Describe types and design of filters used onboard vessels?

24 Jun '17, 02:50

June 24, 2017, 2:50 a.m.
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Mechanical separation of solid contaminants from oil systems (fuel and lubricating) is achieved by the use of filters and strainers. A strainer is usually a coarse filter to remove the large contaminating particles. Both are arranged as full flow units, usually mounted in pairs (duplex) with one as a standby.

The strainer usually employs a mesh screen, an assembly of closely packed metal plates or wire coils which effectively block all but the smallest particles. It is usually fitted on the suction side of a pump and must be cleaned regularly or when the pressure differential across it becomes unacceptable. Where suction conditions are critical the strainer will b fitted on the discharge side of the pump. When cleaning is undertaken the other unit will be connected into the system by changeover valves or levers and oil circulation will continue. The particles of dirt collected on the outside of the strainer element of basket and can be removed by compressed air or brushing. A strainer should be cleaned as soon as it is taken out of the system, then reassembled and left ready for use.

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The wire mesh type filter, however, is rarely made to filter out particles below 125 microns in size. If finer filtration is required, other types of filter unit are used, one such filter unit is the well-known Auto-Clean strainer.

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Auto-Clean strainer is an improvement on the wire gauze strainer. It can be cleaned whilst in operation and it can filter out particles down to 25 microns in size. The dirty oil passes between a series o thin metal discs mounted upon a square central spindle. Between the discs are thin metal star shaped spacing washers of slightly smaller overall diameter than the discs. Cleaning blades, fitted to a square stationary spindle and the same thickness as the washers are between each pair of discs. As the oil passes between the discs, solid matter of sizes large than the space between the discs remains upon the periphery of the disc stack.

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The filter is cleaned by rotating the central spindle, this rotates the disc stack and the stationary cleaning blades scrape off the filtered solids which then settle to the bottom of the filter unit. Periodically, the flow of oil through the filter unit is interrupted and the sludge well is cleaned out. To facilitate this the filters are generally fitted in pairs.

Pressure gauges are fitted before and after the filter unit, these give an indication of the condition of the filter.

Magnetic filters and strainers are often used in lubricating oil systems, where a large permanent magnet collects any ferrous particles which are circulating in the system.

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In the case of magnetic strainer the magnet is surrounded by a cage or basket to simplify cleaning.

Fine filters are used to remove the smallest particles of dirt from oil before the oil enters the finely machined engine parts in either the fuel injection system or the bearings of the rotating machinery. Fin filters are full-flow units which clean all the oil supplied to the unit. The filtering substance may be a natural or synthetic fibrous woolen felt or paper.

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In full-flow filtration systems all the oil passes through the filter on its way to the engine. In a bypass system most of the oil goes to the lubrication system and a part is bypassed to a filter. A higher pressure drop across the filter can then be used and a slower filtration rate. A centrifugal filter can be used in a bypass system where the oil passes through a rotor and spins it at high speed (see figure below). Dirt particles in the oil are then deposited on the walls of the rotor and the clean oil returns to the sump. This type of filter cannot block or clog and requires no replacement elements. It must be dismantled for cleaning of the rotor unit at regular intervals.

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24 Jun '17, 03:50

June 24, 2017, 3:50 a.m.
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