API 653: 35 years ensuring tank integrity

Introduction

API 653, developed by the American Petroleum Institute (API), establishes the minimum requirements for the inspection, repair, alteration and reconstruction of aboveground steel storage tanks. This standard is essential to maintain the integrity and safety of tanks used in the petroleum and other process industries, ensuring that they operate efficiently and safely throughout their service life.

API 653 not only provides technical guidelines, but also promotes a culture of preventive maintenance, which is essential to reduce environmental, economic and safety risks. By applying this standard, companies comply with international regulations and also protect their most valuable assets and the people who work with them (API, 2023).

History and evolution of API Standard 653

The first edition of API 653 was published in 1991 in response to the need to regulate the maintenance and structural integrity of tanks already in operation, many of which had been built under API 650. Unlike the latter, which focuses on the construction phase, API 653 concentrates on the subsequent life cycle of the tank: its operation, maintenance and renovation (API, 2023).

In fact, the API description is very clear in stating: “This standard covers steel storage tanks constructed according to API 650 and its predecessor, API 12C. It establishes the minimum requirements for maintaining the integrity of such tanks after they are placed in service and addresses inspection, repair, alteration, relocation and reconstruction. Its scope is limited to the tank foundation, bottom, shell, structure, roof, fittings and nozzles at the face of the first flange, first threaded joint or first welded connection.

The design, welding, examination and material requirements of API 650 may be applied in the maintenance inspection, classification, repair and alteration of in-service tanks. In the event of apparent discrepancies between the requirements of this standard and API 650 or its predecessor, API 12C, this standard will govern for tanks that have been placed in service. This standard employs the principles of API 650; however, owners/operators of storage tanks, considering the specific conditions of construction and operation”.

What you need to know about API 635 in 2025

Over the years, this standard has been revised and updated several times. Each edition has incorporated significant changes, for example, such as the inclusion of advanced inspection methods, more stringent corrosion criteria, and more detailed procedures for repair documentation. In addition, specific sections on leak testing, differential settlement evaluation and more accurate calculation methods for remaining tank life have been added.

Table of editions and updates

Edition / Addendum	Publication date	Key changes
1st edition	1991	Original publication
2nd edition	1995	Inclusion of reparation criteria
3rd edition	2001	More rigorous tank bottom evaluation
4th edition	2009	Structural review and integrity testing
5th edition	2014	Use of new NDT techniques
Addendum 1	2018	Adjustments in ultrasonic inspection
Addendum 2	2020	Adjustments in regulations for sulfuric acid tanks
Addendum 3	2023	Calculation of the maximum initial internal inspection interval of a tank
Technical Errata 1	2020	Regulatory reference to the specific NACE standard for storage tanks for spent sulfuric acid in oil refinery alkylation units at ambient temperature
Technical Errata 2	2025	Section 4.3.3.1: Clarifies the diameter limit (≤ 61 m or 200 ft) for certain methods. Section 4.3.3.1, listing a): Defines precisely the joint efficiency “E” for USC measurements. Section 4.4.6.1: Modifies the sentence on minimum annular plate thickness with “usually”. Section 6.4.2.1.1.1: The third line of the equation for the initial maximum internal inspection interval, which we had already discussed in your previous question, is added. Section 9.1.3: Requires approval for all alterations and repairs, referring to the specific section. Section 9.1.4, listing b): Specifies that a licensed inspector or tank engineer approves minor/routine repairs not covered in 9.1.4 a). Section 9.9.2: Defines the variable “H” as the height from the centerline of the penetration to the maximum liquid level. Section B.2.2.4: Clarifies the use of measurement points to calculate the optimal cosine curve and iS value.

As can be seen, the corrections and clarifications introduced by Errata 2 of February 2025 are important and contribute to a better understanding and application of API 653 by clarifying specific aspects related to these works, along with other topics such as inspection methods and interval calculations. And let’s see why:

• Clarifications: Many of the changes are clarifications of definitions (such as “E”) or clarifications of the application limits (such as in Section 4.3.3.1). This helps to avoid misinterpretation, but does not introduce completely new methodologies for performing the work.
• Emphasis on approval: Changes to Section 9 (approval of work) reinforce the need for oversight by licensed inspectors or engineers, which is good practice and was already implicit in the standard, but is now further explained.
• Equation correction: The inclusion of the missing line in the inspection interval equation is crucial to the correct application of that section, but it is a correction, not a fundamental improvement in the performance of the repair, alteration or maintenance work itself.
• Variable definition: Clarifying the definition of “H” in Section 9.9.2 is important for specific calculations related to penetrations, but does not radically change how repairs or alterations are made.

Errata 2 contributes to a more accurate and consistent application of API 653 in all its aspects, including repair, alteration and maintenance. By clarifying ambiguous points and correcting errors, it helps ensure that these works are performed in accordance with the intent of the standard. However, it does not introduce fundamental changes to the techniques or procedures for performing and inspecting this work.

Importance of API 653 inspection

Inspection according to API 653 is one of the fundamental pillars to ensure the operability and safety of storage tanks. Its main purpose is to detect conditions that may compromise the structural integrity of the tank before they become major failures (API, 2023).

Inspection is classified into two types: external and internal. The external inspection allows a superficial review, without the need to stop operations, ideal to verify the condition of the coating, identify atmospheric corrosion or visible deformations. The internal inspection, on the other hand, is much more exhaustive and requires taking the tank out of service, which allows examining the bottom and the internal welded joints, critical areas for accelerated deterioration.

In addition, the standard establishes maximum intervals for each type of inspection, which can be modified according to the history of the tank, the products stored, and the environment to which it is exposed. In this way, a flexible and adaptive maintenance strategy is promoted (API, 2023).

In other words, the importance of the API 653 inspection lies in:

• Ensure structural integrity: The inspection detects and evaluates degradation (corrosion, cracks, deformations) of aboveground storage tanks, ensuring that they remain safe to operate.
• Prevent failures and leaks: Regular and periodic inspection helps to identify incipient problems before they become catastrophic failures, preventing product loss, environmental damage and safety risks to personnel and communities.
• Comply with regulations: Many government regulations and industry standards require routine tank inspections according to API 653.
• Optimize service life: By identifying problems early and implementing appropriate repairs, inspection helps to extend tank service life safely and economically.
• Ensure operational reliability: A well-maintained tank is more reliable and reduces the risk of unplanned operational interruptions.

The role of tank inspector

As for the role of the API 653 certified tank inspector, they are a key figure within this system. They are not only responsible for performing inspections in accordance with the standards, but also for correctly interpreting the data obtained and recommending corrective actions. To obtain this certification, it is required to meet rigorous requirements of work experience, technical training and passing a demanding exam covering multiple disciplines (API, 2023).

This professional must have knowledge in non-destructive testing (NDT), structural analysis, metallurgy and safety regulations. Their work has a direct impact on operational safety, but also on the profitability of industrial facilities by avoiding unscheduled shutdowns and costly failures.

In addition, their work includes the preparation of detailed technical reports documenting the condition of the tank, actions taken and future recommendations. The traceability of this information allows planning maintenance based on historical data and projecting the remaining useful life of the equipment. To further specify, your primary responsibilities should include:

• Inspection planning and execution: Determine the scope, methods and frequency of inspections in accordance with API 653 and specific tank conditions.
• Tank condition assessment: Perform visual inspections, non-destructive testing (NDT) and measurements to identify and quantify tank degradation (thickness, corrosion, weld defects, etc.).
• Result interpretation: Analyze the data collected during the inspection and compare it to the API 653 acceptance criteria to determine the tank condition.
• Thorough documentation: Record in detail the inspection findings, including the location and severity of defects, measurements taken, and recommendations for repair or monitoring.
• Corrective action recommendation: Based on the assessment, the inspector should recommend repairs, alterations or future monitoring necessary to maintain the integrity of the tank.
• Repair verification: In many cases, the inspector also has a role in verifying that repairs and alterations are performed in accordance with applicable standards.
• Knowledge maintenance: Be updated on the latest editions, addenda and errata of API 653 (currently February 2025), as well as recommended best practices and industry case studies.
• Effective communication: Clearly communicate findings and recommendations to those responsible for tank operation and maintenance.

The API 653 tank inspector acts as a guardian of tank integrity, using his knowledge and experience to ensure that these critical assets operate safely, reliably and in compliance with applicable standards.

Recent improvements and updates

The most recent API 653 updates have marked considerable progress toward more proactive tank integrity management. For example, inspection technologies such as automated ultrasonic mapping and acoustic emission have been incorporated, allowing for more accurate and less invasive assessment (API, 2022).

The standard’s documentary approach has also been strengthened, requiring stricter control over records of maintenance, repairs and structural changes. This not only improves the traceability of the actions performed, but also facilitates audits and compliance certifications.

Best practices for repair have also been defined, such as the use of liners or internal coatings, partial bottom replacements and the application of reinforcement welds. These techniques allow extending the useful life of the tank without the need for complete replacements, reducing costs and improving operational efficiency.

Conclusions

API 653 represents an indispensable tool for the safe and effective management of storage tanks in various industries. Its constant evolution, based on industry experience and technological advances, has enabled the establishment of world-class standards in the inspection, repair and reconstruction of these critical assets.

The role of the tank inspector and the correct application of the API 653 inspection allows not only to comply with the regulations, but also to prevent catastrophic failures, optimize resources and extend the useful life of the equipment.

Finally, with each revision, this standard consolidates its position as a global technical reference, and its proper implementation is a guarantee of operational responsibility, environmental sustainability and industrial safety.

References

1. American Petroleum Institute. (2023). API Standard 653: Tank Inspection, Repair, Alteration, and Reconstruction. API Publishing. https://www.api.org
2. American Petroleum Institute. (2022). Addendum 1 to API 653, 5th Edition. API Publishing. https://www.apiwebstore.org/standards/653?utm_source=adwords&utm_medium=ppc&utm_campaign=International%20-%20Central%2FSouth%20America&utm_term=api%20standard 
3. American Petroleum Institute. (2023). Errata Sheet for API 653. API. https://www.apiwebstore.org/standards/653?utm_source=adwords&utm_medium=ppc&utm_campaign=International%20-%20Central%2FSouth%20America&utm_term=api%20standard