49 CFR 179.220-3 - Type.
(a) Tanks built under these specifications must consist of an inner container, a support system for the inner container, and an outer shell.
(b) The inner container must be a fusion welded tank of circular cross section with formed heads designed convex outward and must have a manway on top of the tank as prescribed herein. When the inner container is divided into compartments, each compartment must be considered a separate container.
(c) The outer shell must be a fusion welded tank with formed heads designed convex outward.