An economic operation of any ship is not possible without the use of heavy fuel oil. Heavy fuel oil when on ship is passed though a series of equipments to make it of the purest form. This heavy fuel oil used in ship engines has an extremely high viscosity and one sight of it will make you wonder how anyone can possibly use it? It is for this reason that the oil when on board ship, is passed through a series of settling and purification processes to make it usable for engines. To understand how this is done we will go through the whole process of fuel oil system from the start, when the oil is taken on board the ship.
Making Heavy Fuel Oil Usable
When the fuel is first taken on board a ship, it is stored in separate tanks which are generally located at the ship’s double bottom area. In order to reduce the viscosity of the oil so that it flows easily, tanks are continuously heated with steam or any other medium. The heating also facilitates the pumping operation for transferring the heavy oil from tanks which is usually done by positive displacement transfer pumps. The fuel oil is than transferred to settling tanks where it is further heated. The fuel is retained in the settling tanks for a long time so that dissolved solids and water can settle down through gravity at the bottom and taken away later.
From the settling tank, fuel passes through separators and clarifiers for further purification. The separator removes the water, solid impurities and sludge, whereas the clarifiers remove the remaining solid impurities from the oil. This purification process that involves clarifier and purifier is known as ‘two-stage cleaning’ process. It is to note the all the machines that are attached in the fuel line have heaters attached to them for making the purification process easy by reducing the oil’s viscosity. The heaters generally use steam which is maintained at appropriate temperature by a thermostatic control.
The oil before flowing into the clarifier goes through one more heater. Purified oil from the clarifier is transferred to a clean oil tank wherein the fuel is stored until needed by the engine. The clean oil tank is also continuously heated from a heater. The oil from the tank then passes through a surcharge pump which pushes the oil through a high temperature heater which controls the final viscosity of the fuel oil. The steam heater will heat the fuel oil and provide it with the required injection viscosity. The flow and temperature of the steam is maintained by a viscometer which thus maintains the viscosity of the oil leaving the heater. The oil leaving the viscometer then passes through a fine filter before entering the engine.
In this way a high viscosity fuel oil is made of usable quality for the marine engines. It is also to note that some amount of impurities will always remain in the fuel oil even after the purification process. Moreover, soluble impurities like vanadium and sodium compounds dissolves in the oil and thus it is impossible to separate them from the oil. These compounds not only affect the viscosity of the oil but also produce harmful effects on the engine.