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
? 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
? 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:
gDF s )(6 03 ??? ? ??
vdF r ??3?
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
? 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
gu ??
? ?
??
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.
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
? 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
? 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:
gDF s )(6 03 ??? ? ??
vdF r ??3?
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
? 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
gu ??
? ?
??
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.
