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Oily water separator working principles and guidance
Oil/water separators are used to ensure that ships do not discharge oil
when pumping out bilges, oil tanks or any oil-contaminated space.
International legislation relating to oil pollution is becoming more and
more stringent in the limits set for oil discharge.
The oily water separator (OWS) is a very important piece of equipment carried on board. It is used to ensure that water is discharged overboard within legal limits. The OWS must be maintained in full working order and operated according to MARPOL regulations.
Officers using the equipment must be fully familiar with how it operates.
The OWS is frequently inspected at port state inspections. There should also be an OWS operation manual on board and relevant staff should be familiar with the manual and should have practised with the equipment. Please refer to MARPOL Annex 1 and Marlins Course: Oily Water Separators for more information.
Clean water suitable for
discharge is defined as that containing less than 15 parts per million of
oil. Oil/water separators using the gravity system can only achieve 100
parts per million and must therefore be used in conjunction with some
form of filter.
A complete oil/water separator and filter unit for 15 parts per million
purity is shown in Figure above. The complete unit is first filled with clean
water; the oily water mixture is then pumped through the separator
inlet pipe into the coarse separating compartment. Here some oil, as a
result of its lower density, will separate and rise into the oil collection
space. The remaining oil/water mixture now flows down into the fine
separating compartment and moves slowly between the catch plates.
More oil will separate out onto the underside of these plates and travel
outwards until it is free to rise into the oil collecting space. The almost
oil-free water passes into the central pipe and leaves the separator unit.
The purity at this point will be 100 parts per million or less. An
automatically controlled valve releases the separated oil to a storage
tank. Air is released from the unit by a vent valve. Steam or electric
heating coils are provided in the upper and sometimes the lower parts of
the separator, depending upon the type of oil to be separated.
Where greater purity is required, the almost oil-free water passes to a
Fig:Oily water separator
The water flows in turn through two filter stages and the oil
removed passes to oil collecting spaces. The first-stage filter removes
physical impurities present and promotes some fine separation. The
second-stage filter uses coalescer inserts to achieve the final de-oiling.
Coalescence is the breakdown of surface tension between oil droplets in
an oil/water mixture which causes them to join and increase in size. The
oil from the collecting spaces is drained away manually, as required,
usually about once a week. The filter inserts will require changing, the
period of useful life depending upon the operating conditions.
Current legislation requires the use of a monitoring unit which
continuously records and gives an alarm when levels of discharge in
excess of 15 parts per million occur.
Oily water separator regulatory requirement
Other Info pages:
- Compressed Air Systems for various shipboard operations
The main aim of a compressor, as the name suggests, is to compress air or any fluid in order to reduce its volume. Some of the main applications of compressors onboard ships are main air compressor, deck air compressor, AC compressor and refrigeration compressor. Failure to start or control air compressor can be inconvenient, costly and can carry risks, which need to be managed.....
- Marine air compressors working principles
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- Oily water separator and filter unit for 15 parts per million purity
Oil/water separators are used to ensure that ships do not discharge oil when pumping out bilges, oil tanks or any oil-contaminated space. International legislation relating to oil pollution is becoming more and more stringent in the limits set for oil discharge.....
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Marine machineries - Useful tags
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