Fuel Distributors

Why does a high-turnover bulk plant still develop fuel problems?

Fuel turnover helps, but it does not automatically keep a tank clean.

The product being delivered and withdrawn is constantly changing. The material at the bottom of the tank may not be. Water, rust, sediment, and microbial biomass can accumulate below the normal suction point and remain there through repeated deliveries and withdrawals. Eventually something changes—a delivery stirs the tank, inventory gets unusually low, or accumulated material reaches the point where it begins getting picked up—and the problem starts moving downstream.

Water is particularly important because microbial growth depends on it. Free water at the bottom of a fuel tank creates the fuel-water interface where bacteria and fungi can grow. Water can enter through several routes, including contaminated deliveries, leaking fittings or seals, and moisture entering through tank venting and subsequently condensing under the right conditions.

That is why good turnover and good tank housekeeping are not the same thing. A distributor can move a great deal of fuel through a tank and still have a problem developing at the bottom.

What goes wrong in a distributor's bulk storage tank?

Most storage-related fuel problems at a bulk plant fall into a handful of familiar categories.

Microbial contamination and MIC. Bacteria and fungi can grow where fuel and free water meet. The resulting biomass can plug filters and strainers, while microbial activity can also contribute to acids and conditions that promote microbially influenced corrosion (MIC). Depending on the severity of the contamination, the answer may involve biocide treatment, water removal, fuel polishing, tank cleaning, or some combination of them.

Water accumulation and phase separation. Water promotes corrosion and microbial growth and can create operational problems throughout the fuel system. With ethanol-blended gasoline, enough water can also cause phase separation, producing an ethanol-rich water layer at the bottom of the tank and leaving the remaining gasoline off-spec or otherwise unsuitable for normal use.

Oxidation and storage instability. Fuel does not have to contain water or microbes to deteriorate. Diesel fuel and heating oil can oxidize during storage, particularly when inventory sits for extended periods. The products of oxidation can eventually contribute to gums, sediments, darkening, and filter plugging.

Biodiesel blend behavior. Biodiesel blends require some additional attention to storage conditions. NREL notes that biodiesel is more biodegradable than petroleum diesel and may therefore be more susceptible to microbial growth, particularly at higher blend levels. Biodiesel can also affect storage stability and can loosen existing deposits in tanks and fuel systems. None of that means biodiesel blends cannot be stored successfully. It means keeping the tank clean and dry and monitoring stored fuel becomes more important as storage time and biodiesel concentration increase.

How do you know your product is going out in spec?

You test it—and you keep the results.

ASTM D975 establishes the specification requirements for diesel fuel, including properties such as water and sediment, sulfur, distillation, viscosity, cetane quality, corrosion, lubricity, and other parameters. But for a distributor managing stored fuel, there is also value in testing characteristics that can tell you what is happening to the fuel while it is sitting in the tank.

A useful bulk-storage testing program can include:

• D2709 water and sediment. This measures free water and sediment suspended in middle-distillate fuel. Water and sediment can contribute to corrosion, filter plugging, operational problems, and microbial growth.
• D6304 Karl Fischer water. Karl Fischer testing gives a much more sensitive measurement of water content than a simple bottom-water check and can identify increasing water levels before there is an obvious free-water problem.
• ATP-by-filtration. ATP testing provides a rapid indication of biological activity in the sample. More importantly, repeated testing provides a trend. A rising ATP result over several sampling periods tells you considerably more about what is happening in the tank than a single isolated reading.
• D2274 oxidation stability. This is an accelerated test used to evaluate the inherent oxidation stability of middle-distillate fuel. It should not be treated as a literal prediction that the fuel has a certain number of months of storage life remaining, but it can help identify fuel with poorer resistance to oxidation.
• Fuel-specification testing. Distillation, sulfur, cetane quality, copper corrosion, and other appropriate specification tests help confirm that the fuel still meets the requirements that apply to the product being sold.

Testing has another value that is easy to overlook: documentation. If a customer later alleges that a delivery contained bad fuel, dated third-party test results from the supplying tank give the distributor something much more useful than an opinion. Samples can then be taken from both locations and compared to help determine where the problem is actually coming from.

What is the most effective way to keep distributor tanks clean?

Start by identifying the problem. Then match the treatment or service to what the testing tells you.

That sounds obvious, but it is an important distinction. A microbial problem, an oxidation problem, excess water, and a tank containing several inches of accumulated sludge are not four versions of the same problem. Treating all of them by simply adding another chemical to the fuel is not going to produce the same result.

Chemical treatment is useful when there is something in the fuel that chemistry can actually address. A biocide can control an active microbial population. A fuel stabilizer can help slow oxidation. Dispersants and other treatment chemistries can be useful for particular deposit and fuel-condition problems.

But chemistry does not make physical contamination disappear. If a tank contains substantial water, sludge, rust, sediment, or dead microbial biomass, at some point that material has to be removed.

That is where fuel polishing and tank cleaning come in. Polishing removes suspended contamination and water from the fuel through circulation and filtration. Tank cleaning addresses contamination that has accumulated on the tank bottom and other internal surfaces. In more severe cases, the right answer may be a combination: treat the biological problem first, then physically remove the material that remains.

Testing is what tells you which approach makes sense instead of simply guessing at the problem.

Treat rates for bulk plant applications

What does a fuel quality program look like for a distributor?

The important thing is not that every distributor follows exactly the same calendar. It is that every tank is checked often enough to establish what normal looks like and to catch a change before that change becomes a customer problem.

For many bulk plants, a practical program starts with routine checks for bottom water and visible contamination, periodic microbial testing, and a more complete laboratory analysis at an appropriate interval. Tanks containing slower-moving products, fuels with known stability concerns, or products with a history of contamination should be monitored more closely than a clean tank moving fresh product continuously.

The other important part is recordkeeping. Test results should be tied to a particular tank and sampling date so that results can be compared over time. A single test tells you what was found in one sample. A series of tests tells you whether the condition of the tank is stable, improving, or getting worse.

Bell FTS's Fuel Secure programs are designed around that approach, with different testing frequencies depending on the level of monitoring a distributor wants. The objective is to move fuel-quality management away from waiting for a customer complaint and toward finding changes while they are still relatively inexpensive to correct.

That distinction matters economically. Routine testing, water management, and treatment cost money. Emergency polishing, tank cleaning, discarded fuel, credited deliveries, and lost customers generally cost considerably more.

When does a tank need cleaning instead of chemical treatment alone?

The dividing line is usually whether you are dealing primarily with a fuel-condition problem or with physical contamination that has accumulated in the tank.

If testing catches microbial activity early and there is little accumulated biomass or sludge, chemical treatment combined with water removal and better tank housekeeping may be enough. The same general principle applies to fuel that is beginning to show signs of oxidation: catching the problem early gives you more options.

Once significant sludge, sediment, water, or microbial biomass has accumulated, treatment alone is usually not enough. This is particularly important with microbial contamination. A biocide can kill the organisms, but it does not remove the dead biomass from the tank. That material can still be picked up by the fuel and plug filters downstream.

In a heavily contaminated system, the better approach is often to control the active microbial population and then polish the fuel or clean the tank to physically remove the contamination that remains.

There are also fuel-quality problems that additives cannot correct. If testing shows that a fuel is genuinely off-spec in a property that cannot be restored through legitimate treatment or blending, the answer is not to keep adding chemistry. The product has to be handled appropriately based on what the testing shows.

Talk to Bell FTS about your bulk plant

Many fuel-storage problems develop gradually. Water accumulates, microbial activity increases, stability declines, or contamination builds up long before the problem becomes obvious at the dispenser or in a customer's equipment.

That is the advantage of testing the tank before there is a complaint.

Start with the tank that has given you the most trouble, the product that sits the longest, or the tank supplying the customers you can least afford to have a problem with. Bell FTS can sample and test the fuel, evaluate what the results are telling you, and recommend treatment or mechanical service based on what is actually happening in the tank.