Filter is a fine
mesh screen which is used to remove impurities from oil, water and air on ship.
Filters are mounted in pairs as a duplex system so that one can be used and
other is kept on standby at a time. Filter can be used both in low pressure
(suction) and discharge (high pressure) side of the system and is used to
remove the smallest part of dirt which is carried away in the system. The
cleaning frequency of filters depends upon the type of the filter and is
decided by the manufacturer. The schedule is normally included in the planned
maintenance system on board.
Strainer is a type of
filter used mostly on low pressure or suction side and is used to remove large
contamination particles from the system. This is because the mesh screen
openings in the strainer are bigger in size and are similar to those of coarse
filter. If the suction condition is critical, then the strainer can be fitted
on the discharge side. This purely depends upon the make and recommendation of
the manufacturer. Normally strainers are cleaned whenever they are opened or
when the pressure drop on the filter side is high.
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
larger 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 become unacceptable. Where suction conditions are
critical the strainer will be 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 or 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.
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. Fine 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
Figure . 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 element, then the filtered oil
passes down the central tube to the lower 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 as
required. The filter unit shown will be one of a pair which can be alternately
in service .In full-flow filtration systems all the oil passes through the
filter on its way to the engine. In a by-pass system most of the oil goes to
the lubrication system and a part is by-passed 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 by-pass system where the oil passes through a rotor and
spins it at high speed .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 replaceable elements. It must be
dismantled for cleaning of the rotor unit at regular intervals.
·
Magnetic Filters
Apart from the
conventional filters, there are also magnetic filters, which are used to remove
fine iron particles from the system. The arrangement of a magnetic filter is
same as that of a fine filter, but with a magnetic element placed in the
central tube. In such systems, a spring loaded by-pass valve is provided to
ensure the flow of the lubricating oil in case the filter gets chocked.
Moreover, a back flushing system is also provided for cleaning the filter
element while the filter is still inside the assembly. Back flushing is
generally done using compressed air but the back flushing process can be done
only for a specific number of times. If the filter doesn't show any improvement
even after consecutive back flushing, then the filter should be replaced. Both
the filters work alternatively for the filtration process.
·
Types of Filtration Systems
Generally
there are two types of filtration system :
·
Full flow system
·
By-pass system
In a full flow
system , all the oil passes through the filter while flowing to the engine,
whereas in a by pass system, only a part of oil goes to the filter while the
rest goes to the lubrication system. A by pass system is used to attain higher
pressure drop across the filter and also to make the filtration process slower
and efficient. A centrifugal filter can also be used in a by-pass system. The
centrifugal filter consists of a rotor, which rotates at a high speed and
through which the oil passes. The oil is thrown away from the rotor due to the
centrifugal force and the impurities stay at the rotor. The clean oil returns
to the sump and the impurities at the rotor are removed through a special
arrangement. This type of filter never gets chocked and requires no replacement
element.
ref:
Introduction to Marine Engineering by
Great going dude!!
ReplyDeleteGreat going dude!!
ReplyDelete