The Need for Fuel Treatment and Possible Pitfalls
The heavy fuel used today is a residual product. Whilst this provides a cost advantage, today's heavy fuel oil (HFO) is more difficult to manage and contains contaminants which can cause engine damage and must therefore be removed in a suitable treatment system.
The role of the level of cat fines (Al+Si) as an indication of fuel cleanliness after treatment is discussed.
The treatment systems that are onboard most vessels should be designed to reduce the amount of contaminants to an acceptable level prior to injection in the engine. There are however a number of potential pitfalls that can reduce the ability of a treatment system to remove the contaminants. The major points are listed and corrective measures suggested.
Key words: Heavy fuel oil (HFO), contaminants, fuel treatment, cleaning, separation efficiency, cat fines
The heavy fuel used by the marine industry today is a residual product. This is despite the fact that more easily used fuels, such as distillates which are basically clean and easier to use are also available.
The reason is very simple. Heavy fuel oil (HFO) is a low cost material and, in this hard world where costs must be controlled, a low cost alternative cannot be ignored. But as usual there is a down side to this cost advantage.
The handling of HFO is far more complicated. More attention has to be paid to fuel management with the need to avoid mixing different batches of fuel due to incompatibility as just one example. The booster system has to be pressurised and above all, heavy fuel oil has to be cleaned in order to protect the engine.
2. THE CONTAMINANTS IN HFO
There is a wide range of contaminants that have to be removed from HFO in order to avoid abrasive wear, corrosion, deposits and fouling in the engine. Typical contaminants are particles such as sand, rust and catalytic fines. water, sodium and components precipitated from the oil itself. Now and then, abnormal contaminants originating from the dumping of industrial waste in the bunker also show up. Used automotive lubricating oils and polypropylene are some good examples.
The levels given for water, ash and aluminium/silicon (cat fines) in bunker specifications are typically taken as those characteristics that reflect the level of contamination of the oil even though element analysis figures are also sometimes presented.
Except for the amount of water, which can vary widely, the amounts of contaminants to be removed are normally in the order of magnitude of not more than some 100 ppm. This fact makes it sometimes hard to fully understand and follow the efficiency of the different steps in the treatment process. For example, in many cases the removable part of the ash content is only a minor part whereas the rest of the ash is present in oil soluble compounds.
For this reason, the content of aluminium and silicon (Al+Si = cat fines) is often given the role of acting as the model substance to be traced in order to evaluate the efficiency of the treatment system. When there are cat fines present it really is one of the best parameters to use, but when cat fines are not present it does not mean that the oil is clean. Due to the lack of a good definition of oil cleanliness this sometimes gives the perception to the industry that no or a low cat fines content means that the fuel is clean. Nothing could be more wrong as there could still be a number of abrasive particles in the oil, but until a better definition has been established then cat fines will continue to be used as an indication of the level of contamination.
The data in Table 1 is from the May 2000 issue of MER  and gives examples of the levels of cat fines found in fuel oil from different regions. It should be remembered that 80 ppm is the maximum Al+Si level according to the CIMAC specification.
* Alfa Laval Marine & Power AB.
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