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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 filter unit.

Oily water separator

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.



Related Info:

Oily water separator regulatory requirement


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