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)
22.11.2.2 The volumetric capacity of the diked area shall not be less than the greatest amount of ignitible (flammable or combustible) liquid that can be released from the largest tank within the diked area, assuming a full tank.
If the tanks are equalized, they function as a single hydraulic system. We MUST ensure our secondary containment has the capacity to hold the contents of the entire group of interconnected tanks, as a failure at the lowest point of the system (or a breach in a tank) will draw down the contents of all linked vessels.
While standard secondary containment for a single tank is typically sized at 110% of the largest tank’s capacity, interconnected systems effectively treat that group as a single, massive “container.” Under EPA SPCC rules and standard interpretations of NFPA 30, the containment must be designed to accommodate the maximum potential discharge of the connected system.
Our assessment must include the scenario where an interconnecting pipe fails. In this case, the equalization lines themselves become the path for the liquid to reach the containment area. For the purposes of our volume calculations, sum the capacities of all tanks that can equalize into the containment area. Ensure the containment capacity is at least 110% of this total interconnected volume.