Marine fuels that are most used today are light and heavy fuel oils. Heavy fuels are mostly used to drive large marine diesel engines due to economy. The heavy fuel oil system consists of: loading and transfer system, storage system, fuel preparation system and fuel supply system. The fuel loading and transfer system consists of pipelines, purifiers, transfer pumps, valve systems and systems for rapid measurement of the contents of storage tanks in the loading area. The storage system consists of: bunker, settling, service and venting tanks. The fuel preparation system can be divided into two parts: a system up to service tanks and system from service tanks to engine. Fuel treatment devices consist of: separators, homogenizers, heaters, viscometers, pumps and filters. The purpose of separation is to separate two liquids of different densities, which do not mix, and at the same time to remove solid particles. Two separation principles can be applied: gravitational and centrifugal. According to the use, there are two types of separators, a purifier that separates two media of different densities and a clarifier that is used only to remove impurities and small amounts of water. As the next stage in the development of the separator, self-cleaning separators with controlled partial discharge of impurities were constructed. If the fuels have a density higher than 991 kg/m3 at a temperature of 15 oC, a special ALCAP purification system is used. In order to reduce the amount of sediment and exhaust emissions, homogenization of impurities must be performed. A homogenizer is a machine that aids the homogenization process, creating uniform structures of all solid materials contained in heavy fuel. The fuels used on ship at the ambient temperature are too viscous. The viscosity should be reduced by heating so that they can be pumped out of the tanks, sprayed during combustion in the boiler, purified and sprayed into the engine cylinders. The most common heaters on ships are steam heaters. The viscometer is one of the most neglected devices on board because the fuel temperature is considered more important, but the fuel temperature is increased to achieve the desired fuel viscosity. The viscometer measures the viscosity of the fuel and thus controls the supply of steam to heat the fuel. A pump is a machine used to lift liquids from a low to a high point. The most commonly used fuel flow pumps are gear and screw pumps. As a final stage, filters are installed in the fuel treatment system that serve as a safety measure to stop the remaining large particles from reaching the engine.