API 650 Storage Tanks

API 650 Storage Tanks, published by the American Petroleum Institute, is the global industry standard that defines the design, material, fabrication, and inspection rules for field-welded steel tanks designed for the safe and efficient storage of liquids such as petroleum, fuel, chemicals, and water under atmospheric or very near-atmospheric low pressures. These massive structures are critical infrastructure elements in many areas, from refineries and terminals to power plants and industrial facilities, holding the strategic liquid inventory of a country or facility. Although they are often mentioned within the pressure vessels category, the fundamental design philosophy of API 650 Storage Tanks is not to withstand high internal pressure, but to withstand the hydrostatic load (liquid weight) of the stored liquid and external loads such as wind, snow, and seismic activity. In this field, companies like Cryotanx, which have a broad engineering portfolio in both high-pressure systems and atmospheric storage solutions, possess the capability to execute these large-scale and complex projects at the highest level of quality and safety required by international standards.

The API 650 Storage Tanks standard is a reference point in the industrial storage world, and its engineering philosophy is built upon practical, safe, and proven methods shaped by decades of experience. Understanding the scope of the standard correctly is critical to distinguishing these tanks from other storage solutions. API 650 covers steel tanks that are vertical, cylindrical, open or closed-top, flat-bottomed, and field-erected with fully welded joints.

Its most important distinguishing feature is that its operating pressure is very close to atmospheric pressure; it is generally limited to not exceed the pressure created by the tank’s own weight. This fundamentally separates them from pressure vessels designed to withstand high internal pressure, such as EN 13445 / AD 2000 / ASME coded vessels. While the main challenge in an ASME vessel is to contain internal pressure, the main challenge in an API 650 storage tank is to manage the weight of thousands of tons of liquid and external factors. This basic philosophy also determines the design approach. When calculating the shell thickness of the tank’s cylindrical body (shell), one of API 650’s most well-known methods, the “One-Foot Method,” is used. In this method, the thickness of each shell course is calculated based on the hydrostatic pressure at a point one foot (approx. 30 cm) above the bottom elevation of that course and the allowable stress of the material. This practical approach ensures that the lower parts of the tank, where the liquid height is greatest, are thicker, and gradually become thinner towards the upper parts as the pressure decreases. This results in an economical and safe design where the material is used efficiently.

The integrity of an API 650 storage tank depends not only on its steel structure but also on the foundation it rests upon. The design and preparation of the foundation that will carry this massive weight is one of the most critical stages of the project. The bottom plates of tanks, which are generally built on a concrete ring wall or a gravel foundation, also have a special design. “Annular plates,” which are thicker than the standard bottom plates, are placed specifically in the peripheral region where the shell joins the bottom and where stresses are most concentrated. This is essential for the tank’s long-term structural integrity. Furthermore, the external loads that API 650 Storage Tanks are exposed to also play an important role in the design.

Due to their large surface areas, these tanks can be exposed to significant wind loads. Steel reinforcing rings called “wind girders” are added to the upper parts of the shell to prevent the wind from buckling or crushing the shell when the tank is empty. In seismically active regions, special seismic calculations are made to prevent the tank from sliding or overturning during an earthquake and to manage the sloshing effects of the liquid inside, and the tank is fixed to the foundation with anchor bolts. This holistic approach makes API 650 Storage Tanks a reliable solution in all types of geographical and climatic conditions.

The roof of an API 650 storage tank is not just a simple cover that protects the inside of the tank from rain and snow; it is also a critical engineering system that protects the quality of the stored product, prevents evaporation losses, and controls environmental emissions. The API 650 standard permits different roof types depending on the type of product stored, its volatility, and environmental regulations. This diversity allows experienced manufacturers like Cryotanx to offer their customers the most suitable solution. The most basic roof type is fixed roof systems. These also differ among themselves. The Conical roof (Cone Roof) is a relatively simple design to build, usually supported by columns in the center or periphery.

For larger diameter tanks, dome roof (Dome Roof) options, which are structurally more efficient and can be self-supporting without the need for any internal support columns, are preferred. The absence of internal columns is a reason for preference as it keeps the inside of the tank cleaner and provides advantages in some operations (e.g., the use of agitators). These fixed-roof API 650 storage tanks are generally used for storing low-volatility liquids such as water and fuel oil. However, when crude oil, gasoline, or other volatile organic compounds (VOCs) are stored, the situation changes completely. When such products are stored in a fixed-roof tank, they accumulate in the vapor space inside the tank, leading to both serious economic losses (product evaporation) and harmful emissions that cause air pollution.

To solve this problem, “floating roof” (Floating Roof) systems were developed. A floating roof is literally like a giant disk made of steel or aluminum that floats on the surface of the liquid. As the liquid level rises and falls, the roof moves with it, leaving almost no vapor space between the liquid surface and the atmosphere. This reduces evaporation losses and emissions by a rate of over 95%. There are two main types of floating roofs: The External Floating Roof (EFR) floats directly on the liquid inside an open-top tank. The Internal Floating Roof (IFR), on the other hand, is a system where the tank also has a fixed roof, meaning it is a dual-protection system.

These IFR-equipped API 650 storage tanks both protect the product from evaporation and, thanks to the fixed roof, insulate it from external elements like rain and snow. Environmental protection is one of the top priorities in the design of modern API 650 storage tanks. In addition to floating roofs, advanced safety measures such as double-bottom designs with leak detection systems are also implemented to prevent a potential leak in the tank bottom from contaminating the soil or groundwater. This shows that an API 650 storage tank is treated not just as a storage asset, but also with an awareness of it being an environmental responsibility.

The fabrication and erection process of API 650 Storage Tanks is not one that takes place in a factory workshop, but a massive logistical and engineering operation that occurs directly on the site where the project will be located. This process requires special equipment, a highly skilled workforce, and deep experience in project management. The construction of an API 650 storage tank begins with placing and welding the bottom plates onto a meticulously prepared and compacted foundation. The leak-tightness of the bottom welds is precisely checked using methods such as “vacuum box testing.” Afterward, the giant steel plates (shell courses) that form the tank’s cylindrical body are placed one by one with the help of cranes and welded together.

At this stage, especially for large tanks, “jacking-up” systems, where the tank lifts itself upward, are used to accelerate the erection and increase safety. For the horizontal and vertical welds of the shell, modern technologies such as “Automatic Girth Welding” (AGW) are utilized, providing both high quality and speed. All of these processes are carried out within the framework of the strict welding procedures and inspection rules set by the API 650 standard. Quality control throughout the fabrication is of vital importance for the project’s success. A certain percentage of the weld seams is inspected using nondestructive testing methods, such as radiographic (RT) or ultrasonic (UT) testing, to detect internal flaws.

After all welded fabrication is completed and the roof is installed, a “hydrostatic test” is performed to prove the final integrity and leak-tightness of the tank. In this test, the tank is slowly and controllably filled completely with water and held in this state for 24 hours. During this time, it is observed whether there are any leaks and whether any abnormal settlement has occurred in the foundation. An API 650 storage tank that successfully completes this test is then considered ready for service.

Successfully managing such a large and complex project requires not only fabrication knowledge but also strong project management and EPC (Engineering, Procurement, Construction) capability. In this field, Cryotanx serves its customers not just by selling a tank, but as a solution partner that manages all processes from the very beginning to the end of the project. It takes a role in every stage, from detailed engineering and design studies, to the procurement of the right materials, site organization, and the management of quality control and testing processes. The company’s deep experience in the field of pressure vessels and special engineering solutions ensures that the same meticulousness, quality, and safety culture is reflected in these massive field-erected API 650 storage tanks projects. This holistic approach is the guarantee that projects are completed on time, within budget, and at the highest quality.