Design Factors

An oily Water Separator is used to remove oil from the bilge water prior to discharging it overboard. It works on the principle of Stokes law and basically separates the two components utilizing their difference in specific gravity.


The OWS for marine use is optimized to make them smaller in size due to space constraints. Additional components are fitted to help with the separation process but the knowledge of the basic design factors is beneficial.


The rate of rising as per Stroke’s law is as follows – Stoke’s Law generally states that the velocity or rise and hence the separation rate is directly proportional to the difference in density of the oil and the continuous fluid and the size of the droplets of oil. It also states that the rate of rise is inversely proportional to the viscosity of the surrounding fluid.


We infer from the famous law the following:

  • The density of Oil: Light oil is having a higher rate of rising than heavier oil and is therefore easier to separate. This information is useful to adjust and lower the flow rate when heavier oil contamination is suspected.
  • The density of Continuous Fluid: The rate of rising will be higher when continuous fluid is seawater instead of freshwater or condensate. This information is useful and we can discharge the condensate drains into dedicated clean drain tanks instead of bilges.
  • The viscosity of Continuous Fluid:  As the rate of rising is inversely proportional to the viscosity of the continuous fluid the OWS performance is better when the continuous fluid is fresh water. As this deduction is contrary to the above one it is a compromise between the two but it does not concern us much as we don’t really have much control over what goes to the bilges as per the original design.
  • Size of Oil Droplets: The larger the diameter of the oil droplets the better the rate of separation. This information is very helpful and we can assist the OWS by avoiding small drops of oil by mechanical agitation and emulsification.
  • Temperature:  This is another factor that is important as it affects the density and viscosity directly.
    • The low temperature of the continuous fluid hinders the separation of oil due to additional viscous drag in view of the increased viscosity of the continuous fluid in cold temperatures. Oil separation is better in warmer temperatures and slightly increasing the temperature of the bilge water would give better separation.
    • In high temperatures, the formation of emulsion by mechanical agitation is more than in lower temperatures. This information is important as we should not heat the bilge holding tank when the ship is rolling excessively or where we suspect mechanical agitation. 

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