07.01 Should day tanks be single wall or double wall?

Day tanks should always include a containment structure for at least 125% of the tank capacity, and in some locations by regulation 150%. Day tanks are built to a UL 142 standard for Steel Tanks for Flammable and Combustible liquid storage. This standard covers both single wall and double wall tanks.

Common constructions for day tanks are:

(a) single wall UL 142 steel tank within an open-top steel containment basin. This system has the benefit of the open top construction actining also to serve the containment for all of the pipe, valves, and fittings installed on the top of the tank. The drawbacks are problems of excluding rainwater in exterior locations, and the inability to pressure test the containment.

(b) double wall UL 142 steel tanks with the secondary top sealed to the primary tank. This system has the benefit of pressure testing capability for the secondary containment, and exterior installation capability that excludes rainwater. The drawback is that this type of construction may need a means of containment for the pipe, valves, and fittings mounted on the tank.