Do you own, operate, or maintain diesel emergency or standby generators? As an electrical professional, your focus is most likely on all of the electrical aspects of the system. But if your generator’s fuel system is not given the same attention as the critical electrical components, it’s not likely to start when you most need it. Proper maintenance of the diesel fuel in the storage tank should be a critical component of your generator preventive maintenance plan.
Fuel contaminants come in various shapes, sizes, and chemistries. These include water, particulate matter, microbial or biological growth, solids composed of organic salts, soaps, fuel degradation products, polymers, oxides, and eight different silicates. Regardless of the chemistry or origin, fuel contaminants can contribute to engine deposit formation, erosion-wear and fuel filter obstruction. All of these problems reduce the reliability of your critical emergency and standby generating equipment (Photo 1).
Water contamination
Water is the most common contaminant found in fuel and lubricating oil systems. The presence of water in the fuel tank will lead to many serious operating problems if not removed or controlled. Water can enter the fuel during the oil refining process, as rain, as tanker ship ballast water, or as condensation from humid air in storage tanks and equipment. Water can be dissolved in the fuel or suspended as tiny droplets. The amount of water that will dissolve in any given fuel depends on both the composition and temperature of the fuel. The removal of water from diesel fuel is critical, but, in many instances, the removal of accumulated water is performed infrequently. The accumulation of water in a diesel fuel oil tank likely will lead to increased corrosion and the potential for greater microbial activity.
If biodiesel is present in the fuel, water can have a more pronounced effect on the diesel fuel. Water is far more soluble in a biodiesel blend than in pure diesel fuel. When dissolved water is contained in biodiesel, the biodiesel will hydrate (add water molecules) to the fatty acid methyl ester (FAME) molecules. These molecules have different properties from non-hydrated FAME, and react differently and aggressively within the engine fuel. Dissolved water in combination with other contaminants can also lead to hydrolysis of biodiesel. This causes the fuel oil molecule to degraded or break apart, thus causing further reactions with other compounds to form salts, soaps, or peroxides.
The following points are important for the control of water in fuel systems:
• Dissolved water will not change the appearance of the diesel fuel but suspended water (free water) will make the fuel appear cloudy or hazy.
• Water picked up in one part of a fuel system can drop out in another part. This is especially true when the fuel temperature decreases and contaminates the fuel tank or a low spot in a pipeline.
• Humid air that enters a fuel tank during normal temperature cycling (breathing) can result in water condensation in the tank when the ambient temperature reaches the dew point.
• Rain can enter a fuel oil tank through open fill pipes or uncapped vents. This may lead to internal contamination and corrosion of the tank and distribution piping.
• The free water in the fuel oil storage tank can accumulate salts and other compounds over time, and severely impact fuel injection and engine operation.
• The elimination of free water will greatly decrease biological and microbial growth in the fuel oil system.
• The use of corrosion inhibitor additives will significantly reduce the amount of corrosion and particulate generation throughout the fuel oil distribution systems.
Microbial contamination
Numerous types of microorganisms can use diesel fuel as a food source, including:
• Various species of bacteria and fungi, such as yeasts and molds
• Aerobic microbes that require a source of free oxygen to grow
• Anaerobic microbes that only grow in the complete absence of free oxygen
• Facultative anaerobes that live aerobically when oxygen is present and live anaerobically when oxygen is absent.
Like all life forms, microbes require the presence of water. Water collects in low points of fuel systems and can become stagnant. This is often where microbiological contamination occurs (Photo 2). Once again, water is a critical factor for microbial growth. Microbial contamination can block fuel filters or damage engine parts.
How much water is really needed? These microbial organisms do not require large amounts of water. The amount of water necessary typically ranges from 0.5 to 3.0 microns in size and is, therefore, generally less than 1/1,000th the size of a 3-mm-thick film of water. This explains why small spots of condensation on the walls of a tank are more than sufficient to support microbial growth.
Viable organisms are found in the air, soil, and even in the fuel; thus, it’s virtually impossible to keep a fuel system clean without the use of biocides. The effects of these organisms can vary by type of organism and the volume or mass of organisms present in the system.
The most abundant metabolites produced by microbial communities are low molecular weight organic acids. Although these organic acids are not as aggressive as strong inorganic acids, they can readily react with chloride, sulfate, and nitrate to produce strong inorganic acids, such as hydrochloric, sulfuric and nitric, respectively. When present in a system, these strong inorganic acids will cause corrosion and degrade fuel quality. Moreover, microbes living in the tank bottom water commonly produce detergent-like molecules called biosurfactants. These biosurfactants generate an emulsion of water in the fuel. This distributes the water into the fuel and makes the fuel molecules more available as a source of food for the microbes.
Microbial contamination is the colonization of microorganisms that exist and multiply in a free water and mineral oil mixture. As little as 100 ppm of water is sufficient for these microorganisms to multiply, divide, and expand their populations. These organisms exist in a variety of forms such as fungi, molds, viruses, bacteria, and yeasts.
Microbial growth primarily occurs within the zone formed between water and oil. This zone contains the ideal nutrient medium for the growth of these organisms. Depending on the type of microorganisms and ecological conditions, the organism growth can quickly expand in either the oil or water. The source of such contamination can begin at the refinery or any other point along the entire supply chain of fuel delivery and fuel storage, ending with the fuel user.
Prevention techniques
Hire a reputable fuel testing firm in your area to take samples two to three times per year and provide you with the analysis. The fuel in the tank should also be tested and analyzed before and after a scheduled tank fill-up. This method establishes the level of contamination in the fuel tank prior to and after delivery. If contaminated, this method helps to pinpoint the possible contamination source.
Compare the fuel test results against previous test results looking for significant changes in quality or the presence of biological contaminants. Most reports will indicate the changes since the previous sample test and also indicate the seriousness of the change. Possible test results include:
• No significant changes — Continue as normal and file the report for future comparisons.
• Moderate changes (i.e., biological contamination) — Review the report for recommended treatment additives. If none, contact the testing firm and determine the next steps. Early treatment is essential for control and elimination of the biological contaminants.
• Significant changes (i.e., well established biological contamination) — Review the report for recommended treatment additives and implement treatment immediately. Have a second test sample drawn and tested in five to 10 days to determine if the treatment is working properly.
If the treatment is working properly, continue treatment and resampling. If possible, drain and flush the tank and engine fuel system using a biological flushing agent. Then, refill the fuel tank with new, clean fuel. Sample test and add chemical biocide if indicated on the report.
If the treatment is not working, you may need to change the biocide concentration. Certain conditions may require you to drain and flush the fuel tank and engine fuel system. In this situation, a physical and/or chemical decontamination must be employed to destroy the biological contaminants. The chemical biocide selected must be fast-acting against large numbers of microbes when there is substantial organic fouling.
Once the biological contamination problem has been eliminated and cleaned up, the regular implementation of good housekeeping and the continuous use of biocide chemicals can help eliminate a fuel system contamination recurrence.
Tessier is director, insurance engineering, for The Hartford Steam Boiler Inspection and Insurance Co., in Hartford, Conn. He can be reached at [email protected]
