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 system 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, contaminated them and leading to the corrosion problem.
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. It appears that ethanol gasoline remaining in the bottom of the tanker would end up mixing with diesel fuel on a later fuel delivery.
Another study finding was acid problems in the tank were strongly related to diesel tanks with water on the bottom. It is a known fact that diesel fuel suffers from microbial growth problems when water is present. Fungus, mold and different kinds of bacteria live in the water and use the diesel fuel as a food source. The secretions from these tank pests have a detrimental effect on fuel quality and contribute to equipment problems such as plugged fuel filters, resulting in equipment failure.
The conclusions of the study? Cross contamination of diesel tanks with small amounts of ethal gasoline was leading to bacterial contamination (specifically, a kind of bacteria called Acetobacter) of the fuel tanks, leading to acid production and subsequent tank corrosion.
Ethanol's Role in Low Sulphur Diesel Problems
To be a little more specific, the cross contamination of diesel tanks with ethanol matters because this specific kind of fuel microbe (Acetobacter) has an affinity for ethanol. So when you have switch-loading that results in left-over ethanol fuel contaminating a diesel tank, it gives the acetobacter microbe exactly what it wants to multiply and thrive in that diesel tank. The bacteria multiplies and produces acetic acid as a byproduct of its lifecycle. Tank corrosion ensues.
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 the sulfur acted as a natural biocide, but with the deveopment of ULSD, the sulfur is now almost gone. 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 key factor in all of this is the presence of water in the storage tank. As a general rule, no amount of water in a fuel storage tank is acceptable. But life rarely matches the ideal, and all storage tanks get water in them on a regular basis. So it is recommended to get rid of the water as soon as possible. Sticking the tank monthly with water paste test is recommended, and if water is found, it should be dealt with quickly. This can be done by either pumping it out or using fuel treatments that pick up water and allow it to pass harmlessly through a fuel system. If you have no water in the tank, you will have no microbial growth.
A regular maintenance schedule for applying a biocide will also assure that fungus, mold and bacteria will not be able to grow in the diesel tank. It is very important that the biocide have the ability to kill microbes over a long period of time so that when additional fuel is added at a later date the biocide will kill microbes attached to the upper walls of the storage tank to avoid a reinfection of microbes.
Conclusion: Much of what was accepted truth about fuel ten years ago is not relevant now. High sulfur content back then kept microbial growth to a minimum. The EPA has proposed removing the remainder of the sulfur from diesel fuel for emissions purposes. This has led to the problem of corrosion caused by water and microbes in Ultra Low Sulfur Diesel storage and dispensing equipment. It is far cheaper to prevent these problems than it is to repair them after they happen. Prevention requires a remediation program with a regular application of biocide and a water removal program.
Other posts you may be interested in:
- How to Prevent Diesel Fuel Contamination
- 8 Signs of Diesel Fuel Contamination by Microbes, Fungus and Bacteria
- Shrinking fuel distributor margins a problem for the industry
This post was published on March 4, 2014 and was updated on March 20, 2015.