武汉理工大学：《船舶辅机》课程教学资源（PPT课件，英文版）6.1 Principle of operation

6 Oil Water Separators

6 Oil Water Separators

Introduction Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard when pumping out bilges, deballasting or when cleaning oil tanks. The requirement to fit such devices is the result of international legislation The need for such legislation is that free oil and oily emulsions discharged in a waterway can interfere with natural processes such as photosynthesis and re-aeration, resulting in destruction of the algae and plankton so essential to fish life

Introduction • Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard when pumping out bilges, deballasting or when cleaning oil tanks. The requirement to fit such devices is the result of international legislation. The need for such legislation is that free oil and oily emulsions discharged in a waterway can interfere with natural processes such as photosynthesis and re-aeration, resulting in destruction of the algae and plankton so essential to fish life

Introduction Inshore discharge of oil can cause damage to bird life and mass pollution of beaches. 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. In consequence it is important that oil/water separators are correctly installed, used and maintained

Introduction • Inshore discharge of oil can cause damage to bird life and mass pollution of beaches. 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. • In consequence it is important that oil/water separators are correctly installed, used and maintained

6.1 Principle of operation

6.1 Principle of operation

6.1 Principle of operation The principle of separation by which all commercially available oil/Water separators is the gravity differential between oil and water In oily water mixtures. the oil exists as a collection of almost spherical globules of various sizes. The force acting on such a globule causing it to move in the water is proportional to the difference in weight between the oil particle and a particle of water of equal volume. This can be expressed as

6.1 Principle of operation • The principle of separation by which all commercially available oil/Water separators is the gravity differential between oil and water. • In oily water mixtures, the oil exists as a collection of almost spherical globules of various sizes. The force acting on such a globule causing it to move in the water is proportional to the difference in weight between the oil particle and a particle of water of equal volume. This can be expressed as:

6.1 Principle of operation LDO F=rwd The resistance to the movement of the globule depends on its size and the viscosity of the fluid For small particles moving under streamline flow the relationship between these properties is expressed by stokes Law

Fs D ( )g 6 0 3    =  − Fr = 3 vd 6.1 Principle of operation • The resistance to the movement of the globule depends on its size and the viscosity of the fluid. For small particles moving under streamline flow the relationship between these properties is expressed by Stoke’s Law:

6.1 Principle of operation g ()O0-P0)d 18 When separation of an oil globule in water is taking place,Fs F will equal Fr and the above equations can be worked to express the relationship of the terminal (or in this case rising velocity of the globule with viscosity, relative density and particle size

6.1 Principle of operation • When separation of an oil g1obule in water is taking p1ace, Fs will equal Fr and the above equations can be worked to express the relationship of the terminal (or in this case rising) velocity of the globule with viscosity, relative density and particle size: 2 0 )( ) 18 ( d g u    =  −

6.1 Principle of operation In general, a high rate of separation is favoured by a large size of oil globule, elevated temperature of the system(which affects both the specific gravity differential of the oil and water and the viscosity of the water )and the use of sea water Turbulence or agitation should be avoided since it causes re-entrainment; conversely laminar(or streamline) flow is beneficial

6.1 Principle of operation • In general, a high rate of separation is favoured by a large size of oil globule, elevated temperature of the system (which affects both the specific gravity differential of the oil and water and the viscosity of the water) and the use of sea water. • Turbulence or agitation should be avoided since it causes re-entrainment; conversely laminar (or streamline) flow is beneficial

6.1 Principle of operation While heating coils are fitted to all types of separators found at sea (as much to enable high viscosity oils to be removed from the separator as to provide optimum separating conditions), a variety of means are used to encourage laminar flow and the formation of large oil globules, with associated increase in velocity Units also exist which attempt a higher rate of separation by inducing cyclonic flow the theory being that the centripetal force acting on the globule will effectively increase the velocity differential. It is extremely difficult however to induce this type of flow without causing considerable turbulence with resultant re-entrainment

6.1 Principle of operation • While heating coils are fitted to all types of separators found at sea (as much to enable high viscosity oils to be removed from the separator as to provide optimum separating conditions), a variety of means are used to encourage laminar flow and the formation of large oil globules, with associated increase in velocity. • Units also exist which attempt a higher rate of separation by inducing cyclonic flow the theory being that the centripetal force acting on the globule will effectively increase the velocity differential. It is extremely difficult however to induce this type of flow without causing considerable turbulence with resultant re-entrainment