These are all-steel tanks constructed and investigated to UL 58, «Steel Underground Tanks for Flammable and Combustible Liquids,» and are not provided with an external corrosion-protection system investigated by UL. These tank constructions may be primary (single wall) or secondary (double wall) types, and are suitable for containment of general fuels and similar flammable and combustible liquids, as identified in the (*) liquid ratings:
Underground tank for (*) — A steel primary containment tank with one or more compartments.
($) secondary-containment underground tank for (*) — A steel primary containment tank wrapped within an external steel shell that provides both secondary containment and interstitial monitoring.