MARINESHELF RECENT

MARINESHELF

MARINE AUXILARIES

Filters and Strainers

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 o when the pressure differential across it become 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 collect 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.

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-Klean strainer (above).
This 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 stationery 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.
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 indication of the condition of the filter.
The figure illustrates an Auto-Klean filter unit of the type capable of filtering out particles down to 200 microns in size, this type can also be made to filter particles of under 75 microns, but the mechanical strength of the cleaning blades will be low. A more recent type of Auto-Klean strainer has the modified disc stack and cleaning blade arrangement shown. With this modified disc stack particles of under 25 microns in size can be filtered out without impairing the mechanical strength of the cleaning blades.
 Magnetic strainers are often used in lubricating oil systems, where a large permanent magnet collects any ferrous particles  which are circulating in the system. The magnet is surrounded by a cage or basket to simplify cleaning.
Fine filters, again in pairs, 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 engine. The filtering substance may be a natural or synthetic fibrous woollen felt or paper. A felt-type fine filter is shown in the figure above. A steel division plate divides the steel pressure vessel into an upper and a lower chamber. Dirty oil passes into the upper chamber and through the filter chamber and out of the unit. A magnetic filter can be positioned as shown in the central tube. A spring-loaded bypass is shown in the diagram, for lubricating oil filters only, to ensure a flow of oil should the filter become blocked. The cartridge in the design shown is disposable although designs exist to enable back-flushing with compressed air to clean the filter element are required. The filter unit shown will be one of a pair which can be alternately used in service.
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 e 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.