|Asphaltenes in Fuel
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 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 increased 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 causes serious problems in diesel engines, fuel systems, oil recovery, oil-carrying pipelines, and refinery operations, and many of them being related to the presence of aggregates in the heavy fraction
Asphaltene agglomerations cause sludge build-up. 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 the 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 biologicals 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 they were clean enough to eat off of. Then pumped aboard 70k barrels of #2 diesel (3.78 million gallons) of clean filtered fuel and got a couple days out in rough weather and started plugging fuel filters and losing the plant for no apparent reason. You’d look in the centrifuges and there is voluminous amounts of black sludge and some 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 Raycor 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 for 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.
The other problem is caused when changing fuel filters frequently is you tend to lose your prime and get air up in the fuel system. Most yachts are difficult to bleed. Especially if the tanks are located down low and the filters and engine are higher. The small pump on the Raycor only pumps about 10 milliliters and the lift pump on the engine only pumps about 5ml so even though you fill the filters up with clean diesel, you’ll do a lot of pumping and still wind up with air in the system. Naturally when this happens, it is always at the worst possible moment in time. You’re either a mile from a pile of rocks and being set down on it by the wind and the current or making a difficult bar crossing or harbor entrance when the plant quits. I can’t even count the number of times this has occurred on all types of vessels.
Certainly adding an electric fuel pump right after the 30 micron makes it much easier to bleed the system (most commercial vessels do). It is then a matter of just changing the filters out, opening the highest banjo bolt in the system, turning on the electric fuel pump and running out a gallon or so of fuel until all the air bubbles are gone. You don’t need to fool around with filling the filter canisters with diesel or pumping away like a madman on the Raycor’s and lift pump. Or God forbid, sucking on the end of the fuel line with your mouth while praying for prime.
There are asphaltene dispersants and fuel stabilizer like Bycosin to disperse existing sediment and sludge, and prevent new formations of sludge. It also will improve the storage stability of the fuel in combination with a good filtration and priming setup. You should be good to go.
Fair Winds . . .
Improving Heavy Fuel Oil Usage by Homogenization by Stephen R. Burak
Updated - January 2016