Operation, Maintenance, and Safety
Keeping the tank inside its design envelope
Operations questions are really design-envelope questions. The maximum design liquid level is a structural limit; the maximum operating level sits below it with margin for alarm response, and the fill height corresponding to a given volume changes with product specific gravity because shell thickness demand is proportional to G. Loading a tank with a heavier product than it was designed for can overstress the shell even though the level never moves.
Venting: atmospheric and low-pressure tanks need normal and emergency venting sized per API 2000. Blocked vents, plugged flame arrestors, stuck pressure/vacuum valves, or a failed floating-roof drain are classic failure paths - overpressure buckles the shell-to-roof junction, vacuum collapses the roof, and a failed drain can sink a floating roof.
Maintenance: roof drains, rim seals, water draw-off, mixers, heating coils, foundations, and dikes. External cathodic protection (API 651) protects a bottom on grade, not the shell above grade; internal linings (API 652) need their own inspection program.
Safety: tank entry and cleaning per API 2015/2016 - isolation and lockout, gas-freeing, ventilation, oxygen near 19.5% to 23.5%, combustible gas well below the LEL before hot work, and bonding/grounding for static. Dike valves normally closed and secured; secondary containment kept drained and clean.
Traps: treating the design liquid level as the operating limit; assuming vents only handle liquid movement; assuming cathodic protection covers shell corrosion; ignoring the specific-gravity check on a product change.
Rule of thumb: t_min moves one-for-one with G - a product 10% heavier consumes 10% of the shell's thickness margin, so a tank already near t_min has no room for a product change without engineering review.