 2.6 (D, H in m; S in MPa; t in mm). G is the design specific gravity, S the allowable stress (roughly 0.8 x yield or 0.429 x tensile, whichever is lower, with an added cap on the bottom course), and E the joint efficiency (1.0 for full-penetration butt joints).
Other construction concepts: minimum nominal shell thickness by diameter (3/16 in. below 50 ft, up to 3/8 in. at 200 ft and above), annular plates under the shell, shell-to-bottom weld size, nozzle reinforcement, roof type (supported cone, self-supporting, floating), wind girder, seismic provisions in the API 650 seismic appendix, brittle-fracture (Charpy) screening for thick or cold-service plate, and hydrostatic testing of new tanks.
Traps: using H to the top of the shell instead of to the point being evaluated; dropping G; mixing the 2.6 and 4.9 constants; confusing "design thickness" (new plate, with corrosion allowance) with "minimum acceptable thickness" for an existing tank; and assuming API 653 is the new-construction code.
Worked example: D = 100 ft, H = 40 ft, G = 0.9, S = 23,000 psi, E = 1.0: t = 2.6 x 100 x 39 x 0.9 / 23,000 = 0.40 in. (about 10 mm). Because t scales linearly with G, a heavier product eats margin fast.