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*