Storage Tanks (API 620)

A: Governing principle: frangible joints sacrifice the roof to protect the shell. Applied here: if the joint sticks, pressure rises until the next weakest boundary yields, often the lower shell courses per API 620 stress limits. Distractor trap: B appeals to people assuming P/V valves cover all overpressure cases, ignoring rate-of-rise limits.

A: Governing principle: inerting follows dilution, not reaction. Applied here: to drop from 21% to ~2%, one volume change gives the right scale before inefficiencies. Distractor trap: B catches engineers importing purge practices from long pipelines rather than closed tanks.

A: Governing principle: thin shell hoop stress scales with pressure times radius over thickness. Applied here: low pressure but large radius still drives a few millimeters before allowances. Distractor trap: B ignores radius, a common slip from piping intuition.

A: Governing principle: gas contraction lowers pressure at constant volume. Applied here: vacuum relief protects the shell from buckling during cooldown. Distractor trap: B flips the sign, a mistake from confusing heating vs cooling cases.