A normal heavy or intermediate fuel oil treatment process begins with the oil loaded into the bunker tank. Under normal circumstances the fuel will not be used immediately and should be analysed to confirm its specifications match the order and that it is appropriate to burn in the engine.
Assuming the fuel has passed any analysis tests, it will be pumped from the bunker tanks to the settling tank. In the case of HFO it will almost certainly contain some level of cat fines and all fuels will have some amount of water and sludge. In the settling tank, water and the heavier contaminants will migrate to the bottom, but some will remain suspended in the fuel.
Settling tanks have a sloping bottom with a drain valve at the lowest point by which sludge and water can be drained at regular intervals. The feed to the next stage in the process is above the drain cock. The other components include a heating coil so as to be able to control viscosity, high- and low-level alarms and a sight glass or gauges.
Because a ship operates in a dynamic environment the settling tank is not entirely successful in keeping the fuel separate from heavy contaminants and water. In anything but a dead calm, the movements of the vessel will stir up the contents of the settling tank to a greater or lesser degree.
The longer the fuel can remain in the settling tank, the more gravitational settlement there will be and the better the fuel will be prepared for the next stage. From the settling tank the fuel is sent to the service tank. On its way it passes first through a filter to remove the larger contaminants and then to a preheater.
Tank heating & viscosity
Different oil fuel types need differing treatment mostly to control their viscosity. Distillates are much lighter fuels and will usually flow quite well except under conditions of very low temperatures when they may wax. Heavy fuel oils on the other hand have a very high viscosity and normally require heating.
Fuel oil tanks on the most modern ships may need to be in protected locations but on some older vessels there may be just a few millimetres of steel between the fuel and the sea. Even when protected, the location of storage tanks means temperatures will only ever match the ambient outside temperature.
It would be impossible to pump heavy fuel oil at such temperatures because it becomes highly viscous, so heating is essential to bring the temperature above the pour point. For most heavy fuel oils this will be around 40ºC. As the fuel progresses through the treatment system, the temperature will be raised to aid purifying and injection. The usual means of heating are steam coils in large vessels and electric heating in smaller ships.
The steam coils will draw their heat either from boilers or via heat exchangers in the engine exhaust stream. Heat exchangers are also used further along the treatment system. In the settling tank, a temperature of 60ºC is required, thinning the oil sufficiently for heavy contaminants to gravitate to the bottom. The fuel will be heated more before entering the separator, but care needs to be taken to keep the temperature below the boiling point of water otherwise separation will not be carried out efficiently.