Centrifugal PURIFIERS

Basic principles : the purpose of the separation is to To free a liquid of solid particles. To separate two mutually insoluble liquids with difference in density ,

removing any solid at the same time.Separation by gravity : continuous separation & sedimentation can be achieved in the settling tank having the

outlet arranged at levels suitable to their density ratio of two liquid phases. Separation of heavy & solid particles are also achieved over here. Liquids with specific gravity difference can be separated by gravity , the equation like this

Separating force F = (π/6) × D³ ( ρw – ρo) gas

centrifugal separation : In a rapidly rotating vessel the gravity is replaced by the centrifugal force, which can be thousand times greater. Separation and sedimentation are more continuous and faster. As the separation is achieved by the gravity has a smaller separating force differential than the centrifugal force .

the separating force , F= (π/6)× D³× (ρw –ρo)× ω²R where R is effective radius and ω is the angular velocity.Development of a separator The heavier phase (water) had only a short distance to travel before coming to the bowl wall where solids were

deposited and the heavy phase liquid (water) was guided to the water discharge. However, the sludge retention volume and the liquid dwell time for a given throughput could only be increased by lengthening the bowl. This gave rise to bowl balancing and handling problems.

The wide bowl type was able retain more sludge before its performance wasimpaired and was much easier to dean. On the other hand settlingcharacteristics in a wide bowl machine are relatively poor towards the bowlcentre and the distance the water has to travel before reaching the wall is great.To overcome these problems a stack of conical discs (Figure 2.15) spacedabout 2-4 mm apart is arranged in the bowl. The liquid is fed into the bottom ofthe stack and flows through the spaces between adjacent plates. The plates thenact as an extended settling surface, with the heavy impurities impinging on theunder surfaces of the discs. As the particles impinge of the disc surfaces. theyaccumulate and eventually slide along the discs towards the periphery. At thedisc stack periphery, water globules and solid particles continue to move outtowards the bowl wall with the water being sandwiched between the solidsand the oiL which orientates itself towards the bowl centre. The boundaries at

which substances meet are known as interfaces.The oil/water interface is very distinct and is known as the e-Iine. To gaintne fullest advantage from the disc stack the e-line should be located outside ofit. On the other hand if the e-Iine is located outside the water outlet baffle (topdisc) discharge of oil in the water phase will take place.Referring back to gravity separation in a settling tank, if the tank ispartitioned as shown in Figure 2.16 continuous separation will take place.Since the arrangement is a very crude V-tube containing ·two liquids ofdifferent specific gravities, the height of the liquid in the two legs will have therelationshipρoil ( e-l) = ρwtr {e-h}in case of centrifuges ω² ρo(e² -- l²) = ω² ρwtr {e²-h²}The mechanical design of the centrifuge requires that the e-Iine is confinedwithin certain strict limib. However variation~ in gravity will be found dependingupon the port at which the vessel takes on bunkers. It is necessary therefore toprovide means of varying h or l to compensate for the variation in specificgravity. It is usually the dimension h which is varied. and this is done by the use of dam rings (sometimes called

gravity discs) of different diameters. Normally a table is provided in the instruction book for the machine, giving the disc

diameter required for purifying oils of various specific gravities. Alternativelythe disc diameter Dh may be calculated from the following formula which isderived from (5)

Dh =2√ [ l² (ρl / ρh) + e²{1-(ρl /ρh)} ]

The dimension e can be taken as the mean radius of one thin conical plate andthe heavy top conical plate (outlet baffle). If oil is discharging in the wateroutlet the gravity.disc is too large.

Difference between a clarifier & purifier: 1. A clarifier is intended to separate only the solids where as the purifier is intended to separate both solids and

water from the oil.2. In a purifier a dam ring/ gravity disc is provided at the top of the purifier which has an aperature to discharge

water and oil separately. In case of a clarifier the dam ring is replaced by a blank disc.3. No sealing water is required for a clarifier.Basics: Here the fluids with difference in characteristics can be handled by one machinery. Liquid is fed at the bottom of the stack of discs. Centrifugal force tends to separate the two liquids with

difference in specific gravity. This force is very high due to high rotational speed.A purifier is fitted with a dam ring which controls the position of the separationline or interface between the water and oil when the bowl is rotating.:;If the inside of the dam ring is too large in diameter the separation line orinterface moves outward towards the outer periphery of the bowl and some oilwill be discharged with the water from the water outlet. If the inside diameter ofthe dam ring is too small the interface moves inwards and some water will bedischarged with the oil. The diameter of the dam ring is governed by the densityof the oil being treated.If the diameter of the hole in the dam ring is increased, the interface betweenthe oil and the water contained in the bowlmoves outwards. If the diameter increased excessively, oil globules will be discharged with the sludge and water.If the hole diameter is reduced unduly, the interface moves inwards andparticles of water will be discharged with the clean oil.Holes are piaced in the conical plates making up the plate stack to allow theoil to feed upwards into the clearance spaces between the conical discs. Waterand heavy stable emulsions are discharged through the dam ring and spin offthe bowl or are removed from the rotating bowl by a paring disc. Clean oil isdischarged from the bowl and spins off or may be removed by another paringdisc. Heavy solid matter is held in the bowl. (Fig. 3.3(a) )Clarifiers do not have a dam ring, a plain ring (also called a sealing ring) JS

fitted in its place, and the water outlet is sealed off. Solid matter and water thathas passed through the separator are retained within the clarifier bowl until it isopened and the water and sludge are discharged. On modern machinessurveillance and control devices watch out for the build-up of water within thebowl or when a very small amount of water is discharged with the clean oil. Thebowl is then caused to open. Modern centrifuges are operated as clarifiers andcontrolled in this manner.The conical disc stack mav not be fitted with feed holes, but if they are fitted ablank conical disc without feed holes will be fitted at the bottom of the conicaldisc stack.When a centrifugal separator is started up it must be filled with water to

establish a seal which prevents oil leaving the separator at the water outlet.Clarifier bowls do not have to be filled with water after start up.The main purpose of the clarifier is to remove traces of foreign material notremoved when the oil passed through the separators, and to act as a second lineof defence against the accidental passage of contaminant material into the cleanside of the oil storage system. Clarifiers are not normally used to cleanlubricating oil unless the oil is almost free of any water content.

'botch treatment' and 'continuous treatment'

Originally the terms continuous treatment and batch treatment applied only tothe purification of main engine crankcase lubricating oil.In continuous treatment the separator was operated for the whole of the timethe main engine was in operation. Lubricating oil was supplied to the separatorby its own pump taking oil from the main engine drain tank, or was bled fromthe pressure supply to the main engine. The separator was never shut downexcept for short periods when it was being cleaned.Batch treatment referred to the system whereby the engine was shut down andthe whole of the sump lubricating oil charge was pumped up to the dirtylubricating oil tank in the upper part of the engine room. The lubricating oil washeated in the tank and left as long as possible to settle out solids, sludge and anywater. It was then slowly purified in one batch, hence the name.Today the terms have different meanings. Batch treatment centrifugalseparators and clarifiers must be shut down for cleaning after treating a batchof oil.C6ntinuous treatment machines are capable of being cleaned or sludgedwithout being shut down. These machines are also termed automatic self cleaningseparators and clarifiers or automatic separators and clarifiers.This is necessary to heat fuel and lubricating oil prior to treatment in a centrifugalseparator to reduce the viscosity of the oil so that it flows easily into and out of

the separator and does not cause high pumping loads.Heating the fuel or lubricating oil, which is a mixture of oil, water and solids,lowers the specific gravity of the constituent parts. The specific gravity of the oilIs reduced at a greater rate than the specific gravity of the water and solids; thus

the difference in the specific gravities of the constituent parts is greater when themixture is heated.

centrifugal separator used for the purification of fuel and lubricating oils

The main part of a centrifugal separator is the bowl, which is mounted on thetop of vertical spindle supported in two bearings. On the spindle between thebearings is a multiple-tooth helical worm. This worm is driven by a helical'-toothed gear wheel connected to a driving motor through a centrifugally

operated clutch.The bowl is cylindrical and closed at the bottom by a concave conical end.The spindle~fits within the concave space formed by the bottom. On the outsideof the upper end of the cylindrical bowl there is a coarse thread or a breechblocktype thread. The bowl cover nut screws on to this thread. A section acrosstbe diameter of the bowl resembles the letter 'W'.The fuel or lubricating oil is led into the separator bowl by a distributorsimilar to an inverted filling funnel. Radial ribs on the inside of the conical end give uniform clearance between it and the end of the bowl. On the outside of the distribution pipe are four or six longitudinal fins. The conical separator /plates fit over the fins and are driven by them. Radial fins on the conicalseparator plates hold them equidistant.. Over ,the conical separator plates a conical inner cover is fitted. This innercover keeps the separator plates in position. The radial clearance between theouter edge of the conical separator plates and the cylindrical bowl sides isapproximately 30mm in small separators and increases with increase in bowl size. The radial clearance between the inner cQver and the sides of the bowl willbe approximately 5 mm. A sleeve on the top of this inner cover and integral withit surrounds the distributor pipe and extends upwards to some point lower thanthe top of the distributor pipe. The space between the sleeve and the distributorpipe is the purified-oil outlet.The top cover has a conical profile similar to the inner cover. Near itsperiphery is a groove which houses an oil- and heat-resistant rubber O~ring.This makes the seal between the cover and the vertical or sloping side of the bowl. The top cover is held in place by a ring nut which screws on the thread on the side of the bowl. On the top of the outer cover a thread is machined to takeanother ring-nut which holds the specific gravity ring or dam ring in place. A setof these rings having different internal diameters is supplied with every purifier.They are identified by a number or a specific gravity figure.Some separators are fitted with specific gravity plugs-instead of dam ringsand function in a similar manner. The impurities separated from the oil aredischarged over the dam ring or through the specific gravity plug.As good balance is essential, the components of the bowl are designed so thatthey can be assembled in one position only, in relation to one another.The fixed parts of the separator are the frame and the upper and lowerspindle bearings housed in it. The upper bearing bush is held in a flexiblehousing. The lower bearing is arranged to support the weight of the bowl andthrust from the gearing. The frame also houses the bearings for the worm wheeland has a flange to support the flange-mounted driving motor.The gearing is lubricated by splash from an oil bath and the bearings byforced lubrication from an oil pump driven by the motor. The frame alsosupports a flange-mounted tachometer and a hand priming pump for bearinglubrication prior to start-up. ,On the upper part of the frame is fitted a hinged cover with shallow conicalhorizontal partitions inside. The diameter of the hole and height of these partitionsis arranged to suit the various parts of the bowl assembly. The top partitiontakes the liquid spun off from the distributor pipe if an excess amountcauses overflow from the bowl. The second partition takes the purified oilwhich spins off from the upper end of the inner cover. The lower space acceptsthe water, sludge and solid particles discharged over the dam ring or throughthe specific gravity plug. A centrally located nozzle in the cover supplies liquidto the purifier and extends downwards into the distributor when the cover isclosed.In use, the separator is first run up to its operating speed and then water is fedinto the bowl until it discharges from the dam ring. This water provides a sealand prevents discharge of oil from the dam ring. After providing the initialwater seal the mixture of. oil. Water and impurities is fed into the bowl throughthe distributor pipeAfter entry into the bowl the mixture is subjected to a large angular accelerationand then moves wi~h the bowl. The speed of rotation subjects the liquid to'. centrifugal force which breaks up the mixture into its constituents. The heavierparts of the mixture, which include water and solids, move to the outer part ofthe bowl and displace an equivalent volume of water from the dam ring. Themedium weight sludge and oil pass into the spaces between the conical separatorplates; the sludge breaks away from the oil and moves outwards, while thepurified oil continues to move towards the centre of the bowl and then upwardsto the purified-oil outlet.

A stable state develops with the heavier material at the outer part of the bowl,the lighter, purified oil surrounding the distributor pipe, and the mixture withinthe distributor pipe. The process of separation is continuous as liquid is fed intothe bowl. The efficiency of purification will depend on the amount of foreignmatter in the oil and the rate of flow through the separator.Handling the modern day fuels. High Density Fuels In view of the fact that some fuel oil standards incorporate fuel grades without a density limit, and also the

fact that the traditional limit of 991 kg/m3 at 15 oC is occasionally exceeded on actual deliveries, some improvements in the centrifuging treatment have been introduced to enable treatment of fuels with higher density. Since the density limit used so far is, as informed by centrifuge makers, given mainly to ensure interface control of the purifier, new improved clarifiers, with automatic de-sludging, have been introduced, which means that the purifier can be dispensed with. With such equipment, adequate separation of water and fuel can be carried out in the centrifuge, for fuels up to a density of 1010 kg/m3 at 15 oC. Therefore, this has been selected as the density limit for new high density fuel grades. Thus we have no objections to the use of such high density fuels for our engines provided that these types of centrifuges are installed. They should be operated in parallel or in series according to the centrifuge maker’s instructions.

Supplementary Equipment

In a traditional system, the presence of large amounts of water and sludge will hamper the functioning of a clarifier, for which reason a purifier has been used as the first step in the cleaning process. With the new automatic de-sludging clarifiers, the purifier can, as mentioned, be dispensed with. We consider the removal of solids to be the main purpose of fuel treatment. Although not necessarily harmful in its own right, the presence of an uncontrolled amount of water and sludge in the fuel makes it difficult to remove the solid particles by centrifuging. Therefore, additional equipment has been developed:The equipment required for handling and cleaning fuels having a high viscosity,a high density, and a high carbon content is similar to that fitted in oldermotorships (See Question 3.29) except in the areas mentioned below.The fuel system must be fully automated, monitored, and carefully designedto cover all the control functions, cleaning functions and fail safe in the event offailure or shut down of any part.This reduces the risk of any disastrous consequences that may arise if the fuelcleaning system goes out of adjustment and allows improperly prepared fuel tofind its way into the main and auxiliary engines. Such problems can easily occurin modem practice with reduced engine room staff and more particularly iftheir time is fully taken up in dealing with some other crisis.Insulation to heated fuel storage spaces and piping must be increased. Steamtracer lines must be fitted or their capacity increased on connecting pipe lines.The capacity of heaters and their fouling factors should be increased. Themeans to clean oil heaters easily and rapidly should also be provided.Separately driven positive-displacement pumps should be provided forhandling fuel taken from the settling tanks for passage through the cleaningsystem and each machine should have its own pump. The capacity of the pumpsshould be carefully sized to suit the fuel requirements of the engine in conjunctionwith the capacity of the individual pieces of cleaning equipment.

....-.•..--..-----If two or more separators are required to handle the maximum fuel requirement of the engine, the diSCharge from each pump must not be brought into a common line to feed the separators. Each separator should have its own heater or the fuel should be heated prior to being handled by the separator's own . supply pumps. (The lowest-cost fuels supplied today have a high viscosity, density, and ~ carbon content. These kinds of fuel require a high temperature when being.treated in a fuel cleaning system consisting of separators and/or clarifiers. The maximum temperature for heating the fuel prior to cleaning treatment isGoverned by the boiling point of water, which limits the preheating temperature to something little less than 1000C. The manufacturers of centrifugal-type fuel cleaning equipment are well ableTo offer a wide variety of machines, well suited to the fuels supplied today and; future.These machines do not always come within the previously accepted definitionof separator or clarifier as they are not always fitted with a dam ring in the caseof a separator or a sealing ring in the case of a clarifier.The main advantage of these modern machines is that they can be operatedwithout an internal water seal. In this respect they are similar to clarifiers.Control of the cleaning function is governed by monitoring the build up of\olids, sludge, and water contained within the bowl. When solids, sludge andwater build up within the bowl the interface between the water and the partially cleaned fuel moves inwards and reaches a point where the cleaned fuel will contain traces of water.

One manufacturer uses a computer programmed to control the automaticcleaning or dump function of the purifier bowl in conjunction with a deviceextremely sensitive to the smallest trace of water. This device is fitted in theclean oil discharge from the purifier. During normal operation of the purifierthe sludge and water discharge coming from the dirty side paring disc is shut offby a valve fitted in the discharge piping from the purifier.The computer program COvers fuels having a wide range of water content.When cleaning fuel containing limited amounts of water the dump cycle isprogrammed to act at regular fixed intervals. When the fIXed interval expiresthe bowl-cleaning action is triggered by the computer and the solids, sludge andwater are dumped out of the bowl when it opens.If the clean oil outlet shows traces of water before the normal time intervalhas expired, the water-sensitive monitoring device relays a signal to rhecomputer and the closed valve in the water outlet is opened. Water is thendischarged from the bowl through the water paring disc and dirty water line. If the device monitoring the water content in the clean fuel shows a sharp drop inwater content over a shoTt time interval, the valve in the dirty water line remains open for some given period and then closes. The normal dump cycle is thenrepeated at the set interval fOllOWing the previous dump cycle.If the water content in the dirty fuel increases, tile cycle of operations resulting in the discharge of water thrOugh the dirty water line is repeated andwill continue if any water Content is shown in the clean fuel. The dump cycle is again activated after the fixed interval measured from the previous dumping cycle.When this cycle is completed the bowl is clean and a new cycle begins. Theperiod between the bowl dumping or cleaning operation remains the sameirrespective of what may occur during the time between cleaning cycles.If water content in the dirty fuel is more than the purifier can handle, thecomputer measures the rate of change of water content in the clean fuel withrespect to time, and if this is above some accepted value the control system willactivate alarms and cause the water contaminated fuel to be bypassed back tothe settling tank ..One manufacturer's equipment monitors the fluid taken from the lowerparing disc. Again, the time interval between clearing the solids, sludge andwater out of the bowl by opening it is fixed in the computer program and neveralters in amount.When a centrifugal purifier of this type is started the bowl is completely filledwith oil. The oil is discharged from the lower paring disc outlet line andmonitored for conductivity before going through a two-way control valve andpassing back to dirty oil feed to the purifier.As water is removed from the fuel it builds up in the periphery of the bowland willbe discharged during the cleaning or dump cycle. If the water containedin the dirty fuel is above a certain amount, the water will build up in the bowland cause the oil water interface to move inwards to some point where waterwill be discharged from the paring disc outlet. When this occurs the change inconductivity will be relayed by the sensor to the computer. The computercontrol will activate the two-way control valve and divert the water flowingthrough it away from the dirty oil supply and into the alternative route to thewater outlet from the purifier. When the time for the dump cycle is reached, thesolids, sludge and water, are discharged out of the bowl, in the normal mannerand a new cycle commences when the purifier is filled with dirty oil again.The remaining parts of the fuel oil system are similar to those mentioned inthe answer to Question 3.28.Normal separators and clarifiers can be used for cleaning very high-densityfuel oils, but the sealing water in the purifier must have a higher density than thefuel when both are at the operating temperature of the separator. If the densityof the fuel is equal to or higher than the density of the sealing water, theseparator will not function correctly. A liquid having a higher density than thefuel must be selected to create an interface. Liquids having a density higher thanthe "density of the fuel oils now available can be obtained by dissolving one.of avariety of saIts in water for use as the sealing medium. This can provide a sealwith a density and boiling point higher than that of water.The material selected will have to be carefully chosen because of the possibilitiesof setting up corrosion within the fuel cleaning equipment, sludge tanks andsystem pipe work. Corrosion inhibitors will have to be used to give adequatecorrosion protection if it is shown to be necessary.The sealing liquid and water removed from the separator can be recycledAfter testing for density and inhibitor content

High viscosity fuels

Fuel supplied to motor vessels is usually ofthe high-viscosity type. This kind offuel is a blend of low-viscosity distillate and high-viscosity residual oils. Thebunker supplier has tables which give the proportional amounts of each kind ofoil required to obtain some specified viscosity. After the proportions have beenestablished the blend is produced by using two pumps arranged to dischargeinto a common pipe. The size of the pipe is such that turbulent flow takes placeand the two kinds of oil become well mixed. The speed of the two pumps is setso that the proportion of each kind of oil passfng into the common discharge ismaintained correctly.Distillates from one crude stock type do not always blend well with residualsfrom another. This occurs when the smaller part is not soluble in the larger part.1£ incompatible oils are used to produce a blended fuel, precipitation will occur .This shows itself in the operation of the centrifugal purifiers which quickly fillwith asphaltic material and .extreme difficulty may be experienced inmaintaining an adequate throughput of fuel through the purHier for therequirements of the engine. This problem is well known to fuel oil suppliers andthey take every care to blend compatible types of fuel.

HomogenizerA homogenizer is a piece of equipment used to create a material with a stableuniform structure (homogeneous structure) from a mixture of two or morefinely divided solid materials or a mixture of immiscible liquids. It can be usedto break down relatively large water particles within a heavy fuel into ahomogeneous structure or emulsion consisting of water particles of theminutest size uniformly distributed throughout the resulting liquid.A homogenizer can also be used to reconstitute an emulsion that hasseparated out from some heavy fuel, in order to give it some stability.A homogenizer works by severely agitating the mixture being homogenized.The agitation can be carried out by mechanical means such as pumping themixture through very fine orifices, or by acoustic means such as pumping themixture in a thin layer over a surface being agitated at an ultrasonic frequency(above twenty kilohertz). The agitation can be created with any electronicdevice that will create ultrasonic pressure waves.

The homogenizer (Figure 2.26) provides an alternative solution to the problemof water in high density fuels. It can be used to emulsify a small percentage for'injection into the engine with the fuel. This is in contradiction to the normalaim of removing all water, which in the free state, can cause gassing of fuelpumps, corrosion and other problems. However, experiments in fuel economyhave led to the installation of homogenizers on some ships to deal with adeliberate mixture of up to 10% water in fuel. The homogenizer is fitted in thepipeline behveen service tank and engine so that the' fuel is used immediately. Itis suggested that the water in a high density fuel could be emulsified so that thefuel could be used in the engine, without problems. A homogenizer could nofbe used in place of a purifier for diesel fuel as it does not remove abrasives su~

as aluminium and silicon, other metallic compounds or ash-forming sodiumwhich damages exhaust valves.The three disc stacks in the rotating carrier of the Vickers type homogenizerare turned at about 1200 rev/min. Their freedom to move radially outwardsmeans that the centrifugal effect throws them hard against the lining tyre of thehomogenizer casing. Pressure and the rotating contact break down sludges andwater trapped between the discs and tyre, and the general stirring action aidsmixing.Homogenisers Homogenisers are used to disperse any sludge and water remaining in the fuel after centrifuging. A

homogeniser placed after the centrifuge will render fresh water (not removed by centrifuging) harmless to the engine, and eventually lead to the acceptance of fuels with no density limit.

Homogenising may also be a means to cope with the more and more frequently occurring incompatibility problems, which are not really safeguarded against in any fuel specification. Both ultrasonic and mechanical homogenisers are available.

Treatment of heavy fuel oil, & heavy fuel oil and diesel fuel oil separation As a result of experience it is strongly recommended the use of modern centrifuges for the treatment of

heavy fuel oils. The separating effect, i.e. the cleaning effect depends on the throughput and on the viscosity of the heavy fuel oil. As a general rule, the smaller the throughput (m3/h or ltr/h) and the lower the viscosity of the heavy fuel oil, the better the separating effect. It necessitates heating the heavy fuel oil before it enters the centrifuge and maintaining the working temperature at a constant level within a tolerance of _2 _C. The minimum required heat-up temperature depends on the viscosity at 50 _C of the heavy fuel oil in question. This temperature can be read off the viscosity/temperature diagram, please also refer to the instructions of the makers of your centrifuge. For design reasons the admissible heat-up temperature is limited to 98 _C.

For operation, the instructions of the centrifuge makers are to be followed in the first place. The sludge removed by centrifuging must be removed periodically from the separator drum. In the case of self cleaning centrifuges the sequence of the emptying process may be controlled automatically but even in such a plant the correct function and the frequency of proceedings must be kept in control by the operating personnel. Of utmost importance is the unimpeded drain of the sludge from the drum, so that unacceptably high back pressure does not impair the function of separation and thereby of cleaning the heavy fuel oil. This point must be absolutely assured in operation by periodical inspections.

Fuels supplied to a ship must be treated on board before use. Detailed information on fuel oil system layout can be found in the CIMAC Recommendations concerning the design of heavy fuel treatment plants for diesel engines issued in 1987. Practically all fuel specifications refer to fuel as supplied and, as such, serve primarily as purchasing specifications. Furthermore, the data in a standard fuel analysis serves to adjust the onboard treatment and is actually of little use to the operator when referring to the engine operational data. Hence our basic design criterion is that our engines shall be capable of accepting all commercially available fuel oils, provided that they are adequately treated on board.

Centrifuging Recommendations Fuel oils should always be considered as contaminated upon delivery and should therefore be thoroughly

cleaned to remove solid as well as liquid contaminants before use. The solid contaminants in the fuel are mainly rust, sand, dust and refinery catalysts. Liquid contaminants are mainly water, i.e. either fresh water or salt water. Impurities in the fuel can cause damage to fuel pumps and fuel valves, and can result in increased cylinder liner wear and deterioration of the exhaust valve seats. Also increased fouling of gasways and turbocharger blades may result from the use of inadequately cleaned fuel oil.

Effective cleaning can only be ensured by using a centrifuge. We recommend that the capacity of the installed centrifuges should, at least, accord to the centrifuge maker’s instructions. To obtain optimum cleaning, it is of the utmost importance that the centrifuge is operated with as low a fuel oil viscosity as possible, and that the fuel oil is allowed to remain in the centrifuge bowl for as long as possible. A sufficiently low viscosity is obtained by operating the centrifuge preheater at the maximum allowable temperature for the fuel concerned.

For fuels above 180 cSt/50 oC it is especially important that the highest possible temperature, 98 oC, is maintained in the centrifuge oil preheater. The fuel is kept in the centrifuge as long as possible by adjusting the flow rate through the centrifuge so that it corresponds to the amount of fuel required by the engine without excessive re-circulating. Consequently, the centrifuge should operate for 24 hours a day except during necessary cleaning. Centrifuges with separate feed pumps On centrifuges equipped with adjusting with a capacity matched to the engine screws and/or gravity discs, their correct output are to be preferred. choice and adjustment is of special importance for the efficient removal of water. Taking today’s fuel qualities into consideration, the need for cleaning the centrifuges (‘shooting frequency’) The centrifuge manual states which should not be underestimated. disc or screw adjustment should be chosen on the basis of the density of the fuel.

Fuel oil centrifuges

The manual cleaning type of centrifuges are not to be recommended, neither for attended machinery spaces (AMS) nor for unattended machinery spaces (UMS). Centrifuges must be self-cleaning, either with total discharge or with partial discharge.

Distinction must be made between installations for: • Specific gravities < 0.991 (corresponding to ISO 8217 and British Standard 6843 from RMA to RMH, and CIMAC from A to H-grades

• Specific gravities > 0.991 and (corresponding to CIMAC K-grades). For the latter specific gravities, the manufacturers have developed special types of centrifuges, e.g.:Alfa-

Laval Alcap, Westfalia Unitrol, Mitsubishi E-Hidens II . The centrifuge should be able to treat approximately the following quantity of oil: 0.27 l/kWh = 0.20 l/BHPh

This figure includes a margin for: • Water content in fuel oil • Possible sludge, ash and other impurities in the fuel oil • Increased fuel oil consumption, in connection with other conditions than ISO. standard condition • Purifier service for cleaning and maintenance. The size of the centrifuge has to be chosen according to the supplier’s table valid for the selected viscosity

of the Heavy Fuel Oil. Normally, two centrifuges are installed for Heavy Fuel Oil (HFO), each with adequate capacity to comply with the above recommendation. A centrifuge for Marine Diesel Oil (MDO) is not a must, but if it is decided to install one on board, the capacity should be based on the above recommendation, or it should be a centrifuge of the same size as that for lubricating oil.

The Nominal MCR is used to determine the total installed capacity. Any derating can be taken into consideration in border-line cases where the centrifuge that is one step smaller is able to cover Specified MCR.

Centrifuge and pre-heaterThe normal practice is to have at least two centrifuges available for fuel cleaning purposes. Results from

experimental work on the centrifuge treatment of today’s residual fuel qualities have shown that the best cleaning effect, particularly in regard to removal of catalytic fines, is achieved when the centrifuges are operated in series, i.e. in purifier/clarifier mode.

This recommendation is valid for conventional centrifuges. For more modern types, suitable for treating fuels with densities higher than 991 kg/m3 at 15 oC, it is recommended to follow the maker’s specific instructions. In this context, see paragraph on high density fuels. If the installed centrifuge capacity is on the low side, in relation to the specific viscosity of the fuel oil used, and if more than one centrifuge is available, parallel operation should be considered as a means of obtaining an even lower flow rate. However, in view of the above results and recommendations, serious consideration should be given to installing new equipment in compliance with today’s fuel qualities and flow recommendations.

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