What is a Storage Tank? Functions, Materials, and Safety

Storage tanks are essential components widely used across various industries for the safe and efficient storage of a multitude of substances, including liquids, gasses, and solids. Predominantly found in sectors such as oil and gas, chemical processing, water treatment, and agriculture, these tanks play a crucial role in maintaining the continuity of industrial operations. They are designed to hold materials under specific conditions&#;whether at atmospheric pressure, under a controlled temperature, or within certain chemical environments&#;to ensure that these materials are available in sufficient quantity and maintained quality when required. This comprehensive article delves into the core aspects of storage tanks, covering their functions, types, materials, safety considerations, and guidelines for selecting the right tank.

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What is a Storage Tank?

A storage tank is a container, typically large, used to store various types of liquids, gasses, and sometimes solids. These tanks can be found in many different industries such as oil and gas, chemical manufacturing, water supply, and agriculture. Their primary purpose is to hold a substantial amount of material in a controlled environment &#; be it under pressure, at a specific temperature, or within certain chemical conditions &#; to ensure that these materials are available in sufficient quantity and appropriate quality when needed.

The Key Functions of Storage Tanks

Storage tanks serve multiple critical functions:

1. Storage and Conservation: They provide a way to hold large volumes of material until needed, reducing losses that might occur through evaporation or spillage. This allows for efficient inventory management, ensuring a steady supply of materials for ongoing operations without shortages.
2. Mixing and Blending: Certain tanks are designed for mixing or blending different liquids or gasses to create specific formulations.
3. Transportation: Portable storage tanks are used to transport liquids or gasses over short distances.
4. Safety and Compliance: Proper storage of hazardous materials helps in complying with safety and environmental regulations.
5. Buffering: Tanks can act as buffers between the production and the consumption rates, smoothing out fluctuations in demand and supply.

Types of Storage Tanks

In various industries like oil and gas, petrochemicals, water treatment, and food processing, storage tanks play a vital role in securely storing substances. These tanks come in different types, each tailored to specific storage needs. Here is a more detailed overview of the various types of storage tanks:

1. Fixed Roof Tanks: These tanks have a stationary roof, often cone or dome-shaped, and are designed for storing liquids with low vapor pressures. They are commonly used for storing crude oil, refined petroleum products, and water.
2. Floating Roof Tanks: Equipped with a floating roof that adjusts with the liquid level, reducing vapor space and evaporation losses. There are two types of floating tanks which are External Floating Roof Tank (EFR) and Internal Floating Roof Tank (IFR). They are suitable for liquids with higher vapor pressures like gasoline and diesel fuel.
3. Horizontal Tanks: Tanks oriented horizontally, ideal for storing liquids with low vapor pressures such as water and chemicals. They are space-efficient and commonly used when space is limited.
4. Pressure Tanks (High-Pressure Vessels): Designed to store liquids or gasses at high pressures, typically cylindrical with a robust wall to withstand pressure. Widely used in industries like chemicals, petrochemicals, and power generation.
5. Vertical Tanks: Tanks with a vertical orientation, are commonly used for storing liquids with low vapor pressures like water and chemicals. They are space-saving and preferred when the ground area is limited.
6. LNG Tanks: Specialized tanks for storing liquefied natural gas at extremely low temperatures around -160°C. These tanks are often spherical or cylindrical, insulated to maintain low temperatures, and crucial in the natural gas industry for LNG storage and transportation.

Each type of storage tank serves specific purposes and must be selected based on factors like the stored substance&#;s characteristics, volume, pressure, temperature requirements, and safety considerations. Choosing the right tank is essential to ensure efficient and safe storage practices in various industries.

Materials of Storage Tanks

The choice of material for constructing storage tanks is crucial and depends on the chemical properties of the material to be stored, environmental conditions, cost considerations, and the desired lifespan of the tank:

1. Steel: Widely used due to its durability and strength. Different steel grades offer varying levels of corrosion resistance based on the stored material. It can be coated or lined to prevent corrosion and is suitable for a wide range of temperatures and pressures.
2. Concrete: Preferred for large tanks that store water, sewage, and certain chemicals. Concrete offers good structural integrity and fire resistance and it is advantageous in terms of cost for large volumes.
3. Polyethylene and Fiberglass: These are used for chemically resistant tanks or water tanks in small to medium sizes, offering excellent resistance to corrosion. However, they may not be suitable for high-pressure applications.
4. Stainless Steel: Used for tanks that store aggressive chemicals or food products due to its corrosion resistance and non-reactive properties compared to regular steel.

These materials offer different advantages and are chosen based on factors like the type of substance being stored, durability requirements, cost-effectiveness, and resistance to corrosion or chemical reactions.

Safety of Storage Tanks

Safety is a crucial aspect of storage tank management, particularly when storing hazardous or flammable materials like diesel fuel. Proper handling, storage, and maintenance of storage tanks are essential to prevent environmental damage, accidents, and fires.

• Fire safety measures, including fire extinguishers, smoke detectors, and automatic fire suppression systems, should be implemented to prevent fires. Employees handling stored materials should be trained in safe handling, storage procedures, spill response, and emergency response. Regular training and safety meetings can reinforce safe practices and ensure employee awareness of risks associated with storage tanks.
• Regular inspections and maintenance of storage tanks are essential to ensure that they are in good working order and to prevent leaks and spills. Businesses that handle bulk storage are required to maintain records of inspections, maintenance, and testing of the storage tank and associated equipment.
• Leak detection and monitoring systems, such as automatic tank gauging systems or other electronic monitoring devices, should be in place to detect leaks or spills promptly. Secondary containment systems, such as a dike or spill containment basin, should also be in place to prevent spills and leaks from spreading beyond the containment area.
• The storage of other hazardous materials, such as LPG, is also important. LPG storage tanks should be protected from unauthorized access to reduce the chance of intentional or accidental interference. The user of the LPG has responsibilities concerning the tank, including ensuring that it is maintained in a safe condition.

In sum, proper handling, storage, and maintenance of storage tanks, along with regular inspections and employee training, can help prevent accidents, spills, and fires. By following safety guidelines and regulations, businesses can ensure that their storage tanks are safe and compliant with local regulations, protecting their employees, equipment, and the environment from potential hazards.

Differences Between Pressure Vessels and Storage Tanks

While both pressure vessels and storage tanks are designed to hold substances under pressure, they serve different purposes, construction, orientation, mounting, material, size, cost, and heating method:

• Pressure Vessels: Designed to operate at pressures significantly above atmospheric or high pressure and are typically cylindrical or spherical to evenly distribute stress. They are mechanically designed and equipped with provisions for the introduction or removal of heat from the container. Pressure vessels are generally smaller in size and more expensive than storage tanks, as they have to contain higher pressure and temperature.
• Storage Tanks: Primarily used for storing substances at or close to atmospheric or low pressure in a variety of shapes and sizes, depending on the application. They are usually made from thin-walled materials so they don&#;t hold as much pressure as pressure vessels. Storage tanks are larger in size, as they have to store larger volumes of fluid at lower pressure and temperature. However, they are less expensive than pressure vessels as they require less material, labor, and quality control to ensure their functionality and durability.

How to Choose the Right Storage Tank

When selecting the appropriate storage tank, it is essential to consider several critical factors to ensure optimal functionality and safety. Here are key considerations to guide you in choosing the right storage tank:

1. Capacity: Determine the required storage capacity to avoid investing in a tank that is either too large or too small for your needs. This ensures efficient storage without unnecessary costs or limitations.
2. Material Selection: Choose the tank material carefully based on factors like durability, corrosion resistance, and compatibility with the stored substance. Common materials include steel, aluminum, polyethylene, and fiberglass, each offering specific advantages for different applications.
3. Compliance with Regulations: Ensure the selected tank meets all relevant safety and environmental regulations and codes to prevent potential hazards and ensure legal compliance in your area.
4. Safety Features: Prioritize safety by selecting a tank that adheres to safety standards and regulations, reducing the risk of accidents, leaks, and environmental damage associated with improper storage practices.
5. Type of Tank: Consider the various types of tanks available, such as above-ground, underground, portable, ground storage, elevated, and standpipe tanks, to choose the most suitable option for your specific storage requirements and site conditions.
6. Cost Consideration: Factor in the price of the tank within your budget while ensuring it meets your quality and functional needs, balancing cost-effectiveness with performance.
7. Size Alignment: Select the tank size based on available space and storage needs to ensure it fits your site and accommodates the required volume of the stored substance effectively.
8. Availability and Suitability: Be patient in your selection process to find the right tank model that aligns with your budget and requirements, ensuring it is readily available and suited to your specific needs.

By carefully evaluating these factors and seeking expert advice when needed, you can confidently choose the right storage tank that meets your operational needs, safety requirements, and regulatory standards effectively.

Conclusion

Storage tanks are critical components across industries, each designed to meet specific storage needs. Understanding the types of tanks, materials used in their construction, safety measures, and the differences from pressure vessels can guide one in choosing the right tank for their needs. With proper selection, design, and maintenance, storage tanks can provide safe and efficient storage solutions for a wide range of substances.

Container for liquids or compressed gas

This article is about fluid containment vessels. For other uses, see Tank (disambiguation)

Petroleum storage tank near Detroit, United States

Storage tanks are containers that hold liquids or compressed gases. The term can be used for reservoirs (artificial lakes and ponds), and for manufactured containers. The usage of the word "tank" for reservoirs is uncommon in American English but is moderately common in British 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 from pressure vessels.

Tanks can be used to hold materials as diverse as milk, water, waste, petroleum, chemicals, and other hazardous 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 hydrostatic pressure. Tanks built below ground level are sometimes used and referred to as underground 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 a lorry or an articulated lorry trailer. The resulting vehicle is called a road tanker (or simply tanker; tank truck in American English). Tank cars are tanks mounted on goods wagons for rail transportation.

Etymology

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Cylindrical fuel storage tank with fixed roof and internal floating roof. Capacity approx 2,000,000 litres

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.

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• The Arabic verb istanqa`a &#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#; = "it [i.e. some liquid] collected and became stagnant".

Features and maintenance

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While steel and concrete remain one the most popular choices for tanks, glass-reinforced plastic, thermoplastic and polyethylene tanks are increasing in popularity. They offer lower build costs and greater chemical resistance, especially for storage of specialty chemicals. There are several relevant standards, such as British Standard  (), DVS , and ASME 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.

Capacity table for horizontal cylindrical storage tank

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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 containing petroleum products must be protected from corrosion to prevent escape of the product into the environment.[4] The most effective and common corrosion control techniques for steel in contact with soil is cathodic 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 type of foundations keep the tank bottom plates free from water, therefore preventing corrosion.

In addition to their design and application, maintenance and inspection 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 storing hazardous 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 as remote sensing, ultrasonic testing, and robotic inspection tools, like remotely-operated drones, 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 federal Clean 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 the Environmental Protection Agency's AP-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 a bunding, or containment dike, around the tank, so that any leakage may be safely contained.

Types of tank

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Atmospheric

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An atmospheric tank is a container for holding a liquid at atmospheric pressure. The major design codes for welded atmospheric tanks are API 650 and API 620. API 653 is used for analysis of in-service storage tanks. In Europe the applicable design code is EN , which uses load cases from Eurocode 3 (EN ), part 4-2.

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Horizontal, cylindrical shell, elliptical heads carbon steel pressure vessel

In the case of a liquefied gas such as hydrogen or chlorine, or a compressed gas such as compressed natural gas or MAPP, the storage tank must be made to withstand the sometimes-considerable pressures exerted by the contents. These tanks, being pressure 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.

Thermal storage tanks

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One form of seasonal 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.

Milk tanks

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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.

Septic tanks

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A septic tank is part of a small-scale sewage 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 a septic 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.

Mobile tanks

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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 road tankers. Also included are the holding tanks which are the tanks that store toilet waste on RVs, boats and aircraft.

In refineries

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View of Fawley Refinery large atmospheric tanks

Tanks for crude oil and oil-based fuels are chosen according to the flash point of the material. If the material is not a liquefied gas, such as LPG, tanks are atmospheric and generally come in two types:

Spherical gas tank farm in the petroleum refinery in Karlsruhe MiRO

Liquefied gases (such as LPG, butane, propylene, etc.) may be stored in spherical tanks (or Horton spheres).

Typical classification codes used for tanks in a refinery are:

• OSBL &#; outside-battery-limit tankages
  • COT &#; crude oil tanks.
  • PIT &#; product and intermediate storage tanks.
  • DISPATCH &#; dispatch area tanks.
• UTILITIES &#; tanks in a power plant area, for storage of water, etc.
• ISBL &#; inside-battery-limit tankages. These are usually small tanks found in the production units of a refinery, such as neutralization tanks, water tanks, etc.

Use for chemicals

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BASF chemical plant in Portsmouth, Virginia. The plant is served by the Commonwealth Railway.

Chemical tanks are storage containers for chemicals widely used within the chemical industry. They come in a variety of sizes and shapes, and are used for static storage and transport of both raw materials and finished chemical products.

A chemical tank is of necessity designed for a specific chemical. Chemicals have variable corrosion 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 as gaskets and plumbing materials. Other parameters to be taken in consideration are heat, cold, vacuum, pressure, exothermic reactivity and the inherent aggressive nature of the chemical (acids, caustic, etc.). Secondary 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 in polyethylene, fiberglass and metal materials. Secondary containment tank systems are suggested for all aggressive chemicals.

Tank failures

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There have been numerous catastrophic failures of storage tanks, one of the most notorious being that which occurred at Boston on January 14, . A large tank had only been filled eight times when it failed, and the resulting wave of molasses killed 21 people in the vicinity. The Boston 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 ) in compensation to the victims or their relatives.

There have been many other accidents caused by tanks since then, often caused by faulty welding or by sub-standard steel. 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 hold oil or oil products such as gasoline, the atmosphere in the tanks may be highly explosive as the space fills with hydrocarbons. 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 empty bunkers on tanker ships, which are now required to use an inert gas blanket to prevent explosive atmospheres building up from residues.

Images

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See also

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References

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For more information, please visit Industrial Storage Tanks.