API 650 tanks are commonly repurposed to store a different liquid than what intended when the tank was originally built. This happens in every industry imaginable, because companies want to use their capital resources efficiently. The most important step in the process is to have an API 653 tank inspection performed on the above ground storage tank before the new product is introduced. The inspection evaluates the integrity of the tank and calculates the maximum fill height.
When an API 650 storage tank is constructed, the design of the tank is chosen based on the liquid that will be stored in the tank. The crucial detail in determining the needed thickness of the tank steel is specific gravity of the intended stored product. The product-based design allows the tank to be filled to the maximum capacity.
However, when a different liquid will be 
introduced to an API 650 tank, the ability to fill the tank completely may not be possible due to the different product specific gravity. Another factor that can limit the maximum fill height of an above ground storage tank is the integrity of the tank. The best way to determine the capability of the tank with the new liquid product is by performing an API 653 tank inspection.
The API 653 Standard is the corollary to the API 650 Standard. API 650 is for constructing a new above ground liquid storage tank while API 653 is for the maintenance of an API 650 to determine whether the tank fit for continued service. An API 653 inspection includes measurements and calculations to determine the tank’s maximum fill height as well as non-destructive tests to evaluate the integrity of the shell, floor, and roof for fitness in continued service.
The primary concern of the API 653 tank inspector is to look at the measurement data to determine whether the tank’s structural integrity is fit for continued service. This analysis evaluates the corrosion level of the steel plates, the foundation integrity, the corrosion of the tank welds, and other detailed components of the tank. At the end of this analysis, the inspector may conclude that the tank requires and API 653 tank alteration for the tank to meet the operational standard. Such a repair will make the tank safe for operation and enable the highest possible fill capacity based on the specific gravity of the new product.
The final and most important calculation to your business is the maximum fill height that can be safely operated with the new product. This height is based on the structural integrity of the tank as well as the specific gravity of the new product. Filling the tank beyond this height risks possible structural failure of the tank, a scenario no one wants to encounter. So be safe and only fill your tank at or below the maximum fill height.
Another consideration in repurposing an API 650 tank is the required containment depending on the product stored in the tank. We will discuss this consideration in our next blog.
If you have any API 653 tank inspection questions or want to schedule an inspection, just call 800-774-3230 or contact Heartland Tank Services through our website.
