API 620 or API 650? Understanding the Differences

White storage tanks

The American Petroleum Industry (API) has set standards regarding the processes of design, fabrication, and construction of steel storage tanks. In the petroleum industry, there are two common types of tanks, namely API 650 and API 620 tanks. They were named according to the standards they follow. But, how will you know which model of the tank is right for you?

Having a clear understanding of the two standards guiding the design, fabrication, and construction of these tanks is quite eyed opening, advises an experienced API 650 fabricator. Here is a closer look into the standards:

API 620 and Its Applications

API 620 regulates the manufacture of low-pressure oil storage tanks that are larger than 300 feet in diameter. By design, the configuration of API 620 dictates that you should have a flat or elevated bottom to store your tank. The tank should be made of a material with a minimum thickness of 3/16 inch and have a central, vertical axis of revolution.

API 650 and Its Applications

API 650 applies to both aluminum and carbon stainless tanks that are usually used in pipelines and refineries. It sets standards for above-ground storage tanks both open and closed top and of different sizes. By design, these cylindrical tanks should not weigh more than the roof plates, meaning their pressure should not exceed the atmospheric pressure.

Higher internal pressures are only permissible if the tank meets additional requirements. In addition, API 650 covers tanks that have a maximum temperature of 200 °F.

There are fundamental differences between API 620 and API 650 standards for liquid storage tanks. API 620 allows the storage of liquids at a higher pressure but lower temperature, while API 650 allows contents of high temperatures but low pressure. Identifying these key differences will make it easy for your fabricator to choose a suitable standard for your particular temperature and pressure requirements.