Each species has its own characteristics. Living organisms require an ecosystem to survive. The ecosystem provides nutrition, protection, and the opportunity for mobility in order to find more food, more protection, and the potential to reproduce with success. Life on Earth can be found in all environments, from the ultra-hot sulfur vents below the ocean floor to Antarctica; life seems to find a way. Therefore, it is no wonder that your oil sump and fuel tanks harbor life forms. The tank provides a food source, protection, and the fluid provides mobility. In a lubricated system or a fuel tank, bacteria feed on oil or fuel hydrocarbons and double their numbers every 10 to 20 minutes. One bacterium could divide into half a million within 12 hours.
All organisms require other organisms to provide a symbiotic relationship. Even humans require various forms of bacteria to stay alive. Microbes and fungus found in oil sumps and fuel tanks are no different. The following are life sources that might be found in your fuel or oil sump:
SULPHATE REDUCING BACTERIA (SRB)
SRB's are a specific group of bacteria feed on carbon, not fuel or oil hydrocarbons, and require the activity of other microbes in a concert. Aerobic (in the presence of oxygen) or anaerobic (without oxygen) bacteria have a combined effect. The aerobic bacteria (sulphate oxidizing) create a film to consume the oxygen first. This allows the anaerobic (sulphate reducing) bacteria to thrive.
SRB's reduce sulphates and produce hydrogen sulfide (a lethal gas). They are directly involved with many microbial corrosion reactions and can cause sulfide souring of stored fuels and oils. They changes the pH creating an acidic environment which promotes corrosion. They are hard to rid because they form an aggressive adhesion to the tank walls. They produce a sludgy by-product with a strong Sulphur odor (the rotten egg smell from hydrogen sulfide).
IRON REDUCING BACTERIA
These also contribute to corrosion, eating steel and reducing iron or iron alloys to an oxide through a chemical reaction.
Yeasts prefer an acidic environment which is produced by the aforementioned SRB's. Their population can more then double in a few hours.
Fungus growth is typically seen as a vegetative mass plume at the water-petroleum interface.
All of these life forms can cause damage to your oil or fuel by introducing solid as well as fluid contamination, corrosion, and filter blockage. Since micro-organisms also can feed upon the additives within the oil. The functionality of the oil and fuel may be impaired and its viscosity altered. There is an increased in the acid content (lowering of the pH) and increased potential for emulsification and corrosion. Often corrosion is seen explaining why tanks will corrode from the inside-out.
Microbial growth is not just relegated to a fuel tank. Microbial infections can occur in the water phase associated with the lubricant as well. This can occur in hydraulics, gearboxes, turbines, and even crankcase oils in wet engines, particularly those with water-cooled pistons. Filters clog, organic acids are formed, and the oils tend to emulsify.
Sources of microbial contamination within the lubricant are atmospheric air, water from leaks and condensation, the fuel, cooling water, and even seawater. Cooling water in particular is a biological source contaminant of crankcase oil even at operating temperatures ever since the use of chromates as corrosion inhibitors were banned. The use of chromates acted as an effective anti-microbial biocide as did the increased sulfur levels of fuel. With the advent of ULSD and the reduction of Sulfur to under 15ppm, the natural bio-resistance of fuel is no longer present.
As always, a preventive approach is cheaper than a cure. There have been very few microbial infection issues reported in systems having high alkalinity. Bugs do not like a basic environment. Being able to control acid as well as eliminating water will help control a biological infestation. This means controlling oxidation will control acid production. At full operating temperatures, lubricating oils tend to be self-sterilizing. Yet there still remains the possibility of infestation in the tank or sump. If you do have an infection, proper mitigation is required which may include complete emptying followed by sterilization. Consult your fuel or oil supplier for direction.
Think you have an issue? Contact ALS Tribology for information on the tests that can be performed to confirm your suspicions.
Michael D. Holloway, MLE, CRL, LLA I & II, MLT I & II, MLA I-III, OMA 1, CLS
Principle Consultant, Certified Reliability Leader