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| Fuel Oil Purifier |
Why there are impurities in fuel oil?
Crude oil is refined in the refinery and produces various fine products. Marine residual fuel is the last product in terms of purity in the refining process.
Besides to make the process more economic silica-alumina is used to break the heavier molecules lighter. This silica-alumina often is not possible to remove completely.
The amount of impurities depends upon the source of crude oil and quality of refining process.
Principle of operation:
Oil, water and other impurities are of different density. Whenever the oil is subjected to centrifugal force heavier particles (water and solid) will be thrown outside and lighter particles will remain close to the center.
Difference between Purifiers and Clarifiers:
Functional difference: Purifiers can separate two liquids of different densities due to centrifugal force. i.e. water and solid particles from fuel. Purifiers can also remove small amount of solids.
Clarifiers remove solid particles from the fuel oil. Clarifiers will also remove small quantities of water.
Purifiers may be arranged to used as clarifier.
Constructional difference: Purifiers have gravity disc whereas clarifier has no gravity disc.
Clarifier has no sealing water as handles only one liquid i.e: fuel oil. So there is no interface inside clarifier.
Purifiers have two liquid out: one for liquid oil and another for water.
Clarifiers have only one liquid outlet.
Main components of purifier:
Bowl body: Bowl body is connected with the vertical shaft by a lock nut. It houses the main bowl. Main bowl slides within the bowl body to facilitate de-sludge operation and to make seal with bowl hood.
Main bowl: Unpurified oil accumulates in the main bowl. It moves up and down by water pressure to facilitate sludge discharge mechanism.
Bowl hood: The cylindrical shaped hood makes seal with main bowl so that oil cannot leak out. It houses the main seal ring for the sealing purpose.
At the top it houses dam ring/gravity disc, impeller.
Discs:
These discs provide a way for the cleaned purified oils to go upward in a separate path through the holes cut into them.
Gravity disc:
It maintains the interface between oil and water. It is done by the inner diameter of the gravity disc.
If the diameter is increased more water will get its way to escape the bowl. The amount of water will be reduced, so the force exerted by the water will also reduce. The force exerted by oil will push the oil-water interface towards periphery. If the diameter increases more, the interface will move towards a position where oil will escape through water line. i.e: overflow.
Centripetal impeller: Oil collected from purifiers will be drawn inside and delivered by this impeller.
Inlet pipe: As the name implies fuel oil gets inside the bowl by the inlet pipe.
Distributor: Feed oil is introduced through feed inlet pipe and the distributor to the main bowl chamber.
Vertical shaft: The revolution of the horizontal shaft transmitted and increased by vertical shaft gear teeth and the bowl rotates at the top. Bowls are supported by upper bearing housing arrangement.
Purifier rpm and motor rpm:
Motor runs at a rpm of 2000-2500 rpm and purifier bowl rotates around 15000-18000 rpm. This is done by increased gear mechanism of horizontal shaft.
Horizontal shaft: It is connected with motor via frictional block arrangement. Bearings in the bearing housings are provided at two ends of the shaft. Between them spiral gear is fitted.
Water supply device: Underneath the bowl body cylinder water supply device is fitted to supply low and high pressure operating water.
Low pressure operating water used to close the main cylinder.
High pressure operating water used to perform sludge discharge.
Frictional block, frictional pulley and frictional boss : Motor is not directly coupled with the purifier’s horizontal shaft. Friction blocks are used for soft starting and acceleration of the motor and hence avoid the motor being overloaded. There are arrangements to house frictional blocks which will rotate as the same rpm of motor.
During start up the motor will run in slow rpm and so do the friction block. These blocks due to centrifugal force will keep hitting the circular frictional block of the horizontal shaft. So the horizontal shaft will start to move.
Connecting the motor with the horizontal shaft directly would cause huge load on the motor at starting and motor to trip.
To stop the purifiers in emergency this break is used to stop the rotation immediately. The brake linings are pressured against the outer surface of the pulley to stop purifier rotation.
External equipment in the purification system:
Gear pump:
To supply fuel oil to the purifier external feed is fitted. Either the pump is driven by an electrical motor or the pump is attached by a safety joint with the horizontal shaft.
Heaters: Purifiers are fitted with heaters to facilitate fuel oil heating.
Factors influence the interface:
Through put/ Feed rate
Temperature of the fuel
Diameter of the gravity disc
Density of the fuel
Through put:
If the feed rate is excessively increased
- The increased feed rate will increase the mass of fuel.
- The increased mass of fuel will exert more force to push the water out of bowl.
- This will cause the water-oil interface to move towards periphery and cause overflow.
It is of vital importance to run the purifiers with optimum feed rate. High feed rate reduces purification efficiency. Feed rate depends upon fuel consumption of the ship.
Running with low feed rate will cause the fuel oil service tank level to decrease. If feed rate not adjusted for long time
Normally, feed rate should be 110 percent of the fuel consumption.
Temperature of the fuel:
Temperature plays an important role to maintain oil-water interface.
- If temperature increases the density of oil reduces.
- The force exerting by oil reduces.
- Oil-water interface tend to move towards center.
Diameter of Gravity disc:
It is also known as dam ring.
Gravity disc inner diameter determines the amount of sealing water inside the bowl.
Thus it also determines the position of the oil-water interface.
If the diameter increases interface moves toward periphery of the bowl.
If the diameter reduces interface moves toward center.
Density of oil: An increase in density will also results in increase of mass and centrifugal force to push more water out of the bowl. i.e. interface will move towards periphery.
To handle with high density oil gravity disc diameter has to be decreased and temperature has to be maintained.
Operational sequence:
- Gain rpm of the motor and purifier.
- Bowl close: Low pressure operating water will push the main bowl up and make seal with top hood.
- Sealing water: From the top via the inlet line sealing water will be supplied to fill up the full chamber. At this stage full chamber is filled with water.
- Sludge discharge: High pressure operating water will push the pilot valve or spring loaded valve plate to drain the water that used to make the seal with top hood. The seal will break and sludge will be thrown out of the main bowl.
- Again main bowl closes by low pressure water.
- Sealing water supplied to fill the chamber.
- Fuel oil is supplied inlet via inlet pipe through distributor.
- By centrifugal force impurities and water will be thrown outside and purified light oil will get its way through the drilled hole. The light oil will be guided by top disc to the centripetal pump to the service tank.
Sludge discharge/ De-sludge:
An important function of purifier is sludge discharge operation. It removes the accumulated sludge inside the bowl at either set time interval or manually.
Different makers have different methods of discharging. But basic principle is almost same.
Low pressure operating water/ bowl closing water will enter through solenoid valve for predetermined time and then stop.
This water will gain energy due to centrifugal movement of the bowl. The centrifugal force will push the main bowl up and make seal with the top hood.
Sealing liquid will enter inside the bowl through inlet pipe from the top for set time.
The sealing liquid will occupy the full bowl chamber.
High pressure water/ bowl opening water acts on the lower side of the pilot valve. It acts relatively on large surface area than the low pressure water that acts on the upper side of the pilot valve and causes the operating water to drain off through the drain orifice.
This drainage of water will cause the main bowl to fall down. Hence the seal will break. The sludge inside the bowl comes out of bowl with force. High pressure water will be injects for set period of time.
Instead of pilot valves in some designs, spring plates are used.
Safety features of purifiers:
- Low back pressure alarm
- High temperature alarm
- Safety joint of feed pump
Properly tightening:
Some parts of the purifiers are secured by screws and nuts. It is of vital importance to tighten all lock nuts and crews are perfectly. The tally marks on the body of the parts must match perfectly.
- Bowl nut
- Disc nut
- Cap nut of vertical shaft
- Set bolts of frame
- Lock nut of inlet pipe.
Regular checks:
- Fuel temperature and feed rate
- Any abnormal vibration
- Purifier motor load
- Crankcase oil level and color.
- Any leakage.
Dealing with different grade of fuel:
If there is any change of fuel grade the gravity disc has to be changed according to the specific gravity of the fuel. The gravity disc inner diameter can be obtained from the monogram provided in the manual.
Selection of gravity disc inner diameter for fuel oil.
Suppose the specific gravity of fuel at 15 ⁰C is 0.93, operating temperature 80 ⁰C and feed rate is 1200 l/h. Diameter for the operating data will be 72mm.
Here, the blue line represents the fuel oil specific gravity which intersects operating temperature (80 ⁰C, marked yellow) at point C. Draw a parallel line to the X axis (Separating temperature axis) and extends up to 100 ⁰C. Connect the right end of the line with the 1200 L/h point. It will intersect the gravity disc inner diameter line at 72mm.
Next Read: Scrubber System And Maintenance.