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The type of fuel oil used on a ship plays a vital role in determining the quality of combustion, efficiency of the engine and the amount and type of maintenance required. In order to ensure a clean supply of fuel oil, the oil should be free from any type of solid contaminants and impurities. The process of removing solid particles from the fuel oil is known as mechanical separation, which is achieved by using filters and strainers. In fact a strainer is also a kind of a coarse filter used for separating solid particles of a different size.
The main difference between a fuel filter and a fuel strainer assembly is that the latter is usually used to remove large solid contaminant particles while a filter is used for fine filtration. Both, filters and strainers are mounted in pairs, with one as working and the other as standby.
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Construction and Working
The strainer assembly consists of a closely packed cylindrical arrangement of metal plates or wire coils, which is commonly known as mesh screen. The holes of the mesh screen is such that they filter all the large solid particles but allow the smaller ones to pass. The strainers are always fitted on the suction side of the pump. If the pump is placed at a position where it tends to get suction problem, then the strainer is fitted on the discharge side. As the strainers are fitted in pairs, a change-over valve is provided at the top of the assembly to change the strainer when the one is use becomes dirty and chocked. When dirty, the strainer will produce low differential pressure across the pump. Drop in suction pressure is an important signal for changing the strainer. The strainer assembly is so designed that in case a strainer gets chocked with impurities, the system can be put on the standby strainer till the time the chocked strainer is cleaned. During the change over, the system will run normally with continuous oil circulation.
The solid particles gets collected at the outside of the strainer and can be removed with the help of a metallic brush and compressed air.It is also advisable to keep cleaned strainer as standby, in case of an emergency.
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Nowadays, apart from the conventional strainers, magnetic strainers are also used. Lately, it has been seen that heavy fuel oil used in marine engines often contain dissolved iron particles. Some of these iron particles are already present in the oil while some iron particles come off the oil tanks surfaces and walls as a result of corrosion. Magnetic strainers are mainly used for lubricating oil systems. The magnetic strainers consists of a large magnet which attracts and collects any ferrous particles that are suspended in the oil. The arrangement of a magnetic strainer is also same as that of a conventional strainer with the magnet located in the closely held cage or basket. This is done to make the cleaning process easy and for the smooth removal of metallic particles from outside the strainer.
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Introduction of Marine Engineering 2nd Edition by D.A Taylor