Storage Tank Construction

An above-ground storage tank (AST) is a large cylinder that sits on a ground-level foundation and holds a liquid or gas in bulk quantities. ASTs can store products like potable water, diesel, gasoline, chemicals, and hazardous waste materials. Chemical tanks and oil storage tanks must be equipped with secondary containment systems to prevent spills and leaks.

Storage Tank Construction Fundamentals

Storage requirements must be considered for the type and volume of product being stored, with design considerations including an adequate foundation, material compatibility, vapor control, and secondary containment.

Tanks must be designed and constructed to withstand the internal and external pressures and forces they will experience during storage. Regular inspection and maintenance will prevent leaks, corrosion, and other potential hazards.

Regulatory requirements for storage tanks vary depending on the location and type of products being stored. Compliance with these regulations is essential to ensure safety and environmental protection.

Key Components and Their Functions

Steel storage tanks consist of several key components, each serving a specific function.

• Bottom: The bottom of the tank provides support and stability for the stored product. It is usually made of reinforced concrete or steel and is designed to prevent leaks and contamination.

• Shell: The cylindrical shell is the main body of the tank, providing structural support and containment for the stored product. It is typically made of steel and is designed to withstand the internal and external pressures exerted on the tank.

• Floating Roof: If a product has a risk of evaporation and ignition, a floating roof must be installed on the surface of the stored product to reduce the vapor space and minimize the risk.

• Fixed Roof: Tanks without a floating roof use a permanently attached dome or cone-shaped roof to protect the product from contamination.

• Accessories: Components often include vents, level indicators, valves, temperature probes, and gauges. Safe tank access may also require manways, ladders, platforms, and tie-off points.

Materials Used in Above-Ground Tank Construction

Carbon steel is preferred for its cost-effectiveness, while stainless is best used with corrosive or incompatible products.

• Low-Carbon Steel is commonly used for storage tanks designed for chemical, fuel, oil, and water storage. Depending on the stored material, these tanks may be lined with epoxy, polyurethane, or rubber to prevent rust.

• Grade 304 Stainless Steel is widely used in industrial tank fabrication. It resists tarnishing and rust, making it suitable for storing chemicals incompatible with carbon steel. Applications include glycerin, liquid fertilizer, pasteurizing, and maturation tanks.

• Grade 316 Stainless Steel offers excellent corrosion resistance, especially in chloride-rich environments. It is commonly used for tanks storing aggressive chemicals, seawater, and food products. Its resistance to pitting and crevice corrosion makes it a reliable choice for industrial applications.

Planning and Designing Above-Ground Storage Tanks

During the planning phase, site preparation and environmental impact assessments are conducted to determine the feasibility and suitability of the storage tank installation. Factors such as soil conditions, construction access, and proximity to sensitive areas are taken into account to minimize environmental impact and ensure compliance with regulations.

Design Standards and Regulations

API 650 or AWWA D100 are commonly used standards for welded steel tanks. They cover the design, fabrication, and construction requirements for welded above-ground storage tanks.

Regulatory requirements for storage tanks vary depending on the location and the type of products being stored. These regulations may include requirements for secondary containment, leak detection systems, and frequency of inspections.

Site Preparation and Safety Measures

Site preparation helps ensure a stable foundation for the tank. This process involves the following considerations:

• Clearing and Grading: The site is cleared of any obstacles or vegetation that may interfere installation of the tank foundation. The ground is then graded to provide drainage.

• Excavation: The area where the tank will be installed is excavated to create a space for the foundation and to ensure proper drainage.

• Compaction: The soil is compacted to increase its stability and load-bearing capacity, minimizing the risk of settlement.

• Safety Measures: During site preparation, safety measures are implemented to protect workers and prevent accidents. This includes the use of personal protective equipment, proper signage, and adherence to safety protocols.

Environmental Impact Assessments

Environmental impact assessments are an essential part of storage tank construction, ensuring that the project is conducted in an environmentally responsible manner. These assessments include:

• Identification: Environmental impact assessments identify potential impacts on air, water, soil, and biodiversity that may result from storage tank construction.

• Mitigation: Based on the assessments, mitigation measures are developed to minimize the potential environmental impacts. This may include measures such as erosion control, sediment containment, and noise reduction.

• Monitoring: Throughout the construction process, monitoring may be needed to ensure compliance with environmental regulations and to verify the effectiveness of mitigation measures.

By conducting thorough environmental impact assessments and implementing appropriate mitigation measures, storage tank construction projects can minimize their environmental footprint.

The Construction Process

The tank erection process involves fit-up and welding of steel plate and pipe. Skilled welders join the parts of the tank together, following approved welding procedures. These components, including the bottom, shell, and roof, are assembled according to approved construction drawings. This may involve the use of cranes, welding equipment, and other specialized tools.

Foundation Construction

The foundation provide a stable, dry base for the tank. Sitework is needed to prepare the construction site by clearing vegetation, leveling the ground, and ensuring proper drainage. The tank foundation must be designed to support the weight of the tank. Typically a ring foundation is needed to support the tank shell, however a slab foundation may be used if the tank is not too tall. The foundation may use steel piles, reinforced concrete, compacted sand, and other suitable materials. You can read more about foundation design on our website.

Tank Bottom Fabrication

The bottom plates are placed on the foundation, laying them out based on the construction drawings:

• Laser Alignment: Laser alignment can be used tools to verify that the tank’s centerline matches the foundation centerline.

• Measuring Diagonals: When measuring the diagonals of the tank bottom with a tape measure or laser, equal diagonals indicate proper alignment.

• Strapping (Perimeter Measurement): A tape is wrapped around the outer edge of the circle to confirm perimeter and chord dimensions.

• Tack Welding: Tack weld the plates at specific intervals (e.g., every 2 feet) to hold them in place. Check alignment after each row of bottom plates are installed.

After layout and tack welding are complete, the bottom plate seams are fully welded. When three plates overlap at the edge, they must be hammered to eliminate gaps. This step is known as breakdown.

Welding the Tank Shell and Structure

The assembly of the tank shell and the roof structure involves:

• Shell Assembly: The first ring of shell plates are aligned on the bottom plates and against the other shell plates using fit-up nuts. The curve of the shell is checked to confirm that it is straight, and not buckled or warped. When the first ring is fully welded, the remaining rings are stacked and welded using the same method. The placement of plates may involve the use of cranes, welding equipment, and other specialized tools.

• Reinforcement: Additional structural reinforcement, such as stiffening rings or braces, may be added to resist strong winds or internal sloshing. Temporary restraints, such as steel cables, may be needed to protect the tank from high winds during assembly.

• Quality Control: Throughout the assembly and welding process, quality control measures are implemented to ensure the welds are sound and meet the required standards.

Installation of the Tank Roof

The roof of a steel tank serves multiple purposes, including protection from the elements, preventing the entry of foreign materials, and maintaining the structural integrity of the tank. There are different types of roofs used in storage tanks, such as cone roofs, steel domes, aluminum geodesic domes, and floating roofs.

Fixed roof tanks have a stationary roof that does not move with the tank’s liquid level, such as:

• Cone Roof

• Steel Dome Roof

• Aluminum Geodesic Dome Roof

There are several types of internal floating roofs (IFRs) or external floating roofs (EFRs) used in industrial storage tanks:

• Steel Pan

• Steel Open-Top Pontoon

• Steel Pontoon

• Steel Reverse-Slope Pontoon (IFR or EFR)

• Steel Double-Deck Pontoon (IFR or EFR)

• Aluminum Full-Contact

• Aluminum Honeycomb

• Aluminum Skin-and-Pontoon

Read our comparison of floating roof designs for more in-depth information.

Accessories and Insulation

Storage tanks typically have nozzles dedicated to product inlet, outlet, and overflow. Other nozzles may be installed for mixing, temperature monitoring, or gauging. Insulation is required for some tank products. Insulation helps maintain the temperature inside the tank, preventing heat loss and ensuring the contents remain at the desired temperature.

Inspection and Quality Assurance

Inspection and quality assurance play a crucial role in ensuring the safety and compliance of storage tanks.

Types of inspection during AST construction

Inspection is performed during construction to identify poor weld quality and non-compliance with the design drawings. These inspections may include visual inspections, non-destructive testing, and hydrostatic testing.

• Visual inspections are conducted to identify any visible defects. This includes checking the alignment of the tank components, weld quality, and overall compliance with construction drawings.

• Non-destructive testing techniques, such as ultrasonic testing, magnetic particle testing, and radiographic testing, can identify issues such as cracks, incomplete fusion, slag inclusion, and defects within welds.

• Hydrostatic testing is performed to verify the strength and integrity of the tank under load. After the tank is filled with water, the tank is inspected for any signs of leakage or deformation.

Quality control measures and certifications

Quality control measures are implemented throughout the construction process to ensure that the storage tank meets all regulatory requirements and industry standards. These measures include the use of certified materials, adherence to welding procedures, and compliance with design specifications.

Regulatory requirements must be met to ensure compliance with local, state, and federal regulations. These requirements may include specific design standards, safety measures, and environmental considerations.

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