A storage tank is a container, usually for holding liquids or compressed gases. The term can be used for both reservoirs (artificial lakes and ponds), and for manufactured containers. The usage of the word tank for reservoirs is common or universal in Indian English, and moderately common in British English. In other countries, the term tends to refer only to artificial containers.
Storage tanks may often operate under very little or no pressure, distinguishing them from pressure vessels. Storage tanks are often vertical cylindrical in shape, perpendicular to the ground with flat bottoms and fixed domed roofs, hinged or floating roofs. There are many environmental considerations and regulations applicable to the design and operation of storage tanks, often depending on nature the fluid contained within. Aboveground storage tanks differ from underground varieties in the kinds of regulations that are applied.
Storage tanks are available in many shapes: vertical and horizontal cylindrical; open top and closed top; flat bottom, cone bottom, slope bottom and dish bottom; but large tanks tend to be vertical cylindrical, or to have rounded corners to easier withstand hydraulic hydrostatically induced pressure of contained liquid. Most container tanks for handling liquids during transportation are designed to handle varing degrees of pressure.
A large tank is sometimes mounted on a lorry, truck or on an articulated lorry trailer, all are commonly referred to as tankers.
Since most liquids can spill, evaporate, or seep through even the smallest opening, special consideration must made for their safe and secure handling. This usually involves building a secondary containment dike around the tank, so that any leakage may be safely contained.
Some storage tanks need a floating roof in addition to or in lieu of the fixed roof and structure. This floating roof rises and falls with the liquid level inside the tank, thereby decreasing the vapor space above the liquid level. Floating roofs are considered a safety requirement as well as a pollution prevention measure for many industries including petroleum refining.