Monolithic Oil Tanks – Water Tanks – Ethanol Tanks – Other Liquids Monolithic Tanks — The 25,000 barrel insulated concrete oil tank depicted here is approximately 78 feet in diameter and 58 feet high. (1 barrel = 42 gallons) Tank’s Layers — This cutaway shows the tank’s layers from the inside out: Polyurea; primer; concrete; urethane foam insulation; Airform on the exterior.