An Overview of API 650 – Welded Tanks for Oil Storage
API Standard 650, Welded Steel Tanks for Oil Storage, is the definitive guideline for designing, constructing, and maintaining aboveground storage tanks (ASTs) that operate at atmospheric pressure. Developed by the American Petroleum Institute (API), this code is globally recognized and mandated for tanks storing petroleum, chemicals, biofuels, and other liquids. Its rigorous standards ensure structural integrity, environmental protection, and operational safety. This guide provides a deep dive into the code’s scope, technical requirements, and practical applications, offering clarity for engineers, inspectors, and project managers who need a thorough understanding without parsing the full 400+ page document.
Historical Background and Evolution
API 650 was first published in 1961 to unify and standardize the design and construction practices of aboveground storage tanks. Over the decades, it has undergone multiple revisions to incorporate advances in materials, welding technology, and safety practices. The most recent 13th edition was released in March 2020, reflecting the latest industry knowledge and regulatory requirements.
Scope of API 650
API 650 applies to welded carbon or stainless-steel tanks with a storage capacity exceeding 119 gallons (450 liters). These tanks are designed for liquids with a specific gravity ≤1.0 and it focuses on atmospheric and low-pressure storage tanks that operate at internal pressures up to 2.5 psig (17.2 kPa). The standard mandates requirements for materials, design, fabrication, welding, inspection, and testing to prevent leaks, structural failures, and environmental hazards Key applications include:
- Liquids stored: Crude oil, refined petroleum, chemicals, and non-potable water.
- Tank configurations: Fixed-roof, floating-roof (external/internal), and covered-top designs.
- Exclusions: Pressurized tanks (governed by API 620) and cryogenic storage.
Included Applications
- Liquid Types: Crude oil, gasoline, diesel, ethanol, acids, and non-potable water.
- Tank Designs:
- Fixed Roof: Cone, dome, or umbrella-shaped roofs permanently sealed to the tank shell.
- Floating Roof: External/internal roofs that adjust with liquid level to minimize vapor space.
- Covered Tanks: Open-top tanks with secondary containment or geodesic domes.
Exclusions
API 650 does not cover:
- Refrigerated tanks (covered by API 620).
- Bolted or modular tanks.
- Underground tanks or tanks with bolted connections for the shell or bottom.
- Tanks designed for pressures exceeding 2.5 psi.
- Tanks used for compressed gases or cryogenic liquids.
Why These Limits Matter
Atmospheric tanks rely on natural ventilation to equalize pressure. Exceeding pressure limits risks catastrophic failure, such as shell buckling or roof collapse. The code’s exclusion of pressurized designs ensures engineers use specialized standards for high-risk applications.
Applicability of the Code
API 650 is applicable for:
- Tanks with internal pressure ≤ 2.5 psig (17 kPa)
- Tanks fabricated on-site or in large fabrication shops
- Tanks made from carbon steel, austenitic stainless steel, or aluminum
- New tank constructions (not repair/re-rating of existing tanks)
Not applicable for:
- Pressurized tanks (for that, use API 620 or ASME codes)
- Tanks subjected to vacuum pressures without special design provisions
Key Sections of API 650
| Section | Title | Description |
|---|---|---|
| 1 | Scope | Defines the types of tanks and conditions the standard applies to, including storage of liquids at ambient pressure and temperatures. |
| 2 | Normative References | Lists other standards (e.g., ASTM, ASME) that are referenced and mandatory for full compliance. |
| 3 | Terms and Definitions | Provides key terminology used throughout the standard to ensure clarity and consistency. |
| 4 | Materials | Specifies acceptable materials for tank construction, including base metals, welding consumables, and coatings. |
| 5 | Design Requirements | Outlines the minimum design standards for shell thickness, roofs, bottoms, nozzles, and appurtenances, considering pressure, wind, seismic loads, and corrosion. |
| 6 | Fabrication | Details requirements for cutting, forming, welding, and assembling tank components with quality assurance measures. |
| 7 | Erection | Covers tank site preparation, positioning, fitting-up, welding on site, and alignment control during erection. |
| 8 | Inspection | Specifies the types of inspection (visual, NDE) required at different stages of construction to ensure compliance with design and fabrication requirements. |
| 9 | Testing | Requires hydrostatic testing (and sometimes pneumatic testing) to verify tank integrity after construction is complete. |
| 10 | Marking | Defines the mandatory nameplate and stamping information that must be affixed to the completed tank for identification and traceability. |
| Annex A | Optional Design Basis | Provides an alternate design methodology using maximum fill heights and material strength under different loading conditions. |
| Annex E | Seismic Design | Specifies additional design and anchorage requirements for tanks in seismic zones, based on seismic coefficients. |
| Annex F | Higher Internal Pressures | Extends design rules to tanks with internal pressures greater than 2.5 psig but limited to 15 psig maximum. |
| Annex G | Aluminum Tanks | Details material, design, and fabrication rules for tanks constructed from aluminum alloys. |
| Annex H | Stainless Steel Tanks | Provides specific guidelines for tanks fabricated from stainless steels (e.g., Type 304, 316). |
| Annex M | API Monogram Program | Describes the quality assurance requirements for manufacturers participating in the API Monogram Program. |
Special Notes:
- Annex E: Seismic design requirements
- Annex F: Requirements for higher internal pressures (above 2.5 psig but limited)
- Annex G: Aluminum tanks
- Annex H: Stainless steel tanks
Key Points to Remember When Using API 650
- Design Parameters Are Interdependent:
- Shell thickness depends on tank diameter, liquid height, material strength, joint efficiency, and corrosion allowance
- Bottom plate thickness depends on soil bearing capacity and liquid load.
- Roof thickness depends on external loads such as wind and snow.
- Material Certification: All materials must meet specified ASTM/API standards.
- Corrosion Allowance Is Mandatory:
- Typically ranges from 1.5 mm to 3 mm (1/16 to 1/8 inch) depending on the stored liquid and environment.
- Must be added to all thickness calculations to ensure longevity.
- Joint Efficiency: Welding Quality Controls Are Essential.
- Welders and procedures must be qualified.
- NDE must be performed per code requirements.
- Weld defects beyond allowable limits must be repaired.
- Seismic and Wind Design: Must follow API 650 rules and regional building codes.
- Foundations: Must prevent excessive settlement — often involve ringwalls or concrete pads.
- Hydrostatic Testing: Mandatory to confirm tank integrity after construction.
- Inspection: Includes radiographic testing (RT), magnetic particle testing (MT), and visual inspection (VT).
- Documentation and Traceability: Complete tank data sheets must be prepared, documenting design parameters, materials, weld procedures, inspection results, and testing.
Conclusion
API 650 is the foundational standard for welded steel storage tanks worldwide, balancing safety, durability, and environmental stewardship. Its comprehensive requirements for materials, design, fabrication, inspection, and testing ensure tanks perform reliably under operational and environmental stresses.
By understanding the scope, key sections, critical design parameters, and best practices outlined in this guide, engineers and industry professionals can confidently apply API 650 to their projects, ensuring compliance and operational excellence without needing to parse the entire code document.
