Many industries need to store liquids and they do this using either aboveground or underground storage tanks. Gas stations typically have underground storage tanks (USTs) anywhere from 6,000 to 10,000 gallons in size. Aboveground storage tanks (ASTs) can range in size from 500,000 to 12 million gallons, and can be as big as 300 feet in diameter.
American Petroleum Institute (API) 653 is the current industry standard that owners/operators follow for AST inspection, evaluation, repair and testing. Under API 653, regular inspection intervals of ASTs are necessary to evaluate corrosion rates. However, inspections are just one of the reasons why you might need to clean your aboveground or underground storage tank.
When Should You Clean Your Storage Tank?
- Internal Inspections - When it is time for an internal inspection, your tank must be taken out of service, cleaned and prepared for an inspector to enter the storage tank.
- Changing Products - If you plan to store a different product in your tank, a full tank cleaning might be necessary. If you are switching from an unrefined product to a refined product, like from crude oil to gasoline, you would need to remove all residue and impurities.
- Tank Issues - If your tank has a mechanical failure, you likely won’t be able to repair it in service. Depending on the problem, you would need to empty your tank, clean it and make it vapor free before you are able to make the necessary repairs. Typically, the ultimate goal of a tank cleaning is to provide a safe environment for whoever is entering the storage tank after you, whether it is an inspector or a mechanical repair company.
How Do You Clean a Storage Tank?
The owner/operator of the storage tank must pump/drain down the product until the tank has lost suction from the low suction nozzle. This means there will still be several thousand gallons of product on the floor of the tank, below the low suction line, that will need to be removed. Once this is done, a tank cleaning company will come in to perform the following basic steps:
- Lockout/tagout electrical connections on pumps, mixers and similar mechanical equipment.
- Isolate the tank from the system by draining down the lines, removing the valves, and installing appropriately sized blind flanges on tank nozzles and pipe line flanges.
- Remove the remaining product from the tank via vacuum truck or other pumping means.
- Mechanically vent the tank and perform gas freeing to get rid of dangerous vapors, ideally to a lower explosive limit (LEL) of 10 of less.
- Make confined space entry (comply with OSHA’s regulations and API standards) to clean the tank’s interior.
- Remove final product puddles via vacuum truck by squeegeeing the tank floor.
- Powerwash the floor, walls and underneath side of the floating roof of the tank.
- Check the roof pontoons on the floating roof (what keeps the floating tank roof floating) for vapors and clean/vent them if necessary.
- Check seals on floating roof tanks for the presence of vapors and product.
- Inspect for cleanliness and the presence of hidden vapors to confirm your tank is clean and vapor free.
What Issues Arise in Tank Cleaning?
- Unknown conditions inside the tank can make it difficult to formulate a plan ahead of time.
- Petroleum storage tanks pose both a flammability and toxicity hazard that must be recognized and managed to ensure there are no actual or potential atmospheric hazards that can affect the entrants.
- The product isn’t draining properly, it should drain towards the sump (a low point usually for water to drain into at the bottom of the tank).
- Safety risks: you must ensure appropriate personal protective equipment (PPE) is worn and that the tank has been properly ventilated.
- Activities in adjacent areas could affect confined space conditions.
- Weather conditions, both extreme heat and lightning storms can present extra danger.
It's Important to Plan
Due to the amount of steps in the tank cleaning process, many issues can arise throughout the cleaning. Because of this, it is imperative that there is a specific decommissioning plan created for each site as well as a contingency plan if an issue does occur.
Tank cleanings are good standard practice and are highly regulated by local and federal agencies. Routine tank cleanings can be an expensive, drawn out process; however, tank cleanings are not only preventative maintenance but insurance for both the site owner and the land surrounding the tanks.