This article is about Storage Tanks, its types and technical requirements of storage tanks for design and process engineers in plants and oil & gas industries.
What is Storage Tanks?
Storage tanks are containers used to store liquids, gases, or other substances for short or long periods of time. They can be above-ground or underground and come in different shapes and sizes, such as cylindrical or with flat, cone, or dish bottoms. These tanks are often used for holding petroleum, chemicals, water, or hazardous materials.
Regulations and standards are in place to ensure the safe design and operation of storage tanks, and they can vary depending on the type of fluid being stored. Capacity tables are created to measure the volume of the tank at different fill levels. Air emissions from storage tanks are regulated and require compliance with air quality permitting regulations.
Types of Storage Tanks for Petroleum Products
1. Industrial Gas Storage Tanks
Internal Floating Roof Storage Tanks
In the tank world, there are two main types of roofs: fixed roofs and floating roofs. Internal floating roofs can be divided into two categories:
- Tanks with a fixed roof supported by columns: These tanks have a roof that is supported inside the tank by vertical columns. The roof covers the entire open area of the tank and may have openings for various purposes.
- Tanks with a self-supporting fixed roof: These tanks have a roof that is self-supporting and doesn’t require columns inside. The roof completely covers the open area of the tank.
Both types of tanks, whether with a fixed roof or an internal floating roof, serve different functions. Internal floating roofs have a roof that floats above the liquid surface, either in direct contact with the liquid or supported by pontoons above it. The floating roof moves up and down as the liquid level changes, providing flexibility and reducing the empty space above the liquid.
LNG Storage Tanks
LNG (Liquefied Natural Gas) tanks come in various types, including cylindrical and horizontal designs. Cylindrical tanks have a vertical axis and are mostly underground, while horizontal tanks can be used above or below ground. Pressure tanks, on the other hand, are horizontally oriented and have a spherical shape to withstand high pressures. These tanks are raised above the ground. Additionally, there are cylindrical and spherical tanks with variable vapor space, which allows for changes in volume as the LNG expands or contracts.
High Pressure Tanks
High pressure tanks are designed to store liquefied gases or compressed gases under high pressure. They need to withstand the immense pressure exerted by the contents. These tanks, also known as cylinders, are excluded from the classification of “tanks” due to their function as pressure vessels.
2. Industrial Oil Storage Tanks
External Floating Roof Storage Tanks
A floating roof tank is a type of storage tank with a roof that floats on the surface of the stored liquid. It consists of a cylindrical steel shell and a floating roof that moves up and down as the liquid level changes. The floating roof is made up of three components: a deck, fittings, and a rim seal system.
The deck of a floating roof is typically made of welded steel plates and comes in different configurations such as pan, pontoon, or double deck. The trend nowadays is towards pontoon and double-deck roofs. Each configuration offers unique features like complete liquid contact, weight-bearing capability, roof stability, or pontoon arrangement.
Domed External Floating Roof Storage Tanks
Domed external floating roof tanks have a robust deck and a fixed roof that sits on top of the tank shell. The fixed roof serves as both a vapor barrier and a wind block. Another common type of fixed roof is the self-supporting aluminum dome roof, which is securely attached. These tanks are well-ventilated, similar to internal floating roof tanks. The deck fittings and rim seals in external floating roof tanks are generally similar in design and function.
Bunded Storage Tanks
- Bunded tanks are designed with an additional tank or containment dike surrounding the main tank.
- The purpose of the bund is to prevent leaks, oil spills, or contamination from spreading to the environment.
- The containment dike acts as a catch system, providing an added layer of protection.
Single Skin & Double Skin Storage Tanks
- Single skin tanks have a single layer, while double skin tanks have two layers of either plastic or steel.
- Double skin tanks are also referred to as twin-walled tanks.
- Double skin tanks provide enhanced safety and prevent leakage by having an extra layer of protection.
Open Top Tank:
- Open top tanks were used in the past for industrial oil storage but are now limited in use.
- These tanks are not enclosed, allowing for evaporation losses and posing a higher risk of fire hazard.
It is important to note that the use of open top tanks is discouraged due to safety and environmental concerns. The preferred options for oil storage are bunded tanks and double skin tanks, which offer better containment and minimize the risk of spills and leaks.
Fixed Roof Storage Tanks
A fixed-roof tank is a cost-effective option for storing liquids and is commonly used in various industries. It consists of a cylindrical steel shell with a cone- or dome-shaped roof that is permanently attached. While modern tanks are typically welded to ensure they are liquid and vapor tight, older tanks may not have the same level of vapor tightness if they were glued or bolted together.
To regulate pressure and prevent vapor escape, a breather valve, also known as a pressure-vacuum valve, is often installed on fixed-roof tanks. This valve allows the tank to operate at low internal pressures or vacuums, maintaining the integrity of the tank.
3. Water and Liquid Storage Tanks
- Onion Tanks:
- Onion tanks are self-rising tanks that can quickly store water during emergency events.
- They are easy to set up and can be moved easily.
- These tanks have accessible fittings for convenient use.
Pillow (Bladder) Tanks:
- Collapsible tanks, also known as blivet, bladder, or pillow tanks, are temporary storage solutions.
- They can be easily folded and stored when not in use.
- These tanks are reusable and provide a flexible storage option.
Poly Tanks:
- Poly tanks are versatile and commonly used for various applications such as rainwater collection and drinking water storage.
- They come in different types, including wastewater tanks, mobile tanks, and above ground tanks.
- Poly tanks are suitable for domestic and commercial use.
Fiberglass Tanks:
- Fiberglass tanks are durable and resistant to corrosion and rust.
- They can withstand high temperatures and are available with different coatings for specific liquid storage needs.
- These tanks are a reliable choice for long-term storage.
Welded Steel Tanks:
- Welded steel tanks offer a reliable solution for long-term storage requirements.
- They come in various shapes and sizes to accommodate different applications.
- These tanks are known for their durability and can be customized as per project needs.
Corrugated Steel Tanks:
- Corrugated steel tanks are versatile, economical, and have a long lifespan.
- They are made of galvanized corrugated steel and are suitable for rainwater collection, industrial process water, and fire suppression.
- Corrugated steel tanks are available in different sizes and can be customized for specific project requirements.
4. Industrial Chemical Storage Tanks
There are three main types of industrial chemical storage tanks available:
- Stainless Steel Tanks:
- Stainless steel tanks are durable and effective for storing chemicals.
- However, they may not be suitable for certain acids and are generally more expensive than other options.
- Fiberglass (FRP) Tanks:
- Fiberglass tanks are suitable for certain applications.
- They have seams that may potentially leak and require increased maintenance.
- Fiberglass tanks are more prone to human error during the handcrafting process.
- They are also susceptible to specific chemicals like hydrofluorosilicic acid, which can etch the fiberglass material.
- Polyethylene Tanks:
- Polyethylene storage tanks are available in both cross-linked and linear polyethylene.
- They are versatile and widely used in industrial storage.
- Polyethylene tanks are resistant to various chemicals and offer excellent durability.
- They are a cost-effective option and require minimal maintenance.
Among these options, polyethylene tanks are often considered the most versatile and practical choice for industrial chemical storage.
5. Industrial Fuel Storage Tanks
There are two main types of fuel storage tanks: aboveground tanks and underground tanks.
AST Storage Tanks:
- Aboveground fuel tanks (AST) have lower long-term maintenance and upfront costs compared to underground tanks.
- They are easier to install since they don’t require extensive excavation or pavement.
- Aboveground tanks allow for easier inspection, leak detection, and maintenance.
- They are commonly used for storing fuels and chemicals.
UST Storage Tanks:
- Underground storage tanks (UST) are buried underground, with at least 10% of the tank’s volume below the surface.
- USTs storing hazardous materials or fuels are regulated and require EPA registration.
- They are suitable for maximizing space and property value as they can be placed beneath lawns and driveways.
- Underground tanks have a lower risk of explosion, but a higher risk of leaks and pollution due to limited inspection opportunities.
6. Miscellaneous Storage Tanks
Atmospheric Tanks
Atmospheric tanks are designed to hold liquids at atmospheric pressure. They are regulated by codes such as API 650 and API 620 in the United States, and EN14015 in Europe. These tanks are commonly used for storing various liquids.
Septic Storage Tanks
Septic tanks are part of small-scale sewage treatment systems, commonly used in rural and suburban areas. They receive wastewater from homes and businesses and allow solids to settle and scum to float. Anaerobic digestion occurs in the tank, reducing the volume of solids. The treated water is then released into a drain field for further absorption.
Ice and Slush
Tanks Ice and slush tanks are used for short-term storage of cold temperatures. They are commonly used in air conditioning systems to allow refrigeration equipment to operate during off-peak hours when electricity costs are lower. These tanks provide cooling during hot daytime hours.
Thermal Storage Tanks
Thermal storage tanks are used for seasonal thermal energy storage. They are often large surface water tanks that are insulated and covered with earth berms to collect solar-thermal heat. This heat is stored and used for heating purposes throughout the year. Similar technology is used in Danish district heating systems, where large lined pits filled with gravel and water are used as thermal storage.
Milk Tanks
Milk cooling tanks are used in dairy farms to rapidly cool and store fresh raw milk until it can be picked up by a milk hauler. These tanks are made of stainless steel and are designed to sanitary standards. They help maintain the quality and safety of the milk by slowing the growth of microorganisms.
Mobile “Storage” Tanks
Mobile tanks, although not strictly storage tanks, share similar features and purposes. They are designed to handle sloshing loads and the risks associated with transportation. Examples include ocean-going oil tankers, railroad tank cars, road and highway tankers, and holding tanks for toilet waste on RVs, boats, and aircraft.
Plastic Water Storage Tanks
Plastic water storage tanks come in various types and classifications:
Polymer Type: The tanks can be made from different polymers, such as polyethylene, fiberglass, or other materials.
Plastic Type: Plastic tanks can be categorized as either fresh (made from new materials) or recycled (made from recycled plastic).
Shape: Plastic water storage tanks are available in different shapes, including rectangular, cylindrical, and other custom shapes.
Size: Tanks can be classified based on their size, with large tanks holding over 5000 liters, medium tanks ranging from 1000 to 5000 liters, and small tanks holding less than 1000 liters.
Application: These tanks serve various purposes and can be used in industrial, municipal, residential, or commercial settings.
Medium-sized cylindrical tanks are commonly made from recycled polyethylene-based polymers, offering a sustainable storage solution.
Design Technical Requirements of Storage Tanks
A. Sizing Criteria
1. Refer to Company specification for Atmospheric Storage Tanks Q03-S01.
2. Cylindrical tanks for storage of inflammable liquids above or underground shall comply with underwriter (UL) and API-650 specification.
3. Suitable breathing/vacuum arrangement shall be provided for all the chemical storage tanks with vapor dilution/sealing system to prevent collapse of the tank roof.
4. Relief requirement for low-pressure storage tanks shall comply with the requirements of
API-2000.
5. All tanks shall be equipped with proper system for leakage detection to the soil as per API-650.
B. Isolation and Valving
1. Fire safe valves will be used in systems for storage of flammable fluids or fluids stored above their flash point.
2. Fire safe valves with fire safe seats will be used as primary block valves for hydrocarbon and toxic storage vessels containing flammable fluids or fluids above their flash point. The actuation system of the fire safe valves will also be fire safe. Refer to the SSD-27 (HCISS Safety and Security Directives) for storage tank valves and their remote isolation.
3. Fire safe valves will be provided in piping at vessel and storage tank nozzles as follows:
a. In piping at all nozzles below the liquid level of flammable liquid storage tanks.
b. In liquid draw-off lines at the nozzles of process vessels containing 5.7 m or more of a flammable liquid at the maximum normal liquid level when the line does not contain a block valve located within 9 m in a horizontal direction from the vessels.
4. Block valves shall be provided in piping at all storage tank nozzles below the liquid level
of storage tanks.
C. Layout and piping requirement
1. Spacing between storage tanks and adjoining property, between storage tanks and
processing facilities & between adjacent tanks shall be as per SES S01-G01/P01E09/NFPA
30.
2. The design of storage tank suction lines will be based on the required NPSH from the lowest specified liquid level in the tank at which rated (normal capacity plus 10 percent) pump capacity is required. If vortex breakers are used at tank outlet nozzles, the pressure drop through the vortex breaker will be considered.
Ensuring Safe and Secure Handling of Liquids: Storage Tank Considerations
To ensure the safe handling of liquids in storage tanks, measures are taken to prevent leaks or spills. This includes constructing a bunding or containment dike around the tank to contain any potential leakage. In some cases, tanks may have a floating roof that moves with the liquid level to reduce the vapor space above the liquid, which is important for safety and pollution prevention.
In the United States, metal tanks containing petroleum products and in contact with soil must be protected from corrosion to prevent environmental contamination. Cathodic protection is a common technique used to control corrosion in steel tanks in contact with soil. In other parts of the world and certain locations in the United States, elevated tank support foundations with a sand bitumen mix finish are used to keep the tank bottom plates free from water, thus preventing corrosion. These measures help ensure the integrity and longevity of the storage tanks.
Technical Consideration for Storage Tanks in Refineries
In refineries and for storing liquid fuels, different types of tanks are used based on the flash-point of the substance being stored. Fixed roof tanks are designed for liquids with high flash points and come in various shapes such as cone roofs, dome roofs, and umbrella roofs. They are insulated and heated with steam coils. Floating roof tanks, including external and internal floating roof tanks, are used for liquids with low flash-points. They have a floating roof that moves with the liquid level, preventing vapor release. Open roof tanks are commonly used in mining areas for storing ore slurries.
Storage tanks in a refinery can also be classified based on their location, such as crude oil tanks, product and intermediate storage tanks, dispatch area tanks, utilities tanks, OSBL tanks, and ISBL tanks. When the flash-points of fuels are very low, spherical tanks called spheres are used to store substances like LPG, hydrogen, and oxygen. These tanks are efficient and easy to build.
Storage Tanks Failures Possibilities
One of the most well-known incidents involving storage tank failure occurred in Boston, Massachusetts in 1919. A large tank filled with molasses collapsed due to poor design and construction, resulting in the deaths of 21 people. The tank had not been properly tested and had structural weaknesses. The company responsible for the tank paid compensation to the victims.
Since then, there have been other accidents caused by faulty welding and sub-standard steel in storage tanks. New innovations have addressed some of these issues, particularly concerning the tank’s seal.
However, empty storage tanks can also pose a danger. If they previously held oil or petroleum products like gasoline, the atmosphere inside the tanks can become highly explosive as it fills with hydrocarbons. Welding operations in such tanks can easily ignite the contents, leading to disastrous consequences. This risk is similar to empty bunkers on tanker ships, which now use inert gas blankets to prevent the buildup of explosive atmospheres from residue.
Read Also: Shell and Tube Heat Exchanger Design Technical Requirements
FAQs about Storage Tanks
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What is the purpose of storage tanks?
Storage tanks are used to store various substances, including petroleum products, chemicals, water, and gases. They provide a safe and secure storage solution for these substances until they are needed for use or further processing.
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What are the different types of materials used for storage tanks?
Storage tanks can be made of different materials, such as steel, fiberglass-reinforced plastic (FRP), concrete, or polyethylene. The choice of material depends on factors like the type of substance being stored, environmental conditions, and regulatory requirements.
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How are storage tanks maintained?
Regular maintenance is essential to ensure the integrity and safety of storage tanks. This includes inspections for corrosion, leaks, and structural integrity, as well as cleaning, painting, and repair work as needed. Compliance with industry standards and regulations is also crucial.
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What safety measures are in place for storage tanks?
Safety measures for storage tanks include the use of secondary containment systems to prevent spills or leaks from reaching the environment, monitoring systems to detect leaks or abnormal conditions, and emergency response plans in case of incidents. Regular training and adherence to safety protocols are also important.
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How are storage tanks regulated?
Storage tanks are subject to various regulations and standards to ensure their safe operation and environmental protection. These regulations often cover aspects such as design, construction, maintenance, inspection, and spill prevention. Compliance with these regulations is typically enforced by governmental agencies or industry bodies.