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Plates and Surface Processes for Building Oil Storage Tanks

Building Oil Storage Tanks: Plates Selection and Processes

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The construction of oil storage tanks is a critical aspect of the oil and gas industry. These tanks must be designed and built with precision to ensure safety, durability, and efficiency in storing oil products. One of the most important components of these tanks is the selection and processing of plates used in their construction. This blog provides a detailed overview of the plate selection criteria, fabrication processes, and considerations involved in building oil storage tanks.

Importance of Plate Selection

Plates are the primary structural component of oil storage tanks. The selection of appropriate plates is crucial for several reasons:

  1. Safety: The right plate material ensures the tank can withstand internal pressure, environmental conditions, and potential chemical reactions with the stored product.
  2. Durability: High-quality materials enhance the longevity of the tank, reducing maintenance costs and downtime.
  3. Compliance: Adherence to industry standards and regulations is essential for legal operation and environmental protection.
  4. Cost-efficiency: Choosing the correct materials and processing methods can significantly reduce construction and operational costs.

Types of Oil Storage Tanks

Before diving into plate selection, it’s important to understand the different types of oil storage tanks, as each type has specific requirements:

  1. Fixed Roof Tanks: These are the most common type of storage tanks used for storing oil and petroleum products. They are suitable for liquids with low vapor pressure.
  2. Floating Roof Tanks: These tanks have a roof that floats on the surface of the stored liquid, reducing evaporation losses and the risk of explosion.
  3. Bullet Tanks: These are cylindrical tanks used for storing liquefied gases and volatile liquids.
  4. Spherical Tanks: Used for storing high-pressure liquids and gases, providing equal stress distribution.

Plate Selection Criteria

1. Material Composition

  • Carbon Steel: Widely used due to its strength, affordability, and availability. Suitable for most oil and petroleum products.
  • Stainless Steel: Preferred for storing corrosive or high-temperature products due to its corrosion resistance.
  • Aluminum: Lightweight and corrosion-resistant, ideal for floating roof components and tanks in corrosive environments.
  • Composite Materials: Occasionally used for specific applications requiring high corrosion resistance and lightweight.

2. Thickness and Size

  • Thickness: Determined by the tank’s design pressure, diameter, and height. Generally ranges from 5 mm to 30 mm.
  • Size: Plates should be large enough to minimize welding seams but manageable for handling and transportation.

3. Mechanical Properties

  • Tensile Strength: Ensures the tank can withstand internal pressure and external forces.
  • Ductility: Allows for deformation without fracturing, accommodating changes in pressure and temperature.
  • Impact Resistance: Important for withstanding sudden forces, especially in colder environments.

4. Environmental Factors

  • Temperature Variations: Consideration of material behavior in extreme temperatures.
  • Corrosive Environment: Selection of materials resistant to environmental corrosion, especially for offshore or coastal installations.

Material Standards and Grades

When selecting materials for oil storage tanks, it is crucial to adhere to recognized standards and grades to ensure quality, performance, and compliance with industry regulations.

Carbon Steel

  • Standards: ASTM A36, ASTM A283, JIS G3101
  • Grades:
    • ASTM A36: Common structural steel grade used for tank construction due to its good weldability and machinability.
    • ASTM A283 Grade C: Offers good strength and ductility for moderate stress applications.
    • JIS G3101 SS400: A Japanese standard for carbon steel used for general structural purposes, known for its good mechanical properties and weldability.

Stainless Steel

  • Standards: ASTM A240
  • Grades:
    • 304/304L: Offers good corrosion resistance and is used for tanks storing mildly corrosive products.
    • 316/316L: Provides superior corrosion resistance, especially in marine environments, due to added molybdenum.
    • 904L (UNS N08904): Known for its high corrosion resistance, particularly against chlorides and sulfuric acid.
    • Duplex Stainless Steel 2205 (UNS S32205): Combines high strength with excellent corrosion resistance, suitable for harsh environments.

Aluminum

  • Standards: ASTM B209
  • Grades:
    • 5083: Known for its high strength and excellent corrosion resistance, ideal for tanks in marine environments.
    • 6061: Offers good mechanical properties and weldability, suitable for structural components.

Composite Materials

  • Standards: ASME RTP-1
  • Applications: Used in specialized applications requiring resistance to chemical attack and weight savings.

Types of Linings and Coatings

Linings and coatings play a vital role in protecting oil storage tanks from corrosion and environmental damage. The choice of lining and coating depends on the tank’s location, contents, and environmental conditions.

External Coatings

  1. Epoxy Coatings:
    • Properties: Offer excellent adhesion and corrosion resistance. Suitable for harsh environments.
    • Applications: Used on tank exteriors to protect against weathering and chemical exposure.
    • Recommended Brands:
      • Hempel: Hempel’s Epoxy 35540
      • AkzoNobel: Interseal 670HS
      • Jotun: Jotamastic 90
      • 3M: Scotchkote Epoxy Coating 162PWX
    • Recommended DFT (Dry Film Thickness): 200-300 microns
  2. Polyurethane Coatings:
    • Properties: Provide excellent UV resistance and flexibility.
    • Applications: Ideal for tanks exposed to sunlight and varying weather conditions.
    • Recommended Brands:
      • Hempel: Hempel’s Polyurethane Enamel 55300
      • AkzoNobel: Interthane 990
      • Jotun: Hardtop XP
    • Recommended DFT: 50-100 microns
  3. Zinc-Rich Primers:
    • Properties: Provide cathodic protection to steel surfaces.
    • Applications: Used as a base coat to prevent rusting.
    • Recommended Brands:
      • Hempel: Hempadur Zinc 17360
      • AkzoNobel: Interzinc 52
      • Jotun: Barrier 77
    • Recommended DFT: 120-150 microns

Internal Linings

  1. Phenolic Epoxy Linings:
    • Properties: Excellent chemical resistance to petroleum products and solvents.
    • Applications: Used inside tanks storing crude oil and refined products.
    • Recommended Brands:
      • Hempel: Hempel’s Phenolic 35610
      • AkzoNobel: Interline 984
      • Jotun: Tankguard Storage
    • Recommended DFT: 400-600 microns
  2. Glass Flake Coatings:
    • Properties: High chemical and abrasion resistance.
    • Applications: Suitable for aggressive chemical storage and tank bottoms.
    • Recommended Brands:
      • Hempel: Hempel’s Glassflake 35620
      • AkzoNobel: Interzone 954
      • Jotun: Baltoflake
    • Recommended DFT: 500-800 microns
  3. Rubber Linings:
    • Properties: Provide flexibility and resistance to chemicals.
    • Applications: Used for storage of corrosive substances like acids.
    • Recommended Brands:
      • 3M: Scotchkote Poly-Tech 665
    • Recommended DFT: 2-5 mm

Selection Considerations

  • Product Compatibility: Ensure the lining or coating is compatible with the stored product to prevent reactions.
  • Environmental Conditions: Consider temperature, humidity, and exposure to chemicals when selecting linings and coatings.
  • Maintenance and Durability: Choose linings and coatings that offer long-term protection and are easy to maintain.

Fabrication Processes

The fabrication of oil storage tanks involves several key processes:

1. Cutting

  • Mechanical Cutting: Involves shearing, sawing, and milling to shape the plates.
  • Thermal Cutting: Utilizes oxy-fuel, plasma, or laser cutting for precise and efficient shaping.

2. Welding

Welding is critical for joining plates and ensuring structural integrity.

  • Shielded Metal Arc Welding (SMAW): Commonly used for its simplicity and versatility.
  • Gas Tungsten Arc Welding (GTAW): Provides high-quality welds for critical joints.
  • Submerged Arc Welding (SAW): Suitable for thick plates and long seams, offering deep penetration and high deposition rates.

3. Forming

  • Rolling: Plates are rolled into the desired curvature for cylindrical tank walls.
  • Press Forming: Used for shaping tank ends and other complex components.

4. Inspection and Testing

  • Non-Destructive Testing (NDT): Techniques such as ultrasonic testing and radiography ensure weld quality and structural integrity without damaging the material.
  • Pressure Testing: Ensures the tank can withstand the design pressure without leaking.

5. Surface Preparation and Coating

  • Blasting: Cleans and prepares the surface for coating.
  • Coating: Application of protective coatings to prevent corrosion and extend the tank’s lifespan.

Industry Standards and Regulations

Adherence to industry standards is essential for ensuring safety, quality, and compliance. Key standards include:

  • API 650: Standard for welded steel storage tanks for oil and gas.
  • API 620: Covers the design and construction of large, low-pressure storage tanks.
  • ASME Section VIII: Provides guidelines for pressure vessel construction.

Conclusion

The construction of oil storage tanks requires meticulous attention to detail, particularly in the selection and processing of plates. By considering factors such as material composition, thickness, mechanical properties, and environmental conditions, builders can ensure the safety, durability, and cost-effectiveness of these critical structures. Adherence to industry standards and regulations further ensures compliance and protection of the environment. As the oil and gas industry continues to evolve, advancements in materials and fabrication technologies will continue to enhance the construction of oil storage tanks.