ALS Tribology receives regular inquiries regarding problems with prematurely plugged fuel filters. Inquiries have originated from both users of conventional middle distillate fuel and from those using biodiesel blends. The majority of these fuel filters were heavily contaminated with a black substance. Analysis of this black substance revealed that the predominant contamination species noted was carbon particulate.
Carbon contamination can originate from a variety of sources. Carbon flake material can gain entry into the fuel system by seal degradation and wear. Carbon particulate is a contaminant that accumulates in fuel systems as the fuel naturally degrades over time. This degradation is caused by the inherent instability of fuel. Distillate fuel will degrade over time. Soluble gum material will begin to form as fuel ages and develop into insoluble carbon sediment. The degree and rate which this occurs is dependent upon the stability of the fuel in storage. An excess of carbon particulate may build up in fuel filters as the fuel system releases the particulate that developed over time. Biodiesel and biodiesel blends are known to have a solvency effect that may facilitate the release of accumulated carbon deposits in fuel systems and may necessitate more frequent filter changes.
Fuel solvency also plays a role, as fuels are often mixed, and the different solvency characteristics may allow solubilized contaminants to become insoluble and drop out of solution. Two fuels—both with good individual stability—could produce a product with poor stability. This solvency effect is more prevalent when blending biodiesel blends, as the solvency of biodiesel is much greater than distillate fuel. The reduced solvency of the biodiesel blend may facilitate soluble material in the biodiesel portion to become insoluble and drop out of solution, which would increase the filter plugging potential.
Excessive carbon particulate may be generated in the fuel system if the fuel has poor thermal stability, resulting in increased filter plugging potential. In general, fuel systems are designed to return the uncombusted portion of the fuel back to the tank. This returned fuel is has been pressurized and heated, which promotes polymerization and breakdown of the fuel. This creates the carbon particulate that is primarily composed of organic material and has very low ash content. As more fuel is pressurized, heated, and recirculated, the amount of carbon particulate generated increases, causing possible injector failure and filter plugging. To prevent this source of filter plugging, the fuels must have good thermal stability.
Oxidation and polymerization reaction by-products can cause corrosion and accelerate the natural fuel degradation process. This degradation material may be insoluble in the fuel and can accelerate fuel filter plugging. Metals are also known to promote and catalyze oxidation and polymerization reactions of hydrocarbons.
Poor tank housekeeping will also lead to excessive fuel filter plugging. Microorganisms require water to grow and, since most microbial growth occurs at the fuel water interface, keeping fuel systems dry will greatly reduce the likelihood of microbial contamination and its related problems. One of the potential problems with microbial contamination is increased filter plugging potential. The waste created by microbial growth rather than the microbes themselves create a potential cause of premature filter plugging. If there is a known water contamination issue in a fuel system, we recommend that you check for microbial contamination.
David Doyle, CLS, OMA I, OMA II
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