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Storage tanks are containers that hold liquids or compressed gases. The term can be used forreservoirs (artificial lakes and ponds), and for manufactured containers. The usage of the word "tank" for reservoirs is uncommon inAmerican English but is moderately common inBritish English. In other countries, the term tends to refer only to artificial containers. In the U.S., storage tanks operate under no (or very little)pressure, distinguishing them frompressure vessels.
Tanks can be used to hold materials as diverse asmilk,water,waste,petroleum,chemicals, and otherhazardous materials, all while meeting industry standards and regulations.[1] 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. Large tanks tend to be vertical cylindrical, with flat bottoms, and a fixed frangible or floating roof, or to have rounded corners transition from the vertical side wall to bottom profile, in order to withstand hydraulic hydrostaticpressure. Tanks built below ground level are sometimes used and referred to asunderground storage tanks (USTs).
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.
Tanks can be mounted on alorry or anarticulated lorry trailer. The resulting vehicle is called aroad tanker (or simply tanker; tank truck in American English).Tank cars are tanks mounted ongoods wagons for rail transportation.
The word "tank" originally meant "artificial lake" and came from India, perhaps viaPortuguesetanque. It may have some connection with:
Whilesteel andconcrete remain the most popular choices for tanks,glass-reinforced plastic,thermoplastic andpolyethylene tanks are increasing in popularity. They offer lower build costs and greater chemical resistance, especially for storage ofspecialty chemicals. There are several relevant standards, such asBritish Standard 4994 (1989), DVS 2205, andASME RTP-1 which give advice on wall thickness, quality-control procedures, testing procedures, accreditation, fabrication and design criteria of final product.
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 vapour 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.
In order for volume measurements from the tank to be used, it typically has a capacity table created using appropriate standards.[3] Each row of capacity table contains fill level value and corresponding volume value (and other related data).
In the U.S., metal tanks in contact with soil and containingpetroleum products must be protected fromcorrosion to prevent escape of the product into the environment.[4] The most effective and common corrosion control techniques for steel in contact with soil iscathodic protection. Outside the United States and at some locations in the United States, elevated tank support foundations with a sand bitumen mix finish are often used. These types of foundations keep the tank bottom plates free from water, therefore preventing corrosion.
In addition to their design and application,maintenance andinspection of storage tanks play a critical role in ensuring their safety and efficiency. Regular inspection is essential for identifying potential issues such as corrosion, leaks, structural weaknesses, and compliance with environmental regulations. These inspections can vary in frequency and detail depending on the type of tank, the material stored, and the regulatory requirements applicable in the location where the tank is used. For instance, tanks storinghazardous materials may require more frequent and thorough inspections compared to those used for non-hazardous materials. Maintenance protocols, including cleaning, repairs, and preventative measures, are equally important to prolong the lifespan of the tanks and prevent environmental contamination or accidents. Advanced technologies, such asremote sensing,ultrasonic testing, and robotic inspection tools, like remotely-operateddrones, are increasingly being employed to enhance the effectiveness and safety of these inspection processes. Understanding and implementing appropriate inspection and maintenance schedules is paramount for operators of storage tanks to ensure operational reliability and adherence to safety standards.[5]
Several environmental regulations apply to the design and operation of storage tanks, often depending on the nature of the fluid contained within.[1] In the U.S., air emissions are typically required to undergo air quality permitting under the federalClean Air Act. Quantification of potential emissions from tanks for permitting purposes is most often accomplished by applying emission equations published in chapter 7.1 of theEnvironmental Protection Agency'sAP-42 (Compilation of Air Pollutant Emissions Factors from Stationary Sources).
Since most liquids can spill or seep through even the smallest opening, special consideration must be made for their safe and secure handling. This usually involves building abunding, or containment dike, around the tank, so that any leakage may be safely contained.
An atmospheric tank is a container for holding a liquid atatmospheric pressure. The major design codes forwelded atmospheric tanks areAPI 650 and API 620. API 653 is used for analysis of in-service storage tanks. In Europe the applicable design code isEN 14015, which uses load cases fromEurocode 3 (EN 1993), part 4-2.
In the case of a liquefied gas such ashydrogen orchlorine, or a compressed gas such ascompressed natural gas orMAPP, the storage tank must be made to withstand the sometimes-considerable pressures exerted by the contents. These tanks, beingpressure vessels, are sometimes excluded from the class of "tanks". Container tanks for handling liquids during transportation are often designed to handle varying degrees of pressure.
One form ofseasonal thermal energy storage (STES) is the use of large surface water tanks that are insulated and then covered with earth berms to enable storage of seasonal solar-thermal heat that is collected primarily in the summer for all-year heating.[6] A related technology has become widespread in Danish district heating systems. The thermal storage medium is gravel and water in large, shallow, lined pits that are covered with insulation, soil and grass.[7]
Ice and slush tanks are used for short-term storage of cold for use in air conditioning, allowing refrigeration equipment to be run at night when electric power is less expensive, yet provide cooling during hot daytime hours.
A bulk milk cooling tank is a storage tank located in a dairy farm's milkhouse used for cooling and holding fluid milk at a low temperature until it can be picked up by a milk hauler. Since milk leaves the udder at approximately 35 °C, milk tanks are needed to rapidly cool fresh raw milk to a storage temperature of 4 °C to 6 °C, thereby slowing growth of microorganisms.[8] Bulk milk cooling tanks are usually made of stainless steel and are constructed to sanitary standards. They must be cleaned after each milk collection. The milk cooling tank may be the property of the farmer, or may be rented by the farmer from a dairy plant.
A septic tank is part of a small-scalesewage treatment system often referred to as a septic system. Septic systems are commonly used to treat wastewater from homes and small businesses in rural and suburban areas.[9] It consists of the tank and aseptic drain field. Waste water 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-goingoil tankers andLNG carriers; railroadtank cars; and roadtankers. Also included are the holding tanks which are the tanks that storetoilet waste onRVs,boats and aircraft.
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Tanks forcrude oil and oil-based fuels are chosen according to theflash point of the material. If the material is not a liquefied gas, such asLPG, tanks are atmospheric and generally come in two types:
Liquefied gases (such as LPG,butane,propylene, etc.) may be stored inspherical tanks (or Horton spheres).
Typical classification codes used for tanks in a refinery are:
Chemical tanks are storage containers forchemicals widely used within thechemical industry. They come in a variety of sizes and shapes, and are used for staticstorage and transport of bothraw materials and finished chemical products.
A chemical tank is of necessity designed for a specific chemical. Chemicals have variablecorrosion potentials, so the size and features of chemical tanks are diverse. Chemical resistance is usually the first priority in designing chemical tanks. Selected materials have to be as resistant to the chemical stored as design and economics allow. This includes selection of smaller features such asgaskets andplumbing materials. Other parameters to be taken in consideration are heat, cold, vacuum, pressure, exothermicreactivity and the inherent aggressive nature of the chemical (acids, caustic, etc.).Secondary spill containment is a back-up strategy sometimes used to mitigate potential failure of the primary container. The typical profile of a vessel with secondary containment is a primary vessel with an exterior vessel shell encompassing the primary vessel with at least 100% capacity. Secondary vessels are available inpolyethylene,fiberglass and metal materials. Secondary containment tank systems are suggested for all aggressive chemicals.
There have been numerous catastrophic failures of storage tanks, one of the most notorious being that which occurred atBoston on January 14, 1919. A large tank had only been filled eight times when it failed, and the resulting wave ofmolasses killed 21 people in the vicinity. TheBoston molasses disaster was caused by poor design and construction, with a wall too thin to bear repeated loads from the contents. The tank had not been tested before use by filling with water, and was also poorly riveted. The owner of the tank,United States Industrial Alcohol Company, paid out $300,000 (nearly $4 million in 2012) in compensation to the victims or their relatives.
There have been many other accidents caused by tanks since then, often caused by faultywelding or by sub-standardsteel. New inventions have at least fixed some of the more common issues around the tanks' seal.[10][11] However, storage tanks also present another problem, surprisingly, when empty. If they have been used to holdoil or oil products such asgasoline, the atmosphere in the tanks may be highly explosive as the space fills withhydrocarbons. If new welding operations are started, then sparks can easily ignite the contents, with disastrous results for the welders. The problem is similar to that of emptybunkers ontanker ships, which are now required to use aninert gas blanket to preventexplosive atmospheres building up from residues.
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