Introduction
Storage infrastructure is one of the most strategic links in the hydrocarbon value chain. The structural reliability of storage tanks has a direct impact on operational safety, environmental integrity and profitability of industrial assets. Within this context, the American Petroleum Institute’s API 650 standard has established itself as the international reference standard for the design, fabrication, inspection, and testing of welded atmospheric storage tanks.
API 650 defines the best engineering practices to ensure the safe and efficient performance of tanks, and establishes a technical framework on requirements for materials, welds, test procedures, and acceptance criteria. This provides a common platform for operators, manufacturers, insurers, and regulatory bodies. Its adoption is a prerequisite for obtaining operating permits, liability insurance, technical and aligned with international industrial safety standards.
This article provides a detailed analysis of the historical origin of API 650, its regulatory evolution, its specific technical scope and the tangible benefits it brings to modern petroleum facilities.
What is API Standard 650?
API Standard 6501, known as Welded Petroleum Storage Tanks, was developed and published by the American Petroleum Institute (API) to establish minimum requirements for the design, fabrication, construction, and testing of welded storage tanks. This specification applies exclusively to tanks seated on uniformly supported bottoms and intended to operate in non-refrigerated service conditions, with a maximum design temperature of 93 °C (200°F) or lower. Originally published in 1973, the standard has evolved through multiple revisions, with the 13th edition being the most recent, issued in march 2020.
The following video shows a video on the assembly and construction of a storage tank. Source: FYRESA GROUP.
Projects for the oil industry.
Origins and history of API 650
The emergence of API 650 is closely linked to the boom of the oil industry in the 20th century, a time when the accelerated expansion of crude oil production and refining highlighted the need for safer and more standardized storage systems. Before the existence of the standard, tanks were designed under empirical criteria, which resulted in technical inconsistencies, catastrophic accidents and high maintenance costs.
The American Petroleum Institute (API), founded in 1919, took the lead in developing technical industry standards, promoting safety, efficiency, and interoperability. In 1961, the first edition of API 650 was published, consolidating unified criteria for the design of welded tanks intended for atmospheric storage of liquids at limited internal pressures.
Since then, the standard has evolved through multiple revisions incorporating advances in materials science, welding techniques, nondestructive testing methods, seismic analysis and design requirements for extreme environmental conditions. Each update reflects a technical response to new operational challenges, as well as an assimilation of lessons learned from relevant incidents in the industry.
Technical scope of API Standard 650
API 650 establishes guidelines for the design, construction, inspection, and testing of cylindrical, welded, flat-bottom steel tanks intended for the storage of liquids at internal pressures not exceeding 2.5 psig (17.2 kPa) and at typical operating temperatures of -40 °C to 93 °C (-40 °F to 93 °F). Their field of application covers small, medium, and large volume tanks used in refineries, export terminals, chemical, and processing plants2.
Among the most relevant technical aspects, the following stand out:
- Structural design criteria: Based on allowable stresses using linear elastic analysis methods, considering hydrostatic loads, wind loads (according to ASCE 7), seismic loads (according to API 650 Appendix E or alternatives based on spectral response analysis) and accidental loads. Minimum thickness requirements, form factors for conical or vaulted roofs and provisions for differential ground settlement are defined.
- Material selection: The standard specifies acceptable materials for construction, which include carbon steels, low alloy steels and austenitic steels for cryogenic service (per Appendix Q). Certification of mechanical properties to applicable ASTM standards is required, as well as strict chemical composition limits to mitigate phenomena such as embrittlement.
- Welding procedures: Requirements are established for welding procedure qualification (WPS) and operator qualification (WPQ) in accordance with ASME Code Section IX, including tensile, bend, Charpy impact and Brinell hardness tests, depending on the criticality of the service and location of the joints.
- Inspection and Non Destructive Testing: API 650 prescribes the application of NDT methods such as ultrasonic, magnetic particle, liquid penetrant or radiography, depending on the type of joint and its accessibility. In addition, guidelines are established for final pneumatic or hydrostatic testing as an indispensable condition for acceptance.
- Special accessories and components: The standard regulates the design and installation of nozzles, manholes, hatches, pressure/vacuum relief valves, bottom drains and accidental overpressure protections.
Manufacturing processes, testing and acceptance testing of API 650 storage tanks
The construction of API 650 storage tanks involves a series of critical steps that ensure structural integrity, fabrication quality and compliance with regulatory requirements. From welding procedures to acceptance testing and coating application, each phase must be rigorously executed and verified to ensure the long-term reliability of assets in the petroleum industry. In this context, it is important to comply with the following steps during the fabrication process and inspection of storage tanks:
Welding procedures: Welding of tanks is performed according to qualified procedures (WPS) and performance specifications (PQR) endorsed under codes such as ASME Section IX or equivalent specifications. The selection of welding techniques – such as submerged arc welding (SAW) or shielded metal arc welding (SMAW) – depends on factors such as plate thickness, welding position and base material. Each welded joint must meet strict requirements for penetration, bead profile and freedom from discontinuities.
Liquid penetrant inspection: The liquid penetrant (PT) method is used to detect surface discontinuities in critical welds, such as cracks, open porosities or lack of fusion. This test is mainly performed on tank bottom welds and fitting connections, following procedures based on standards such as ASTM E165 or ISO 3452.
As built of welds and location of radiographs: The “as built” record of welds documents each joint executed, specifying its location, identification number, type of test applied and results obtained. In high criticality joints, industrial radiographs (RT) are performed according to API 650 and ASME V, to validate the internal quality of the weld, ensuring the absence of unacceptable defects such as slag inclusions or lack of fusion.
Gas oil test: The gas oil test is used as a preliminary leak test to detect leaks in the bottom welds. It consists of applying a low viscosity liquid (gas oil) on the opposite side of the joint while visually observing the possible appearance of the fluid on the opposite side, detecting micro-defects that might not be evident in conventional visual inspections.
Bottom vacuum test: The vacuum box test is performed on bottom welds, especially on plate joints. A vacuum box is used which, by creating a differential pressure over the inspected area, makes it possible to identify any air leakage, indicative of sealing defects in the weld.
Hydrostatic test: The hydrostatic test on tanks is a fundamental requirement for final acceptance, whereby the tank is completely filled with water up to the maximum design level. This test verifies the structural strength and tightness of the tank, according to the procedures established in Section 7 of API 650. Parameters such as fill rate, pressure dwell and acceptance criteria based on visual observations of leaks must be controlled.
Internal coating application: Once the integrity tests have been passed, the internal coating is applied, using epoxy paint systems, polyurethane or specialized anticorrosive coatings, depending on the type of product to be stored in the tank. The process involves surface preparation by shot blasting (with controlled roughness according to NACE/SSPC) and the application of coatings with specified thicknesses, verified by magnetic induction thickness gauges.
Exterior cleaning and painting: The exterior cleaning and painting system is part of the protection against atmospheric corrosion. Standards such as SSPC-SP10 are followed for surface preparation and multi-layer protection systems are applied, including primer, intermediate coat and finish, selected according to the aggressiveness of the environment (rural, industrial or marine).
Calculation of optimal inspection frequency: Finally, to ensure in-service risk management, an optimal inspection frequency is established based on Risk-Based Inspection (RBI) methodologies and API 653 guidelines. Factors such as corrosion rate, operating conditions, previous inspection results and component criticality analysis are considered, allowing the definition of internal, external, and background inspection intervals.
Benefits for the oil industry
The adoption of API 650 offers strategic benefits to the oil industry, including:
- Improved operational safety: The standardization of design criteria reduces the probability of structural failures, minimizing the risk of leaks, fires or explosions that could result in human losses and irreversible environmental damage.
- Economic efficiency: Optimized design based on proven safety parameters allows the construction of more efficient tanks in terms of materials and execution time, positively impacting investment (CAPEX) and operation (OPEX) costs.
- Facilitation of inspection and maintenance: The API 650 regulatory framework establishes a solid foundation for periodic structural integrity inspections (API 653), allowing for predictive maintenance schedules that extend the useful life of assets.
- International recognition: Compliance with API 650 is a globally recognized endorsement of technical quality, which facilitates the homologation of projects with international clients, insurance companies and regulatory bodies.
- Flexibility of application: The standard allows adaptations for specific operating conditions, such as storage of products with high volatility, exposure to corrosive environments.
Current challenges and considerations
The energy transition, industrial digitalization and the tightening of environmental regulations require storage facilities to be designed under new premises of resilience, energy efficiency and sustainability. In this regard, API 650 must coexist with additional criteria such as.
- Implementation of on-line monitoring systems for early detection of structural failures.
- Adaptation for tanks intended for storage of new energy products (biofuels, liquid hydrogen, etc.).
- Strengthening of secondary containment requirements and mitigation of accidental releases.
Continued review and modernization of API 650 will be key to maintaining its technical relevance in the new global energy paradigm.
Conclusion
API 650 represents a technical compendium of decades of knowledge regarding innovation and commitment to industrial safety. Its correct application guarantees the construction of reliable storage tanks adapted to the most demanding operational scenarios. Facing the emerging challenges of the industry, maintaining technical discipline in the design and construction under API 650 will be a decisive competitive advantage for oil and energy companies that aspire to lead the market in the coming years.
References
- API Standard 650, “Welded Tanks for Oil Storage”, latest edition.
- Sánchez, M Ángel; Diseño y Cálculo de Tanues de Almaenamiento. https://harvard.academia.edu/Miguel%C3%81ngelLuceroS%C3%A1nchez
