Last Updated on June 1, 2020 by Amit Abhishek
A high-speed centrifugal purifier is used on a ship to separate unwanted contaminants from lube and fuel oil. On the ship, we use separate marine purifiers for lube and fuel oil. While for lube oil only use centrifuge but for fuel oil, we use both centrifuge and gravity separation.
The gravity separation uses a settling tank for the process. The centrifugal purifier uses the principle of density difference to separate water and other contaminants from oil mixture.
While the separating force of these mixtures under gravity separation is defined by formula Fs = ∏/6*D3*(ρw-ρo)*g. Now by replacing gravity ( g ) with centrifugal force ω2 r we able to simply increases the rate of separation by a significant factor; making it a better way of purification.
- What Are Purifiers And Why To Use Them On Ship
- Difference Between Centrifugal Purifier And Clarifier
- Construction Features of Centrifugal Purifier
- Centrifugal Purifier Working Principle
- Maintenance procedure
What Are Purifiers And Why To Use Them On Ship
A purifier is an equipment with the sole purpose of purifying or cleaning the given media. On the ship, they are also referred to as Centrifuge ( equipment using centrifugal action to separate two medium ).
Marine fuel oil taken from bunker contains solid contaminants such as ash, carbon, cat fines and dust that if remained unchecked will accelerate normal wear and tear damaging cylinder liner, piston, piston rings and fuel injector.
Further we have water particles in the bunker fuel. Which if remained unchecked will assist in chemical reaction between vanadium and sodium forming sticky liquid salts during combustion process.
It may also leads to situation such as cavitation of injectors, microbial formation, ignition delays and subsequent reduction in quality of oil.
After receiving the fuel oil in bunker tank we transfer them to first storage tanks and then to settling tanks where the heavier contaminants and water gets separated under the effect of heat.
The fuel is then further purified using a centrifugal purifier to remove sludge, water, and solid contaminants completely.
Difference Between Centrifugal Purifier And Clarifier
It is a type of centrifuge that can separate two liquid substances based on their difference in density.
It is a type of centrifuge that can only separate solid contaminant from a liquid medium.
Gravity disc is used and thus separates both solid and water.
No gravity disc in the system. Thus it only separates solid contaminants.
Sealing water is required for operation.
No such requirement of sealing water.
Lower discs have holes and have two outputs at the top; one for water and other oil.
The lower disc has no holes and there is just one outlet at the top for oil.
It Can or cannot be used in a series of centrifugal purifiers.
Can only be used in series with a centrifugal purifier on ship due to presence of water.
Construction Features of Centrifugal Purifier
A centrifugal purifier consists of various parts such as the bowl, disc stack, gravity disc, electric motor, inlet-outlet ports, rotating shaft, friction pads, shafts, gearbox, and automation devices.
A purifier consists of an electric motor which drives the various component, such as Bowl, Nodge(rotating counter), Tachometer, Gear-pump(via pinion gear)
The complete purifier assembly is too heavy for a motor to take the full load from start without overheating. Thus to safeguard and protect the electric motor from damage friction clutch are installed on the horizontal shaft.
These pads gradually move out under centrifugal force connecting the motor directly to the bowl once the system attends a preset required momentum. To start the revolution of the Bowl, it is necessary that the Friction clutch engage with the Drum.
It acts as a safeguard against heavy starting current. If there would have been a solid coupling, then due to a large starting current the winding of the motor will simply overheat and burn.
So friction coupling allows for some time, and after certain R.P.M is being achieved by the motor the coupling gets engaged.
The Gear Box
The gearbox assembly in a centrifugal purifier consists of a motor shaft, friction clutch, the breaking lever, spur gear, worm gear, and attached gear pump.
The Bowl & Disc Stack
The bowl is a solid assembly that operates within the purifier frame made of high tensile steel with ample space to accumulate and discharge sludge.
Purifier and clarifier bowls have conical disc stack, which may be numbered from 150 to 200 and are separated from each other by a small gap.
Small holes are provided on these disc stacks where the interference line is formed and separation takes place. The centrifugal force cause oil to move inward and water and solid particle to move outward.
Rotating Shaft & Automation
The bowl is driven by an electric motor using bevel gear to produce centrifugal action. The vertical shaft rotated the bowl at high speed and is connected with horizontal shaft using a spur gear.
The horizontal shaft is driven directly by the electric motor through a clutch plate or friction plate. Apart from other mechanical devices a centrifuge also have an automation circuit consist of transducers, monitor, detectors, solenoid valve and control panel.
Gravity Disc & Pairing Discs
Also known as the damn ring it is fitted on top of the disc stack to create an oil-water interface based on density difference. It is another disc fitted at the neck of the bowl converting rotational energy into pressure energy.
Centrifugal Purifier Working Principle
The dirty oil is supplied to the center from the settling tank. Then due to centrifugal force, the separation between oil and water took place due to their different density.
Water being heavier than oil is thrown out toward the wall of the bowl whereas the oil is positioned in the center. Water is applied first, because if the oil is supplied directly to the purifier, it will come out from the waterside, so water is supplied first and it is checked that it is coming from the waterside, gradually the seal water is shut off.
If we supply seal water quickly, it may come to the oil side. An interference line is formed between Water and oil, this line is also known as E-line.
The position of this interface within the purifier is very important for correct operation, this interface is achieved by gravity disk at the outlet.
A number of gravity disc is provided with the purifier. This Gravity disk (a.k.a Regulating Ring) that is used has a different diameter for the different density of oil to be purified, so if the oil is being changed the gravity disk has to be changed.
As a thumb rule, a large-diameter gravity disk that does not break the seal should be used. A blind disc is at the top disc stack.
- Particular care should be taken when moving the main bowl parts so to avoid damage to the sealing faces and prior opening it must be ensured that the purifier is a complete stop and not rotating.
- All seals to be renewed.
- Clutch drive to be checked and lining renewed if required.
- Gearbox to be checked for abnormal wear. Bearing to be within makers running period limit. Gearcase lubricating oil to be renewed.
- Prior to reassembly all sliding faces to be suitably lubricated. Bowl height to be measured and adjusted according to makers limits. Record to be taken.
- Before closing top cover, turn the bowl by hand. To test if it’s free of obstruction.
- After running up to normal speed the operation of the purifier shall be checked especially the separating efficiency of the purifier with regard to the selection of the correct gravity disc or plug.
- Correct torque should be employed as mentioned by the maker.
- Necessary entries shall be made in the record.
A centrifuge is a perfectly dynamically balanced piece of equipment with very high R.P.M, so the care should be taken while dismantling and boxing it up and also during maintenance.
***Always refer to maker manual before carrying out any maintenance***
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