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Oily Water Separator

Oily Water
MARPOL has a regulation under ANNEX I which limits the oil content in the bilge
water that vessel can legitimately discharge into the sea. It is now a requirement
for all vessels to have an oil discharge monitoring and control system along with
an oil filtering equipment known as the Oily Water Separator (OWS).
A ship engineer may work with 5-10 different makes of marine engines, but he/she
is more likely to encounter many more types and makes of OWS in his/her career
span. Even for PSC inspectors and surveyors, oily water separator (OWS) has
always been a preferred choice of machinery on the ship for inspection. Hence, it
is imperative to know and understand the basics of oil and water separator design
and how an oil and water separator works.
As the name indicates, the function of oily water separator is to separate
maximum amount of oil particles from the water to be discharged overboard
from engine room or cargo hold bilges, oil tanks and oil contaminated spaces. As
per maritime regulations, the oil content in the water processed from the OWS
must be less than 15 parts per million of oil.
Oily Water Separator (OWS),
MARPOL Annex - 1 : Every ocean going ship must have
Oily water separator.
MEPC (marine environment protection committee)
suggests OWS for all ocean going ships.
Function: separates oil from bilge water as per
regulations and also used in de-ballasting or when
cleaning oil tanks.
OWS & Centrifuges:
Both are used for separating oil and water.
In OWS, there is large quantity of water and less amounts of
oil is removed.
In Centrifuges, there is large amount of oil and less amounts
of water is removed.
In OWS, all components are static but in Centrifuges
rotating components are available.
An Oil Water Separator is a piece of equipment
used to separate oil and water mixtures into their
separate component. There are many different
types of oil-water separator. Each has different oil
separation capability and are used in different
industries. Oil water separators are designed and
selected after consideration of oil separation
performance parameters and life cycle cost
considerations. ”Oil” can be taken to mean
mineral, vegetable and animal oils, and the
many different hydrocarbons.
Oil water separators can be designed to treat a
variety of contaminants in water including free
floating oil, emulsified oil , dissolve oil and
suspended solids. Not all oil separator types are
capable of separating all contaminants. The
most common performance parameters
considered are
Oil droplet size (in the feed of the separators)
Oil density
Water Viscosity (temperature)
Discharge water quality desired
Feed oil water flow (daily and peak hourly)
An oily water separator (OWS)
(marine) is a piece of equipment
specific to the shipping or marine
industry. It is used to separate oil and
water mixtures into their separate
components. This page deals
exclusively oily water separators
aboard marine vessels. They are found
on board ships where they are used to
separate oil from oily waste water such
as bilge water before the waste water
is discharged into the environment.
These discharges of waste water must
comply with the requirements laid out
in Marpol 73/78
Bilge water – is a nearunavoidable product of
shipboard operations. Oil leaks
from running machinery , such as
diesel generators, air
compressors, and the main
propulsion engine . Modern OWS
have alarms and automatic
closure devices which are
activated when the oil storage
capacity if the oil water separator
has been reched.
The primary purpose of a shipboard oily water separator (OWS) is to
separate oil and other contaminants that could be harmful for the oceans.
The International Maritime Organization (IMO) publishes regulations through
the Marine Environment Protection Committee (MEPC). On July 18, 2003,
the MEPC issued new regulations that each vessel built after this date had
to follow. This document is known as MEPC 107(49)[ and it details revised
guidelines and specifications for pollution prevention equipment for
machinery space bilges of ships. Each OWS must be able to achieve clean
bilge water under 15 ppm of type C oil or heavily emulsified oil, and any
other contaminates that may be found. All oil content monitors (OCM) must
be tamper-proof. Also whenever the OWS is being cleaned out the OCM
must be active. An OWS must be able to clear out contaminants as well as
oil. Some of these contaminating agents include lubricating oil, cleaning
product, soot from combustion, fuel oil, rust, sewage, and several other
things that can be harmful to the ocean environment.
Gravity plate separator
A gravity plate separator contains a series of plates
through which the contaminated water flows. The
objective of the design is to allow oil droplets in the water
to coalesce on the underside of the plate eventually
forming larger oil droplets which floats off the plates and
accumulates at the top of the chamber. The oil
accumulating at the top is then transferred with some
en-trained water to a waste oil tank. This type of oily
water separator is very common for many industrial
applications as well as in ships but it has some flaws that
decrease efficiency. Oil particles that are sixty
micrometers in size or smaller do not get separated. Also
the presence of chemicals and surfactants in the water
greatly reduce oil droplet coalescence, impeding the
separation effect The variety of oily wastes in bilge water
can limit removal efficiency especially when very dense
and highly viscous oils such as bunker oil are present.
Plates must be replaced when fouled, which increases
the costs of operation.[2
Centrifugal oily water separator
A centrifugal water–oil separator, centrifugal oil–
water separator or centrifugal liquid–liquid
separator is a device designed to separate oil and
water by centrifugation. It generally contains a
cylindrical container that rotates inside a larger
stationary container. The denser liquid, usually
water, accumulates at the periphery of the
rotating container and is collected from the side of
the device, whereas the less dense liquid, usually
oil, accumulates at the rotation axis and is
collected from the center. Centrifugal oil–water
separators are used for waste water processing
and for cleanup of oil spills on sea or on lake.
Centrifugal oil–water separators are also used for
filtering diesel and lubricating oils by removing the
waste particles and impurity from them.
Hydrocyclone oily water separator
An oil water separation hydrocyclone is a device designed to
separate oil from water by the use of a strong vortex. These separators
are passive (no moving parts) and resemble long tapered pipes. They
typically contain an inlet section, long tapered section and a long
outlet section. In operation the strong vortex is created when the oily
water is injected tangentially into the inlet end of the separator. This
creates a centrifugal force, that accelerates as it moves down the
tapered cone. The centripetal and centrifugal forces separate the
heavier water component to the outside of the vortex while the lighter
oil droplets are forced to the centre. The separated oils are removed
through an orifice at the inlet end of the cone and treated water is
discharged through the opposite end. The centrifugal forces
generated inside the vortex of the better de-oiling hydrocyclone
separators are of the order of 1,000 times the force of gravity. This is
why smaller emulsified oil droplets as low as 15 microns can be
Oil removal hydrocyclones, or de-oiling hydrocyclones, are very
different in geometry, design and operation compared to the more
common solid removal hydrocyclones. When correctly designed and
operated oil removal hydrocyclones Hydrocyclones are very useful for
removing both large oil droplets and smaller emulsified oil droplets in a
broad range of applications across many industries. The technology
has been successfully applied to treat oily water produced in the
mining industry, meat processing, dairy manufacturing,
petrochemical, oil refining, oil marketing and oil production
Separation force & Resistance force
Main principle of separation, is the density difference between oil and water.
In oily water mixtures, oil exists as a collection of globules of various sizes.
Force acting on these globules, causing it to move in the water is proportional to the difference
between the weights of oil particle and water particles.
Resistance to the movement of globule, can be given by stoke's law.
Now, separating force is equal to resistance to movement.
Above equation shows, separation of oil depends on various factors.
Note : The entrance area in OWS is made large so that flow is slow and large slugs of oil can move to
the surface quickly. Faster rate of separation is obtained with large size oil globules or slugs.
Separate, What & What not?
Oil droplets from water
HC & Vegetable oils
Solid particles
Not, dissolved contents
Oily Water Separator
Working can be divided into 2 stages : Separation (1st Stage Filter) & Filteration (2nd Stage
Filter) or Coalescer unit.
In separation stage oil content is reduced in 100 ppm and after Coalescer unit oil content
is reduced in 15 ppm (i.e desired ppm level).
Now, the oily water (Bilge water) enters the separator in the upper half of the unit and is
directed downwards to the Baffle plates.
Large globules of oil separate out in the upper part and smaller globules are carried out
by the water into the spaces between plates.
Raising velocity of globules carries them upwards where they become trapped by the
under surfaces of the plates and coalesce until the globules have sufficient rising velocity
to travel along the plate surface and break away at the periphery.
Particles are settled down in compartment when passes through baffles.
Through control valve, oil is collected in sludge tank or any desired tank.
Vent valve releases air whenever required.
Now, from this separating unit oily water is sent to 2nd stage filter unit. First of all oil is
filtered by dedicated filter, particles settle down in compartment
Coalescer is used for de-oiling finally. Coalescer is just a reverse of Emulsifier,
separate emulsions into their components.
Oil rises above the water and it is collected through control valve.
Finally oily water mixture (very less oil) is controlled and monitored by OCM Unit
(oil content monitoring). When mixture has less than or equal to 15 ppm oil
content , then it is discharged through 3-way control valve.
If oil content is more than 15 ppm then alarm is automatically activated and
mixture is sent to Bilge holding tank again.
In Simple :
1st Stage - Oily mixture (Bilge water) enters into separator unit
and passes through baffles - larger particles settle down - oil rises
up due to less density - oil is collected through control valve mixture is passed via a pipeline to Filter unit
2nd Stage - Oily mixture is filtered by filter and particles are
separated - Coalescer breaks the oil-water emulsion in its
components - now, oil rises up due to less density - oil is collected
through control valves in desired tank - now ppm level is very less
and it is monitored by OCM - OCM monitors the required ppm
level and controls over it
Effective separation:
Large size of globules (oil) - easily rises up and separated
Temperature (high) - density decreases and tends to
Use of sea water - having more density compared to fresh
water, easily separated
Pumping system - effective pumping always helps (reduces
Very less maintenance required because of static parts
That's why it serves for long time
But for effective running, we have to perform some
maintenance also
Oil absorbent material should be changed regularly
Cleaning of chambers
Checking of Oil Monitoring Discharge System (ODM) or
Checking of valve etc.