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A storage tank is a construction or a container, usually for holding liquids, sometimes for 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.

In the USA, storage tanks operate under no (or very little) pressure, distinguishing them from pressure vessels. Storage tanks are often cylindrical in shape, perpendicular to the ground with flat bottoms, and a fixed or floating roof. There are usually many Environmental regulations applied to the design and operation of storage tanks, often depending on nature the fluid contained within. Aboveground storage tanks (AST) differ from underground (UST) varieties in the kinds of regulations that are applied.

Reservoirs can be covered, in which case they may be called covered or underground storage tanks or reservoirs. Covered water tanks are common in urban areas.

Container tanks can be many shapes, but large tanks tend to be cylindrical, or to have rounded corners, to easier withstand hydraulic pressure of contained liquid. Most container tanks for handling liquids during transportation are pressure vessels.

A large tank is sometimes mounted on a lorry or on an articulated lorry trailer, which is then called a tanker.

Contents 1 Special features 2 Types of tank 2.1 High pressure 2.2 Milk tank 2.3 Septic tank 2.4 Mobile "storage" tanks 3 Materials of Construction 4 See also 5 Images 6 Etymology 7 External links 8 References

Since most liquids can spill, evaporate, or seep through even the tiniest opening, special consideration must made for their safe and secure handling. This usually involves building a bunding, or 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.

Main article: Gas cylinder

Cylinders are not storage tanks at all, but pressure vessels. In the case of a liquefied gas such as hydrogen or natural gas, or a compressed gas such as compressed natural gas or MAPP, the storage tank must be made to withstand the sometimes immense pressures exerted by the contents. These tanks may be called cylinders

Main article: Bulk tank

In dairy farming a bulk milk cooling tank is a large storage tank for cooling and holding milk at a cold temperature until it can be picked up by a milk hauler. The bulk milk cooling tank is an important milk farm equipment. It is usually made of stainless steel and used every day to store the raw milk on the farm in good condition. It must be cleaned after each milk collection. The milk cooling tank can be the property of the farmer or being rented to the farmer by the dairy plant.

Main article: Septic tank

A septic tank is part of a small scale sewage treatment system often referred to as a septic system,. It consists of the tank and a septic drain field. Wastewater enters the tank where solids can settle and scum floats. Anaerobic digestion occurs on the settled solids, reducing the volume of solids. The water released by the system is normally absorbed by the drain field without needing any further treatment.

While not strictly a "storage" tank, mobile tanks share many of the same features of storage tanks. Also, they must be designed to deal with a heavy sloshing load and the risk of collision or other accident. Some of these include ocean-going oil tankers and LNG carriers; railroad tank cars; and the road and highway traveling tank trucks. Also included are the holding tanks which are the tanks that store toilet waste on RVs and boats.

While steel remains one of the most popular choices for tanks, GRP and thermoplastic tanks are increasing in popularity. They offer lower build costs and greater chemical resistance, especially for storage of speciality chemicals. There are several relevant standards, such as British Standard 4994 (1989) and DVS (German Welding Institute) 2205 which give advice on wall thickness and quality of final product.

• Ballast tank
• Irrigation tank in India. Some Indian language words similar to "tak" or "tank" mean "reservoir for water".
• BS 4994: British Standard 4994:1987 specification for design and construction of vessels and storage tanks in reinforced plastics
• Fuel tank
• Oil depot
• Pressure vessel
• Rainwater tank
• Tank blanketing
• Underground storage tank
• Vacuum flask
• Water heating
• Bunding

Oxygen and MAPP gas cylinders	Steel pressure vessel	Milk cooling tank	Fuel tanks
Septic tank	Tanker lorry	Several large tanks at an airport. For scale, note concrete highway barriers.

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The word "tank" originally meant "artificial lake" and came from India, perhaps via Portuguese tanque. It may have some connection with:

• Some Indian language words similar to "tak" or "tank" and meaning "reservoir for water". In Sanskrit a holding pond or reservoir is called a tadaka. Gujarati talao means "man-made lake". These uses of the word were incorporated into the English language.^[1]
• The Arabic verb istanqa`a اِسْتَنْقَعَ = "it [i.e. some liquid] collected and became stagnant".