The water contamination in fuel aviation and their solutions – Edition 2007 (página 2)

These situations can be managed by keeping the relative amount of contamination small enough that it doesn't significantly alter the specification properties. However, even small amounts of contamination can be a problem for aviation fuels if the other product is dyed or contains additives or impurities, that because they are surfactants, degrade water separability.

Microbes   Aviation fuels are sterile when first produced because of the high refinery processing temperatures. But they become contaminated with microorganisms that are omnipresent in air and water. Microorganisms found in fuels include bacteria and fungi (yeasts and molds). The solids formed by biogrowth are very effective at plugging fuel filters. Some microorganisms also generate acidic by-products that can accelerate metal corrosion.

Since most microorganisms need free water to grow, biogrowth usually is concentrated at the fuel-water interface, when one exists. Some organisms need air to grow (aerobic organisms), while others grow only in the absence of air (anaerobic organisms). In addition to food (fuel) and water, microorganisms also need certain elemental nutrients. Jet fuel can supply most of these; phosphorus is the only one whose concentration might be low enough to limit biogrowth. Higher ambient temperatures also favor biogrowth.

Microbial contamination in avgas is much less common than with jet fuel, although it does occur. Presumably, the lower occurrence in avgas is due to the toxicity of tetraethyl lead.

The best approach to microbial contamination is prevention. And the most important preventive step is keeping the amount of water in the fuel storage tank as low as possible.

Biocides can be used when microorganisms reach problem levels. But biocides have their limits. A biocide may not work if a heavy biofilm has accumulated on the surface of the tank or other equipment, because then it doesn't reach the organisms living deep within the biofilm. In such cases, the tank must be drained and mechanically cleaned.

And even if the biocide effectively stops biogrowth, it still may be necessary to remove the accumulated biomass to avoid filter plugging. Since biocides are toxic, any water bottoms that contain biocides must be disposed of appropriately.

Cleanup     Contamination with particulate matter, and to a lesser extent, with water is unavoidable during distribution. Therefore, the aviation fuel distribution system includes processes to remove these contaminants:

• Filtration to remove particulate matter.
• Filter/separator, water-absorbing media, and salt drier to remove water.
• Clay treatment to remove surfactants.

One or more of these processes may be used at each stage in the distribution chain: at the refinery, at the inlet or outlet of terminal tanks, at the inlet or outlet of airport storage tanks, and in equipment dispensing fuel into aircraft.

Filtration   Passing a petroleum product through a filter with a pleated paper or synthetic fiber medium removes solids with particle sizes larger than the pore size rating of the filter.Typically, filters with a nominal pore size of five micrometers (microns) are used for avgas and one micron or two microns are used for jet. These filters are commonly called pre-filters, because they are typically used before filter/separators, or micronic filters, because they are rated by the size of particulate removed, e.g., two microns.

The filter elements are hollow cylinders that screw into a base plate in the filter vessel. Clean fuel flows out of the center of the filter elements into the bottom of the vessel.

In most fuel cleanup systems, particulate removal precedes water removal for cost reasons. If the particulate matter isn’t removed, it would shorten the life of media used in the subsequent water removal process. Particulate removal media are less expensive than water removal media.

Filter/Separator   The filter/separator is the workhorse of the processes used to remove free water from aviation fuels. Two media are involved. First the fuel passes through a water-coalescing medium, which is composed of fibers with a hydrophilic surface that serves to combine small drops of water into larger drops (see Figure 1). Then it passes through a water-separation medium, which has an outer hydrophobic surface that rejects the larger water droplets. The rejected water is collected in a sump. A filter/separator does not remove dissolved water. Figure 2 is a cutaway drawing of a typical filter/separator.

Figure 1

Figure 2

Filter/Separator

Bibliography

1.- Analysis of the water contamination in the fuel aviation, problems and their solutions. MSTUCA 2006.

About the Author

Vihelmo Velapatino

Doctor of Science in Aviation and Space Technology USA*