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.
