Oily Water Separator Used Onboard Ship

Marine Auxiliary Machine

Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard mainly when pumping out bilges.

The requirement to fit a OWS is due to the regulation of International Maritime Organization.
What will happen if there is no regulation?
Legislation was needed because free oil and oily emulsions discharged in a waterway can interfere with natural processes such as photosynthesis and re-aeration, and induce the destruction of the algae and plankton so essential to fish life. Inshore discharge of oil can cause damage to bird life and mass pollution of beaches.
What is the Regulation of IMO?
Ship cannot discharge water in the sea more than 15ppm. Ships found discharging water containing more than 100 mg/litre of oil or discharging more than 60 litres of oil per nautical mile can be heavily fined, as also can the ship’s Master.
What is the difference between OWS and purifier ?
Oil/water separators and centrifuges are both employed for the purpose of separating oil and water but there are major differences. Oil/water separators are required to handle large quantities of water from which usually, small amounts of oil must be removed.

Various features are necessary to aid removal of the oil from the large bulk of water particularly when the difference in densities is small.
Centrifuges are required to remove (again usually) small quantities of water from a much larger amount of oil. Additionally the centrifuge must separate solids and it must, with respect to fuel, handle large quantities at the rate at which the fuel is consumed.

Working Principle of Oily Water Separator

Water containing less than 15 parts per million of oil is allowed to throw out of the ship into the sea.
Oily Water Seperator(OWS) works in the three stage of purification of water.

In the first stage purity of water will be acheived 100 parts per million or less.
First Stage:-

  • First of all the unit is completly filled with clean water, then the oily water mixture is then pumped through the separator through the inlet pipe into the coarse separating compartment.
  • Due to the lower density oil, 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.
  • 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.

Second and Third Stage :-

  • 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 tojoin 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.
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