When flammable liquid tanks are equalized — meaning they are interconnected by piping that allows fluid levels to equalize between them — we MUST account for this in our secondary containment design. From a regulatory and engineering standpoint, the governing principle is to protect against the worst-case single-event release. If tanks are interconnected, a failure in a single tank or a failure in the interconnecting piping can result in the drainage of the entire interconnected system into the containment area. NFPA’s language in NFPA 30, Chapter 22, Storage of Ignitible (Flammable or Combustible) Liquids in Tanks — Aboveground Storage Tanks is pretty clear when we read it with an open-mind… (emphasis by me)