What is a Marine Oily Water Separator?
A marine Oily Water Separator (OWS) is a piece of equipment used onboard ships to remove oil from water. They are commonly used to separate oil from bilge water in order that the water can be discharged overboard, and the oil retained onboard; this protects the marine environment from pollution. An OWS is also sometimes called a Bilge Water Separator.
Oily Water Separator (OWS) Parts
Good to know – bilge water may be mixture of seawater, oily residues, and pollutants, that collect in the bilge (the lowest part of a ship's hull). Typical bilge water contaminants may include various types of oil (fuel oil, lubricating oil, hydraulic oil etc.), grease, detergents, solvents, chemicals, and/or solid waste.
What are the different types of Oily Water Separator?
There are two main types of OWS, these are the Gravity Separation type and Centrifugal Separation type.
Gravity Oily Water Separators
A gravity OWS uses density difference and the immiscibility of oil and water to separate the two fluids. Oil is lighter (has a lower density) than water and will thus be displaced by the heavier (higher density) water, forcing the oil to rise above the water.
Gravity Separation Principle
In a typical gravity oily water separator, bilge water is pumped into the top of the OWS and moves downwards due to gravity. Small particles of oil become trapped on catch plates and over time coalesce to form larger oil particles; the larger particles then become free of the plates and move to the top of the separator (due to the density difference) where the oil is pumped away. Oily water separate designs do however differ depending upon each manufacturer. Common parts seen within OWSs include filter elements, conical plates, resistive heaters, and absorption media, but the parts used depend upon the OWS’s design.
Gravity Oily Water Separator Parts
Good to know – the term immiscible refers to two liquids which do not mix or dissolve into each other. Instead, the two liquids remain separate, forming distinct layers or phases.
Good to know – the term coalesce refers to the creation of larger particles from smaller ones. In liquids, coalescence is often observed with oil and water mixtures. Oil molecules tend to coalesce due to the weaker intermolecular forces between them compared to the stronger bonds between water molecules; this causes the oil droplets to attract to each other and merge into larger particles.
Centrifugal Oily Water Separators
A centrifugal OWS uses centrifugal force to separate oil from water. Bilge water is fed into a centrifugal OWS, which rotates at high speed. The high rotational speed of the OWS causes the heavier phase (water) to be thrown outwards towards the periphery of the OWS bowl, whilst the lighter phases (oil etc.) remain closer to the centre rotating axis of the bowl. Centrifugal separators are more compact and efficient than gravity separators although less common.
Fuel Oil Purifier
Good to know – an item that uses centrifugal force to separate a fluid into its various parts is referred to as a centrifuge. Purifiers and clarifiers are types of centrifuges. Purifiers are typically used to separate mixtures of immiscible liquids, such as oil and water, while clarifiers are used to remove solid impurities from liquids.
Good to know – some oily water separators use chemical additives to enhance oil droplet coalescence and improve separation efficiency; these chemicals are typically biodegradable and environmentally friendly.