Asphaltenes in Fuel

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Asphaltenes in Fuel

The Gunk in the Tank

by David Gower, submitted by Jeff s/v Calypso

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)