Low Sulphur Diesel Problems: Corrosion in Storage and Dispensing

In 2006, the US Environmental Protection Agency (EPA) mandated the switch from Low Sulfur to Ultra Low Sulfur Diesel (ULSD). This was a reduction of sulfur in the fuel from 500 parts per million to 15 parts per million - a 97% reduction overnight and a 99.7% reduction from the high-sulfur diesel fuels in use before the 1990s. There were initial concerns about lubricity and other potential problems, but they turned out to be fairly minimal.  Not because removing sulfur from the fuel did not lower its lubricity rating, but because the industry came up with ways to add lubricity additives at the refinery level to head off potential problems.

In 2007, ethanol-gasoline blending mandates hit the US market in full force.  Shortly after this, there were reports across the country of extreme cases of fuel storage tank corrosion - not in ethanol systems but in diesel storage tanks, fuel dispensing pumps, and associated plumbing. Corrosive damage that normally took years in the past was now happening in months.

In response, a study was conducted across the nation by the Clean Diesel Fuel Alliance and it found that there were no geographic areas immune to this problem and that the problem was pretty consistent across the country.  Fuel samples were taken and analyzed and something they did not expect was found. Diesel tanks suffering from extreme corrosion contained trace amounts of ethanol and had low pH values in the 2.5 range.  A low pH value means the fuel is high in acid. Further research found several ways that ethanol fuels could have been introduced into these diesel tanks, contaminating them and leading to the corrosion problem.  

how did we get here?

So how did the industry get to that point? To understand this, you have to understand the practice of "switch loading". Fuel-hauling tanker trucks are not used exclusively for only one particular fuel. Today, they could be hauling ethanol-blended gasoline and tomorrow they could be hauling diesel fuel in the same tanker. The chances are pretty high that some ethanol gasoline remaining at the bottom of the tanker will end up mixed with diesel fuel during a later fuel delivery.

Another study revealed that issues with acid in fuel tanks were mainly linked to diesel tanks with water at the bottom. It's well-known that when water mixes with diesel, it creates an environment where microbes like fungi, mold, and various bacteria can thrive. These microorganisms live in the water but feed on the diesel, releasing substances that lower the fuel's quality and cause trouble for equipment, such as clogged fuel filters, which can lead to breakdowns.

What did the study conclude? It found that mixing even a little bit of ethanol gasoline into diesel tanks could introduce a specific type of bacteria, known as Acetobacter, into the fuel. This bacteria then produces acid, which can eat away at the tank and cause damage.

Ethanol's Role in Low Sulphur Diesel Problems

The issue with mixing ethanol into diesel tanks gets to the heart of the problem because of a specific fuel-loving microbe called Acetobacter. This microbe has a particular fondness for ethanol. So, when remnants of ethanol fuel end up in a diesel tank, perhaps due to switching fuel types without thoroughly cleaning (a process known as switch-loading), it creates an ideal environment for the Acetobacter to flourish. Once there, the bacteria multiply rapidly and, as part of their life cycle, produce acetic acid. This byproduct is harmful and leads directly to the corrosion of the tank.

Corrosion Not So Much A Problem In Certain Tanks

The study also noted that corrosion was not much of a problem in tanks that were on a regular biocide treatment program to kill tank microbes. In the past, there was a mistaken notion that the sulfur acted as a natural biocide (it turns out that was a dramatic misunderstanding). With the development of ULSD, the sulfur is now almost gone while the composition of the fuel has been altered to make it more hospitable for microbial growth.  Since there's nothing now to prevent bacterial growth, the use of biocides to treat the fuel and prevent microbial growth that normally would occur otherwise. 

The presence of water in fuel storage tanks plays a pivotal role in the quality and stability of the stored fuel. Ideally, a fuel tank should be completely free of water, as even a small amount can cause significant problems. However, it's common for storage tanks to accumulate water over time due to condensation or other factors, deviating from this ideal state.

To manage this, it's advised to regularly check for water in the tanks using a method known as the water paste test, conducted monthly. If water is detected, it should be promptly removed either by pumping it out or by treating the fuel with additives that can encapsulate the water, allowing it to safely pass through the fuel system without causing harm. This step is crucial because a dry tank environment inhibits microbial growth, thus preventing contamination and the resultant issues.

Moreover, maintaining a diesel tank involves not just water management but also the strategic use of biocides. Biocides are essential for eliminating and preventing the growth of harmful organisms such as fungi, mold, and bacteria within the tank. For effective long-term protection, it's important to choose a biocide that continues to act over an extended period, ensuring that any new fuel added doesn't reintroduce microbes that could cling to the tank's upper surfaces.

In conclusion, the approach to fuel storage and maintenance has evolved significantly. What was once considered acceptable practice for fuel storage, such as the tolerance of high sulfur content to inhibit microbial growth, has changed due to environmental regulations like those from the EPA aimed at reducing sulfur in diesel fuel. These regulations have inadvertently increased the susceptibility of Ultra Low Sulfur Diesel to water-related corrosion and microbial contamination. Thus, implementing a proactive maintenance strategy that includes regular biocide application and water removal is not just advisable but essential for preventing costly damage and ensuring the integrity of fuel storage and dispensing systems.

Other posts you may be interested in:

• How to Prevent Diesel Fuel Contamination in Farm Fuel Storage
• 8 Signs of Diesel Fuel Contamination by Microbes, Fungus and Bacteria
• Shrinking fuel distributor margins a problem for the industry