An Overview of API 650 – Welded Tanks for Oil Storage
Updated: 3-Aug-2025
for oil storage. Developed by the American Petroleum Institute (API), this globally recognized code is mandated for aboveground storage tanks (ASTs) operating at atmospheric pressure that store 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 of API 650 Code
The API 650 standard was first introduced in 1961 by the American Petroleum Institute to provide a unified framework for the design, fabrication, erection, and inspection of welded, aboveground storage tanks. Before its publication, tank construction methods varied significantly, leading to inconsistencies in safety, reliability, and quality across the industry. API 650 addressed this gap by establishing consistent criteria based on sound engineering practices and practical field experience, particularly for tanks storing oil, gas, and other liquid products.
Over the years, the standard has undergone numerous updates to reflect advancements in technology, materials, and safety standards. Each revision has incorporated improvements in areas such as welding procedures, nondestructive testing, structural design methods, and seismic considerations. The most recent 13th Edition, released in March 2020, introduced enhanced guidelines for low-temperature applications, expanded material options, stricter inspection protocols, and a stronger emphasis on health, safety, and environmental performance—ensuring the code remains relevant to current industry demands.
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:
Included Applications
API 650 is widely used for the design and construction of welded, aboveground storage tanks intended to store a broad range of liquid products under atmospheric or low-pressure conditions. The standard is applicable to various tank configurations and service environments commonly found in the oil, gas, petrochemical, and industrial sectors.
Typical Stored Liquids:
API 650 covers tanks used for storing:
Crude Oil and Refined Products: Including gasoline, diesel, jet fuel, and other hydrocarbons.
Ethanol and Biofuels: Alcohol-based fuels used either standalone or blended with gasoline.
Industrial Chemicals and Acids: Compatible with the material limitations outlined in the standard.
Non-Potable Water: Such as firewater, process water, stormwater, or wastewater used in industrial operations.
Supported Tank Designs:
Fixed Roof Tanks: Tanks equipped with cone-shaped, dome, or umbrella-style roofs that are permanently attached (welded) to the tank shell. Suitable for storing liquids with low vapor pressure.
Floating Roof Tanks: Designed with internal or external floating roofs that rise and fall with the liquid level, effectively minimizing vapor space and reducing product loss due to evaporation. Commonly used for volatile hydrocarbons.
Covered/Open-Top Tanks: Includes tanks with open tops that may be fitted with geodesic domes, secondary containment structures, or other types of covers to control emissions, limit contamination, or provide weather protection.
Exclusions of API 650
While API 650 provides comprehensive guidelines for the design and construction of welded, aboveground storage tanks operating at atmospheric pressure (or up to 2.5 psig), it specifically excludes certain types of tanks and applications that fall outside its intended scope. These exclusions are important to ensure proper application of relevant standards and safety practices.
API 650 does not apply to the following:
- Refrigerated or Low-Temperature Tanks: Tanks designed to store liquefied gases or refrigerated liquids at low temperatures and higher internal pressures. These are covered under API 620, which provides design requirements for tanks operating at pressures up to 15 psig, including cryogenic service.
- Bolted or Modular Tanks: Storage tanks assembled using bolted panels or modular components rather than being fully welded.
- Underground Storage Tanks: API 650 is intended only for aboveground tanks. Tanks intended for buried installation fall under different standards due to different structural and environmental considerations.
- Tanks with Bolted Shell or Bottom Connections: Any tank utilizing bolted joints in critical areas like the shell or bottom plates is excluded, as API 650 assumes continuous welded construction for structural integrity.
- High-Pressure Tanks: Tanks designed to operate at internal pressures exceeding 2.5 psig (17.2 kPa) are outside the scope of API 650. These should be designed using API 620 or pressure vessel codes like ASME Section VIII.
- Tanks for Compressed Gases or Cryogenic Liquids: API 650 is not suitable for storage of compressed gases, liquefied gases, or cryogenic fluids due to the specialized requirements for pressure containment, thermal insulation, and material selection.
Why These Limits Matter
API 650 is intentionally limited to atmospheric or low-pressure storage tanks (up to 2.5 psig) because such tanks are not designed to withstand significant internal pressure or vacuum. These tanks rely on natural ventilation and external relief systems to manage pressure fluctuations during filling, emptying, or thermal expansion. Exceeding these pressure limits can lead to severe structural failures, such as shell buckling, bottom uplift, or roof collapse—posing safety hazards and environmental risks.
By clearly excluding tanks intended for pressurized service, cryogenic storage, or underground installation, API 650 ensures that each tank design follows the most appropriate standard. For example, API 620 is used for low-temperature and moderately pressurized applications, while ASME Section VIII governs high-pressure vessels. This distinction is critical for maintaining structural integrity, optimizing material selection, and complying with regulatory safety requirements.
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
Interdependent Design Parameters:
API 650 mandates that shell thickness calculations must consider tank diameter, liquid height, material tensile strength, welding joint efficiency, and corrosion allowance. The code provides explicit formulas to calculate minimum thickness based on these parameters.
For bottom plates, thickness depends on soil bearing capacity and the weight of liquid, which the engineer must assess through geotechnical studies. Roof thickness must accommodate external loads such as wind and snow, per the code’s load tables and regional codes.
Engineers must integrate all these interdependent factors for safe design and document the assumptions clearly.Material Selection and Certification:
API 650 does not prescribe a single material for specific liquids like water or diesel but requires that materials be selected from those approved in the standard, which includes carbon steel grades typically used for atmospheric storage tanks.
The code specifies materials must comply with referenced ASTM/API standards for chemical composition, mechanical properties, and toughness. For example, tanks storing water or diesel generally use carbon steel (ASTM A36, A283, A516 grades), but highly corrosive liquids or acids may require specialty materials or coatings—this selection is at the engineer’s discretion, based on chemical compatibility and corrosion considerations.
Material certification is mandatory: Mill test reports and traceability documentation must accompany all materials to verify compliance with ASTM/API specs.Corrosion Allowance:
API 650 requires a corrosion allowance to be added to all thickness calculations to ensure structural longevity. The minimum corrosion allowance is typically 1.5 mm (1/16 inch), but can be increased to 3 mm (1/8 inch) or more depending on the stored liquid’s corrosivity and environmental factors such as soil conditions or presence of salts.
This allowance is not fixed by the code but must be justified by the engineer based on operating conditions and maintenance plans.Joint Efficiency and Welding Quality:
The code requires welding joint efficiencies to be incorporated into thickness calculations. This means that the quality of welds directly affects required thickness—higher quality welds allow for thinner shells.
Welders and welding procedures must be qualified under recognized standards (e.g., ASME Section IX).
API 650 mandates non-destructive examination (NDE) methods such as radiographic testing (RT), magnetic particle testing (MT), or ultrasonic testing (UT) for welds based on the tank class and criticality.
Weld defects exceeding allowable limits must be repaired to ensure safety and code compliance.Seismic and Wind Design:
API 650 includes detailed provisions for designing tanks to withstand seismic and wind loads, using formulas and coefficients based on location and tank size. These are mandatory where applicable.
Additionally, local or regional building codes must be consulted and followed if they impose stricter requirements.
The engineer is responsible for verifying site-specific loading conditions and applying the correct design rules.Foundation Design:
API 650 requires foundations to support the tank and prevent excessive settlement but does not prescribe a specific foundation type. Common solutions include ringwall foundations or concrete pads.
Soil bearing capacity must be determined by geotechnical investigation. The foundation design is therefore left to the engineer’s judgment but must ensure structural stability and compliance with relevant standards.Hydrostatic Testing:
Hydrostatic testing is mandatory under API 650 after construction and erection to verify the tank’s structural integrity and leak tightness. The test pressure and procedures are specified in the code.
This step is critical before placing the tank in service.Inspection and Documentation:
API 650 requires thorough inspection at various stages: visual inspection (VT), radiographic testing (RT), magnetic particle testing (MT), and other NDE methods as appropriate for weld quality and tank safety.
All inspections must be documented in detail, and a complete record of material certificates, welding procedures, test reports, and design calculations must be maintained for traceability and quality assurance.Engineer’s Role and Judgments:
While API 650 clearly prescribes mandatory requirements (e.g., minimum thickness, testing, NDE, material standards), it leaves critical decisions such as corrosion allowance selection, foundation design, and material compatibility to the engineer’s professional judgment based on site conditions, product stored, and operating environment.
Understanding this balance is essential for engineers to design safe, compliant, and cost-effective tanks.
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.
