Marine Purifier

MARINE PURIFIER
Mohammud Hanif Dewan, Senior Engg. Lecturer,
International Maritime Academy, Bangladesh

Separation
 Separation as a means of removing impurities from a fuel
can be undertaken by means of gravity in a settling tank
or by means of centrifuging the fuel. Both methods work
on the same principles that by subjecting the fuel to a
constant force, the denser components of the fuel
i.e water and dirt will be separated from the lighter
components i.e. the fuel itself.
 Gravity acting on the fuel as it passes slowly through the
tank will separate the denser components from the fuel
where they will accumulate at the bottom of the tank. The
contaminants can then be remove by separation of
sludge from the tank.
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Principle of separation
 If an oily water mix is placed into a tank then separation of
the two parts will begin with the lighter element rising to
the top. The rate the separation occurs is governed by
several factors including the difference in specific gravities
and the force of gravity acting upon it.
 For mixes placed into a settling tank there is little that can
be done about the gravity but the difference in the specific
gravities can be increased by heating. This because water
density changes at a much reduced rate when compared to
oil.
A wide shallow tank will increase the rate of clarification
over a tall thin tank
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Principles of Separation
 When a volume of light oil is placed into a tank
contain a weir and a quantity of water the fluids will
tend to arrange themselves as shown above. The height
of the water in the weir rises to a point governed by the
volume (and thereby relative height) and specific
gravity of the light oil.
 Knowing this it is possible to form a rudimentary
purification system.
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Principle of separation
 As a oil/water mix is fed into the tank separation begins
with heavy particulates falling to the base of the tank along
with water which joins the other water excess overflowing
the heavy phase weir. Hopefully clear oil passes over the
light phase weir. The problem arises that to ensure their is
suffcient time to allow for full (separation of the oily mix
the flow would have to be very small relative to the size of
the tank.
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Centrifuging
 Centrifuging is the process by which the effects of
gravity can be amplified by the use of centrifugal
force to the extent that the separation process
becomes rapid and continuous. Centrifuges work by
rapidly spinning a bowl containing the liquid, thus
producing the required centrifugal force to produce
separation.
 The principle of operation of the centrifuge is simple.
When a bowl containing impure fuel is rotated,
centrifugal forces will throw any item with density
greater than the fuel oil density (solids and free
water) to the periphery of the bowl.
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Centrifuging
 Centrifugal separators used for the separation of two
liquids of different densities (fuel and water) are
known as purifiers and those used for separating
solid impurities are known as clarifiers. Purifiers will
also remove some solids and clarifiers will also
remove small quantities of water.
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Principle of seperation in centrifuge containing
angled plate stack
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Fluid moving between two plates has a velocity
greatest at mid point and minimum approaching
the plates.
a particle entering into the plates will tend to be
pushed upwards by the fluid flow. All the time
centrifugal force tends to retard the horizontal
component of the movement causing the partical
to approach the underside of the top disc. As it
approaches the fluid flow velocity reduces.
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The centrifugal force eventually overcomes the
force acting on the particle due to fluid
movement and the particle starts to move
towards the outer rim. The centrifugal force
acting on a particle is proportional to its mass
therefore a small particle will tend to move
further under the influence of fluid flow. Indeed a
particle small enough will be carried through the
plates and out with the discharge. In this way it
can be seen that reducing the flow rate to a
purifier will tend to increase the quality of the
output.
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Types of Centrifuge:
 There are normally two types based on the
application:
1) Clarifier &
2) Purifier
1) Clarifier:
When a centrifugal is arranged to remove only impurities
and small amount of water, it is called as clarifier. Since
it is used mainly for that fluid where mostly solid
impurities are to be removed, gravity disc is not used in
clarifier; instead a sealing ring is used to keep the
impurities intact unless desludged.
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Clarifier
 The particle is pushed upwards with the oil stream
towards the centre while the centrifugal force directs it
to the periphery. The residual force on denser particles
(impurities) will drive them towards the periphery, while
the less dense particles (oil) will be directed towards
the centre of the bowl and raise to the outlet
connection.
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Clarifier
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2) Purifier:
When a centrifuge is arranged for separating two liquids
of different densities, for e.g. water from oil, it is known
as a purifier. The main component of purifier is correct
size gravity disc or dam ring which is responsible to
create interface between the oil and water.
The basic operations of clarifier and purifier are:
- It contains stack of disk numbering up to 150 and are
separated from each other by very small gap. A series
of holes are aligned in each disk near the outside edge
which permits the entry of dirty oil.
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- Due to difference in gravity and centrifugal force, the
heavier impure liquid (water) and particles moves outside
and lighter clean oil flows inwards and get separated.
- The collected sludge and impurity can be discharged
continuously or at a time intervals, depending upon the
construction, automation and system incorporated.
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Purifier
 When a centrifuge is set up as a purifier, a second
outlet pipe is used for discharging water as shown. In
the fuel oil purifier, the untreated fuel contains a
mixture of oil, solids and water, which the centrifuge
separates into three layers. While in operation, a
quantity of oil remains in the bowl to form a complete
seal around the underside of the top disc and, because
of the density difference, confines the oil within the
outside diameter of the top disc.
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Purifier
 As marine fuel oil normally contains a small
quantity of water, it is necessary to prime the
bowl each time that it is run, otherwise all the oil
will pass over the water outlet side to waste. The
water outlet is at greater radius than that of the
fuel. Within the water outlet there is a gravity
disc, which controls the radial position of the fuel
water interface
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Purifier
 A set of gravity discs is supplied with each machine
and the optimum size to be fitted depends on the
density of the untreated oil. When the fuel centrifuge
is operating, particulate matter will accumulate on the
walls of the bowl. If the centrifuge is set as a clarifier,
the particulate matter will be a combination of water
and solid material.
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Purifier
 If it is set as a purifier, the free water is continuously
discharged, therefore, the particulate matter will
consist of solid material. In older machines it is
necessary to stop the centrifuge to manually clean the
bowl and disc stack, however, the majority of
machines today can discharge the bowl contents while
the centrifuge is running.
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Purifier
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Purifier Operation
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The following factors are of importance when
understanding the function of the purifier
Increasing the sg of the oil will tend to push the
interface outlet and cause overflow from the heavy
phase outlet untill the equilibrium is restored. Should
the interface be moved so far as to breach the dam oil
will be issued from the heavy phase outlet and an
alarm will sound.The ideal position for the interface is
to lie over the distribution holes
Reducing the sg of the oil will tend to bring the
interface towards the axis, this reduces the force of
separation on the oil mix and reduces the efficiency of
the unit possibly leading to contaminants and water
carryover with the light phase outlet
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- The "gravity" disc are changeable on virtually all
purifers. Their centre bore is governed by the sg
of the oil being centrifuged. The largest bore
should be used without risking overflow.
- The flow rate of a purifer should be set to
optimise removal of whole system impurities. The
lower the oil feed the greater the time for
impurity removal and the more efficient the
purification. The higher the rate the greater the
amount of system oil is treated per unit of time.
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Choosing Gravity Disc
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The graph shown above is one typical of one found in a
purifier instruction book for selecting appropriate gravity
disc size. Shown on the diagram is an example of an oil
of sg 0.93 at 0'C. The sg at 15'C for use with this graph is
found by projecting along a horizontal line to 15'C. This
step would be omitted if the sg at 15'C was already
known. A line is then drawn parallel to the pre-drawn
sloping lines. Where the drawn sloping line cuts the
appropriate oil supply temperature isothermal then This
becomes the selection point for the disc. This is found
simply by ascertaining which size band the point lies in.
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PARTS OF A PURIFIER
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Basic components of the centrifuge are as follows:
 Exterior framework:
The exterior frame work is normally made up of caste iron
which supports the internal bowl and disk parts and
carries water line, feed line and outlet line connections.
 Bowl and disk:
There are bowls inside the frame, which can be a solid
assembly operating non continuous and have space
enough to retain the separated sludge. There can also be
an arrangement in which the upper and lower parts are
separate for discharging the accumulated sludge by a
continuous operation. These parts are normally made up
of high tension stainless steel.
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Vertical shaft:
The Vertical shaft is used to transform the electrical motor output into
rotational motion for rotating the bowl in high speed through spur
gear and horizontal shaft or belt. material used for vertical shaft
construction is an alloy of steel.
Horizontal shaft or belt drive:
The electrical motor drives the horizontal shaft through clutch pads
and is used for transmitting the rotational motion to bowl assembly. A
special belt having elastic character is used in some models in place
of horizontal shaft, thus removing the use of the gear assembly. The
horizontal shaft material is a special alloy of steel.
Attached Gear pump:
A general construction of centrifuge consists of a horizontal shaft
driven attached supply or discharge gear pump. In some system an
external supply pump may be installed in place of the attached
pump.4/8/2014 28

Spur gear:
A spur gear is placed between the horizontal and vertical
shafts for the transfer of rotational motion. These gears are
manufactured by special aluminum bronze material.
Clutch or friction pads:
An electric motor will get overloaded if it is connected directly
to the bowl assembly for the rotation of the same as the
complete assembly is very heavier. To avoid this, clutch or
friction pads and drum assembly are installed on the
horizontal shaft. Normally the number of pads varies from 2
to 4 depending upon the frequency supply to the motor.
As the motor starts, the pads inside the drum moves out
gradually due to centrifugal force and cause friction in the
internal wall of the drum resulting in rotation of the shaft and
the bowl gradually without overloading and damaging the
motor and gears.4/8/2014 29

Any Question?
Thank you!
4/8/2014
International Maritime Academy, Bangladesh 30

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