Asphaltenes in Fuel
What is that Sludge?
Asphaltenes are what cause sludge formation and deposits in fuel tanks and oil lines.
Sediment and sludge are formed in oil tanks and fuel lines due to water separation from the fuel
oil and when the asphaltenes, waxes and other materials flocculate (stick together) and ultimately
settle to the bottom.
A certain amount of water is always found in fuel oil and additional water can come from condensation
in the storage tank and during tank filling. The water normally separates out and remains at the bottom of
the fuel tank in direct contact with the metal surface. Under the right conditions, microorganisms can grow
at the water/oil interface and contribute to an increase in the amount of sediment in the tank.
Asphaltenes, the most polar and heaviest compounds of oil, associate themselves in solution to form
complex colloidal structures. Asphaltenes cause serious problems in diesel engines, fuel systems,
oil recovery, oil-carrying pipelines, and refinery operations; many of them being related to the
presence of aggregates in the heavy fraction.
Asphaltene agglomerations cause sludge buildup. Flocculation of asphaltenes occurs, not
only from the natural oxidation and aging of the fuel, but also as the result of mixing oils
from different crude sources. This can occur when blending fuels or mixing into a storage
tank that contains fuel.
Asphaltene solubility is also affected by the aromaticity and type (and concentration)
of resins in the blend components. When the fuel blend components are mixed, the asphaltenes
may precipitate and form sludge.
It is these compounds as well as other biological matter that cause the blockage of fuel filters on
yachts and commercial vessels. I've seen it happen on new ships where we walked the tanks in the
yard and found them clean enough to eat off of. Then pumped aboard 70k barrels (3.78 million gallons)
of #2 diesel of clean filtered fuel. A couple days out in rough weather started plugging
fuel filters and losing the plant for no apparent reason. You'd look in the centrifuges and there
was voluminous amounts of black sludge. Some of it being so fine that it would get through the
centrifuges and plug the 30 micron primaries solid.
This has happened to me on numerous ocean yacht deliveries, where even though we've taken on
filtered fuel, we'd start plugging filters once we got out in the ocean into the agitate cycle.
Upon inspection, the tanks and sumps were clean. We'd pipette out a mid-column fuel sample
and it would be unremarkable as well. Yet, there would be sludge in the bottom of the Racor bowls
on the 30's and on the 10's, eventually causing us to lose the plant. This is the reason that we
insist on having many fuel filters. Mysteriously, this never seems to happen to the owners of
these yachts, either because they don't take them very far, or they don't sail them very hard.
Whatever the reason, it is a problem.
(also Asphaltene - Wikipedia)