March 29

Storage Tank Corrosion Oversight

Storage tank corrosion continues to be a concern among home heating oil and diesel fuel distributors. In order to avoid costly damage to product, equipment and reputation, blame must be removed from the individual fuel providers, and measures for improvement must be embraced collaboratively by the complete supply chain.

The principal cause of corrosion in storage tanks is phase separation, which itself occurs as the result of the difference in product temperatures between blended fuels. This was first seen with the introduction of E10 gasoline (a fuel mixture of 10% anhydrous ethanol and 90% gasoline). Because ethanol absorbs water, a layer of ethanol/water mixture forms creating a warm and wet environment where bacterial growth occurs. This growth, coupled with E10’s high acidity, degrades the tank bottom. Phase separation of this kind isn’t only restricted to the bottom of the tank, however. It can also occur at other points, so readings should be drawn from both the middle and the bottom of the tank.

The initial concern was low sulfur diesel’s lubricity, which the industry soon addressed with lubricity additives or the use of B2 to B5 biodiesel blends, a route that in turn only aggravated the issue of bacterial growth and particulate matter. Then the industry mandated the use of 10 micron filter elements, which resulted in extensive clogging, especially during the winter months, because of excess water content and waxing. The issue of water accumulation has only grown, due to conservation venting, ambient temperature, and the difference in product temperatures between fuel and B99 biodiesel. Often, the B99 is stored at temperatures of 60°F to 80°F. Using that B99 to reach blends from B2 up to B20 at a fuel temperature difference greater than 20°F will result in water moisture in the tank.

Removal of water and prevention of bacterial growth are key, but attempting to absorb water buildup with fuel dispenser filter/separators should be the final step in a series of preventative measures. Filter/separators used in the aviation sector present an option. Capable of removing down to 0.5 micron-sized droplets, these filter/separators can remove water to sumps within the vessel, which in turn have control valves called water slug valves. The valves will close when the accumulated water reaches and trips a float device. Filter/separators of this caliber can also arrest bacteria before they are introduced to the storage system. When considering implementing such a system, bulk storage must first be completely cleaned out. Another frequently-considered solution lies in the introduction of additives, either during input into bulk storage, or during the transfer to delivery trucks. However, use of these additives lowers pH levels and creates additional acidic buildup, which only accelerates corrosion. Another step to take is the use of desiccant breather vents, which strip the surrounding air of contaminants, both moisture and dirt. These vents can address humid ambient air and expose your product to this air during product transfer.

No matter where the solution may lie, it is clear that storage tank corrosion has detrimental industry-wide effects, and as such, measures to prevent its occurrence must be worked towards by the entire supply chain as a whole. It is only through this collaborative effort that it can be addressed. As we continue to further examine the nature and chemistry of storage tank corrosion, more solutions and strategies may present themselves.