LESSON 6

• PURIFICATION(净化 ) OF FUEL OILS

•Fuels vary considerably in viscosity, quality and chemical composition. They also contain impurities from producing, transferring and storage.

•All heavy fuels contain insoluble(不能溶解的 ) impurities such as mineral salts, asphalt(沥青 ), foreign matter and some liquids not miscible(易混合的 ).

•These impurities can to a great degree(extend) be removed by centrifugal(离心的 ) and filtration(过滤 ) treatments.

•Failure of any cleaning equipment could cause very serious troubles to the mechanical parts of the fuel injection equipment,

•therefore standby duplicate cleaning equipment must be provided and so arranged that continuous operation can be maintained should the working unit fail.

•The most common method of cleaning fuel is by centrifuging and there are various makes of purifiers(分水机 ) and clarifiers(分杂机 ) in use

•such as Alfalaval, Titan, Sharpies, etc., the basic principles of operation and design are similar.

gravity disc

Dirty oil

Clean oilwater

bowl

Cylindrical interface

Separation disc

•The separation discs(分离盘 ) in purifiers and clarifiers are conical in shape and are made of stainless steel,

•the outer periphery(外围 ) is perforated(打孔 ) by a series of holes through which the fuel passes, the distance between each disc is approximately 0.5 mm and the thickness is about 1 mm.

•When the fuel is pumped into an operating centrifuge the centrifugal force generated causes the heavier fractions(部分 ) such as sludge and water to be forced along the underside of the discs to the outer periphery.

•A cylindrical interface(圆柱分界面 ) is formed between the heavier fractions (water) and the lighter fractions (oil). The lighter fraction is forced inward and up to the clean oil outlet.

•The impurities and sludge can be collected in the bowl(分离筒 ), sludge space, or in some designs discharged at regular intervals to a sludge tank.

•The best results are obtained when the interface zone is close to the periphery of the bowl and it must be outside the disc stack(堆叠 ) areas.

•The specific gravity of the fuel determines the size of the gravity rings(比重环 ) to be fitted and it is important to refer to the maker’s instruction manual for verification.

•A higher throughput(处理能力 ) and more efficient process can be achieved if the fuel is heated to reduce its viscosity.

•Heaters are, therefore, installed in the system. Recommended temperatures for high viscosity fuels are between 80 to 95℃ depending on the viscosity of the fuel.

•If the purifying of relatively clean fuel is needed, a centrifuge fitted with a clarifier bowl should be used.

•There is only one outlet for clean oil, no gravity discs(rings) are fitted for the separation of water.

•Consequently, the maximum cleaning efficiency is achieved, as the oil feed will at all time be outside the disc stack zone.

•Experience has shown excellent results from a two-stage centrifuge treatment—purifying followed by clarifying.

•The fuel is pumped into the purifier fitted with gravity rings and with separate discharges for oil, water and sludge.

•The relatively clean oil is then pumped into a clarifier which extracts(分离出 ) any small amount of water or impurities still left in the oil.

•Unacceptably large quantities of impurities are left in the oil after the purifying process if the oil is insufficiently heated, or the throughput is too high; or the wrong type of gravity disc is fitted.

•Improvements in oil, oil purifying techniques and centrifuges have led to the development of single phase purifiers which have in themselves been aided by improved temperature control, heaters, flood alarms and other ancillary equipment.

•A further development has been the self-cleaning purifier which operates unattended(无人照管[监视 ]的 , ) for long periods and discharges its own sludge.

•The self cleaning unit can be stopped and started at any time without dismantling(拆卸 ) for manual cleaning which is a dirty and time consuming job.

•If the unit is not self-cleaning, then a strict maintenance routine must be followed.

•Poorly maintained units are a continuous source of trouble and allowing a machine to run beyond its recommended time can result in a choked bowl, especially if very dirty fuel is being purified.

•The efficiency of centrifugal unit decreases rapidly when the accumulation of impurities reaches a certain point.

•For self-cleaning purifiers used in unmanned machinery spaces controls are arranged that if a centrifuge fails the standby unit will automatically comes into operation.

•The purifiers are fitted with electric timers for controlling the period between bowl emptying operations and sequential regulating devices control the various' emptying operations;

•The units must incorporate an audio-visual alarm system which come into operation when any running condition is out of order.

•When an alarm condition prevails with the unit, it should be arranged to shut down or the fuel oil recycling valve should open and the oil flow should be passed to the outer circle thus preventing the loss of oil.

•In order to control the interface zone of the bowl a constant pressure control valve is fitted at the clean oil outlet.

•By applying a high back pressure the interface moves outward and with a low back pressure the opposite results.

•It is important to note that when an excessive amount of water is present in the fuel the temperature for centrifuging must be kept below the boiling point of water, otherwise foaming(起泡 ) and, agitation(搅拌 ) of the fuel takes place.

•Under these circumstances it will be necessary to purify at a very low rate to achieve a reasonable efficiency.

READING MATERIAL

• A. OIL TREATMENT

•Both fuel oils and lubricating oils require treatment before passing to the engine.

•This will involve storage and heating to allow separation of water present, coarse and fine filtering to remove solid particles and also centrifuging.

•The centrifugal separator is used to separate two liquids, for example oil and water, or a liquid and solids as in contaminated oil.

•Separation is speeded up by the use of centrifuge and can be arranged as a continuous process. Where a centrifuge is arranged to separate two liquids, it is known as a 'purifier'.

•Where a centrifuge is arranged to separate impurities and small amounts of water from oil it is known as a ‘clarifier’.

•The separation of impurities and water from fuel oil is essential for good combustion. The removal of contaminating impurities from lubricating oil will reduce engine wear and possible breakdowns.

Centrifuging

•A centrifuge consists of an electric motor drive to a vertical shaft on the top of which is mounted the bowl assembly.

•An outer framework(机壳 ) surrounds the assembly and carries the various feed and discharge connections.

•The bowl can be a solid assembly which retains the separated sludge and operates non-continuously, or the bowl can be arranged so that the upper and lower parts separate and the sludge can be discharged while the centrifuge operates continuously.

•The dirty oil is admitted into the center of the bowl, passes up through a stack of discs and out through the top.

The Purifying Process

•The centrifugal separation of two liquids such as oil and water, results in the formation of a cylindrical interface between the two.

•The position of this interface within the centrifuge is very important for correct operation.

•The setting or positioning of the interface is achieved by the use of dam ring(阻水环 ) or gravity disc at the outlet of the centrifuge.

•Various diameter rings are available for each machine when different densities of oil are used.

The Clarifying Process

•Cleaning oil that contains little or no water is achieved in a clarifier bowl where the impurities and water collect at the bowl periphery.

•A clarifier bowl has only one outlet. No gravity disc is necessary since no interface is formed.

•The bowl therefore operates at maximum separating efficiency since the oil is subjected to the maximum centrifugal force.

The Bowl Discs

•Purifier and clarifier bowls each contain a stack of conical discs. The discs may number up to 150 and are separated from one another by a small gap.

•Separation of impurities and water from the oil takes place between these discs. A series of aligned(排列的 ) holes near the outside edge permits entry of the dirty oil.

•The action of centrifugal force causes the lighter components (the clean oil) to flow, inwards and the water and impurities flow outwards.

•The water and impurities form a sludge which moves outwards along the undersides of the discs to the periphery of the bowl.

Selection of Gravity Disc

•The interface between the liquid seal (water) and the oil should be positioned as close as possible to the bowl periphery.

•However, the interface must not be located so far from the bowl center that the oil will pass the outer edge of the top disc, breaking the liquid seal and discharging with the water.

•Factors influencing the interface position are:•Oil viscosity and density. A high oil density will position the interface closer to the bowl periphery than will a low density.

•Throughput and back pressure. As a rule the interface will be located closer to the bowl periphery at a high throughput than at a low one.

•The same effect is produced by a high back pressure, and a low one respectively, in the clean oil outlet.

•Gravity disc. The location of the interface is adjusted by altering the outlet for the water, i.e. exchanging the gravity disc.

•Changing to a gravity disc with larger hole diameter will move the interface towards the bowl periphery,

•whereas a disc with smaller hole diameter will position the interface closer to the bowl center.

•The nomogram(诺莫图 ) is an aid to select a tentative(暂定的 ) gravity disc when the density of the oil at a given temperature is known.

•The hole diameter of the disc to be tried first appears directly from the nomogram.

•However, in practical operation the best result is obtained by using the gravity disc with largest hole diameter that will not cause a break in the liquid seal in the bowl or an emulsification(乳化 )in the water outlet.

Discharge of Sludge

•Modern centrifuge designs enable continuous operating, over a considerable period of time.

•This is achieved by an ejection process which is timed to discharge the sludge at regular intervals.

•The sludge deposits build up on the bowl periphery as separation-continues, and the ejection process is timed to clear these deposits before they begin to affect the separation process.

•To start the ejection process the oil feed to the centrifuge is first shut off and the oil remaining in the bowl removed by admitting flushing water(置换水 ).

•Water is then fed into the hydraulic system in the bottom of the bowl to open a number of spring-load valves.

•This 'operating' water causes the sliding bowl bottom to move downwards and open discharge ports in the bowl periphery.

•The sludge is discharged through these ports by centrifugal force. Closing ‘operating’ water is now feed in to raise sliding bowl up again and close the discharge ports.

•Water is fed into the bowl to remake the liquid seal, the oil feed reopened, and separation continues.

Maintenance

•The bowl and the disc stack will require periodical cleaning whether or not an ejection process is in operation.

•Care should be taken in stripping(拆卸 ) down the bowl, using only the special tools provided and noting that some left-hand threads are used.

•The centrifuge is a perfectly balanced piece of equipment, rotating at high speeds; all parts should therefore be handled and treated with care.

• B. OPERATION OF AN OIL SEPARATOR

Before Starting

•The bowl should be well cleaned and assembled according to the instruction. Check particularly: The brake is released.

•The collecting covers (frame hood respectively) are clamped(夹紧 ) with the hinged(装铰链 ) bolts. The oil level in worm gear (蜗轮 ) housing is somewhat above the middle of the gauge glass.

•The operating liquid tank is full and the control valves are closed (operating liquid feed turned off; bowl open).

•Note: The oil level must never be allowed to sink below the lower edge of the gauge glass. If the glass is provided with corrugation(沟纹 ) they should be vertical.

•Keep the gauge glass clean, otherwise a line, which could be mistaken for the oil level, will in time build upon the inside of the glass.

•If the machine has been idle (for instance during a night), screw out the drain screw some turns and drain off any water.

Starting the Motor

•If the process liquid is to be preheated, circulate it through the preheating until suitable temperature is reached.

•Shortly after starting it may occur that the bowl begins to vibrate more than normal.

•The cause is generally lack of balance due to bad cleaning of the bowl. Stop the machine and clean the bowl if the vibrations are very heavy.

•Heat is always generated in the clutch(离合器 ) coupling during the running-up period.

•This will be noticeable, especially when the pads(垫 , 衬垫 ) on the friction block are new, through smoke and smell of burning. Like the sliding sound, this is quite normal and has no importance.

•During acceleration the power consumption is higher than in normal operation.

•The acceleration time may vary somewhat depending, for instance, on the condition of the friction pads in the clutch coupling(离合器 ).

Bowl Closing

•After attaining the right speed, the bowl should be closed.

•Before starting the closing operation, the number of revolutions of the speed indicator must be checked against the speed table in the instruction book.

•The acceleration time can vary somewhat depending, i.e. on the wear of the friction pads.

•Close the bowl by opening the corresponding control valve. Wait till the bowl has closed. Then fill it.

Filling

•The filling procedure differs for purification, clarification and concentration(浓缩分离 ), i.e. for machines provided with purifier, clarifier and concentrator bowl respectively.

Clarification (clarifier bowl)

•Set the flow regulator to wanted throughput and open the process liquid feed valve.

Purification (purifier bowl)

•Supply liquid, usually water, to form the liquid seal. This liquid should preferably have the same temperature as the process liquid and must be supplied quickly.

•Shut off the feed of sealing liquid when this begins flowing out and becomes visible in the sight glass.

Set the Flow Regulator to Desired Throughput

•Slowly open the process liquid feed valve. A certain quantity of sealing liquid will now escape, until equilibrium is reached.

•If the valve is opened too quickly, the liquid seal could be forced away to the effect that light liquid phase is discharged in the wrong way,

•i.e. through the outlet for the heavy liquid phase. When this occurs with the filling procedure has to be repeated.

•Adjust to suitable back pressure in the conduit for light phase (For machine with equipment for interface disposition see Selection of gravity disc)

Concentration (concentrator bowl)

•The liquid seal builds up automatically. Adjust to wanted throughput-see Purification above.

Running

•Check particularly: that throughput and working temperature are constant; that oil does not leak from the worm gear housing (oil level at operative height);

•that, in purification, light phase is not escaping together with the heavy one, thereby indicating that the bowl is clogged or the liquid seal is broken.

•If so, a sludge discharge must be carried out immediately and henceforth the interval between discharges be reduced.

•If sludge had packed(塞满 ) between the bowl discs it may be necessary to stop the machine for manual cleaning of the bowl.