In 2024, the American Petroleum Institute (API) initiated the development of a new standard: API 626 – Inspection and Evaluation of Refrigerated and Cryogenic Storage Tanks. The driving force behind this standard came from regulatory authorities and increased demand for LNG. Most LNG tanks in the United States are well beyond their originally contemplated design life.
A recent government-funded study revealed that the industry lacks consensus on best practices for LNG storage tank integrity, prompting the API standard to initiate this new standard. The standard will fill a gap in the industry by providing a reliable consensus document on inspection and evaluation best practices for all stakeholders: owner/operators, inspection companies, repair contractors, engineering firms and regulators.
The mechanical integrity and repair of refrigerated tanks are critical activities, and without industry best practices could result in significant incidents. The new standard will apply to cylindrical, aboveground, refrigerated storage tanks, essentially those originally designed to API refrigerated tank construction standards. The most notable storage classes are LNG and ammonia. However, the standard can also be applied to a more diverse class of refrigerated liquids, such as liquefied industrial gases. The first edition of the standard will not include repair best practices due to the accelerated schedule, but subsequent editions will address this topic.
U.S. Government initiative
The U.S. government consulted with the industry to determine whether there was sufficient guidance on the standards for LNG tanks and common best practices, as well as whether there were standards in other countries that could provide comprehensive inspection and repair guidelines. The answer was “no.” More details and the rationale can be found on the PHMSA project page. To fill this gap, the API standard created a working group to initiate the drafting of this standard: API 626. The first meeting of the group was held in St. Louis in May 2024.
Internal inspection of refrigerated tanks
How often should refrigerated tanks be removed from service for internal inspection? This is one of the most important questions facing the API 626 working group. API 653 certified tank inspectors inspect typical petroleum storage tanks on a regular schedule of approximately 20 years.
Some refrigerated tank companies rarely, if ever, remove tanks from service for inspection, while others do; there is no standardized practice. Age-related damage mechanisms, such as corrosion, behave differently in refrigerated tanks. For example, corrosion is a chemical reaction, and the extreme low temperatures of refrigerated and cryogenic tanks slow down the rate of this reaction. The product environment inside the tanks is devoid of oxygen and water, which virtually eliminates internal corrosion.
The question of when to inspect refrigerated tanks is being resolved by consensus by the API 626 working group, based on the condition and events affecting the storage tank, which will allow for variable, some even indefinite, internal inspection intervals. Arbitrarily removing refrigerated tanks from service requires more tanks to meet demand, which is costly, plus many refrigerated tanks do not require regular internal inspections due to virtually non-existent corrosion.
Today, the industry has a sufficient understanding of damage mechanisms, and the API 626 working group has the necessary basis for deciding when to remove tanks from service for internal inspections. The prevailing consensus is to keep the tank in service until there is a reason or event that justifies taking it out of service, which we call an event-triggered inspection.
Possible future problems without API 626
To date, the industry has not recorded serious fatigue failures in a refrigerated tank, although fatigue could contribute to stress corrosion cracking damage in ammonia tanks, where failures have occurred.
Metal fatigue is a process that weakens metal until it eventually breaks down due to repeated stresses. Whenever a refrigerated tank is heated to ambient temperature, thermal excursion can cause damage in the form of crack growth. Fatigue failures are a major concern for older tanks, tanks subjected to frequent cycling, manufactured with different materials and techniques, or susceptible to environmental cracking mechanisms. Fatigue may be one of the most common motivations for removing a tank from service in event-triggered inspections.
While modern tanks can last for centuries without significant mechanical integrity problems, older tanks from the 1950s to 1970s may be at significant risk of fatigue failure. The tanks studied have exhibited a characteristic “break-before-leak” fracture mode, which is of further concern as fatigue damage accumulates over time.
Participation of regulator and owner/operator committees
Developing consensus on these issues requires reliable data. Owners should collaborate by providing information and participating in the discussion to determine the fatigue life of tanks based on materials and design. Regulatory authorities should be involved to express their concerns and learn the industry’s position on internal inspections. In this way, a universally accepted standard can be published.
No country or Standards Development Organization (SDO) has complete inspection standards for refrigerated tanks. For this reason, the U.S. Government asked API to develop such a standard. API is a leader among SDOs in the power, oil and gas industry, and is in a strong position to produce the fundamental API 626 standard for inspection and evaluation of refrigerated and cryogenic tanks.
Working group progress
The API Task Group is chaired by Andrew Yearwood (PEMY Consulting) and has started its work quickly, driven by the participation of 100 members. The evaluation chapter will provide guidelines on interpretation of inspection findings and is expected to be consistent with the Fitness-for-Service principles of API 579, which already addresses evaluation of corrosion, fatigue, deformation and other aspects.
While the new API 626 will share similar principles to the well established API 653 the character of the standard will be different. API 653 is more prescriptive, with explicit rules and clear criteria. API 626, on the other hand, will be more performance-based, with inspections planned by a multidisciplinary team of inspectors and engineers. In many cases, the results will have to be evaluated according to engineering criteria developed on a case-by-case basis, i.e., it will be more engineering-focused. The goal is to complete the standard by 2028.
Conclusion
Creating a worldwide refrigerated tank inspection standard that meets the needs of regulators, owners, operators and the public is vital for energy self-sufficiency, regardless of the country.
This article was developed by specialists from PEMY Consulting and published as part of the sixth edition of the Inspenet Brief September 2025, dedicated to technical contents of the energy and industrial sector.