catalytic fines - safe operation

8/20/2019 Catalytic Fines - Safe Operation...

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written by Alfa Laval, BP Marine and MAN B&W Diesel

Marine diesel engines,

catalytic fines and

a new standard to ensure

safe operationSeparation Performance Standard

MAN B&W Diesel


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A joint effort

– good for the industry

This publication is intended to provide an overview of catalytic

fines and their effect on marine diesel engines, marine residualfuel oils and fuel cleaning on board. It has been written bymembers of the European Committee for Standardization (CEN)Workshop No. 20 for separation performance standard, thereference of which is CWA 15375:2005 Separators for marineresidual fuel – Performance testing using specific test oil.

Authors of this publication are Gunnar Åström of Alfa Laval, Adrian Danielsof BP Marine and Kjeld Aabo of MAN B&W Diesel, whose collaborative effortshave been key in establishing a new standard for separation performance.

All three companies, together with others, have been driving the marineindustry towards a standard for separation performance through their work to establish and gain approval for a CEN Workshop Agreement (CWA).

Kjeld Aabo

MAN B&W Diesel

Gunnar Åström

Alfa Laval

Adrian Daniels

BP Marine


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Inside view

Summary ..................................................................................................... 2

Marine residual fuel ................................................................................... 4

Residual fuel oil .............................................................................................. 4

Refining processes ....................................................................................... 4

Catalytic fines .............................................................................................. 11

ISO 8217 Fuel Standard .............................................................................. 12

Measuring catalytic fines content ................................................................. 13

Catalytic fines in residual fuel oil – future trends ........................................... 13

Marine diesel engines .............................................................................. 14

Catalytic fines and engine performance ....................................................... 15

The importance of fuel cleaning ................................................................... 15

Engine wear and damage from catalytic fines .............................................. 17

Case study: Land-based power plant .......................................................... 19

Fuel cleaning systems ............................................................................. 20

The modern approach ................................................................................. 20

Stokes’ Law ................................................................................................ 21

Operating parameters .................................................................................. 21

How fuel oil quality affects separation .......................................................... 23

Series or parallel operation? ........................................................................ 24

New standard for separation performance ........................................... 26

Model test method ...................................................................................... 27

Correlation to actual operating conditions .................................................... 29

Why a model test method ........................................................................... 29

Type approval .............................................................................................. 30

Economical aspects .................................................................................... 31

Dimensioning of fuel cleaning systems ................................................. 32

Sizing of centrifuges .................................................................................... 32

Certified Flow Rate vs. Maximum Recommended Capacity ......................... 32

Conclusion ................................................................................................ 34


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Summary

In general, marine diesel engines burn residual fuel oils. The

quality of the fuel oils varies widely, depending on the gradeand processing of the fuels. Some may contain higher levels of contaminants, such as water and abrasive solids, than others.

To achieve reliable and cost-effective operation of the marine diesel enginesit is necessary to clean all residual fuel oils before injection into the engines.

The leading cleaning method used on board ships today is centrifugal separa-

tion. For years engine builders, ship owners and classification societies havebeen missing reliable performance criteria to be able to compare how onemanufacturer’s centrifuge performs against that of another manufacturer withregard to the removal of hard abrasive particles such as catalytic fines frommarine residual fuels. If unchecked, these catalytic fines can endanger shipsafety by causing engine wear and damage.

According to the ISO 8217 Fuel Standard for Specifications of Marine Fuels,the maximum allowable content of catalytic fines in bunkered fuel, expressed

as the total content of aluminium and silicon, is 80 mg/kg (ppm). However,engine builders generally anticipate that this amount will be reduced by thefuel cleaning system onboard to a maximum of 15 ppm before the fuel oil isinjected into the engine.

Manufacturers of centrifuges now supply buyers with their own maximumrecommended capacity (MRC) tables as guidelines to select a fuel cleaningsystem. However, buyers cannot be sure that the centrifuges that are speci-fied and installed using these tables actually ensure the safe removal of harmful solids from bunker fuels.

In June 2004 the European Committee for Standardization (CEN) began toestablish an operational specification of a method to measure separationperformance of centrifuges with regard to the presence of solid particles inresidual fuel oils.

The CEN Workshop finalised an agreement, called CWA, in August 2005 that

effectively established a new industry standard that defines the performance

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of centrifuges installed on board ships. This standard is based on controlledartificial particles, which however are still considered as reliable performancecriteria, which establish a fuel oil centrifuge’s ability to remove solid, abrasiveparticles from marine residual fuels to safe operational levels.

Hereby objective comparisons between centrifuges from different supplierswill be possible.

The participants will investigate the prospects of developing the agreement toa full international standard recognized by the International Organization forStandardization.

The reference number of this document is CWA 15375, Separators formarine residual fuel oil – Performance testing using specific test oil, dated2005-08-17.

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Marine residual fuel

Marine residual fuel, also called bunker fuel or residual fuel oil, is

the name given to the primary energy source used in the marineindustry.

This fuel is consumed on board the vessel both by the main engine, whichtransports the vessel through the water, and by the auxiliary engines, whichprovide all the electrical energy for onboard systems, such as lighting, heatingand engine auxiliary equipment.

Residual fuel oilResidual fuel is essentially a refinery by-product, blended to satisfy marketdemand for a cheap source of energy. Some of the residual fuel manufacturedis used by the marine industry. The main drivers in the refining industry are theproduction of light and middle distillate grades, used to formulate motor gaso-line, jet fuel, automotive diesel fuel and chemical feedstock. Over the past15 years the worldwide demand for distillate grades has continued to rise,whereas the production of residual fuel oil has declined slightly (Figure 1).

As a consequence of the relentless increase in demand for distillate fuels,refineries have incorporated more sophisticated processes in order to extractas much as possible of distillate feedstock from a barrel of crude oil. The neteffect is a reduction in the percentage of the crude oil barrels produced asresidual fuel oil. In 1990 approximately 20 percent of a barrel of crude wasprocessed into residual fuel oil, compared to 14 percent today. The decreaseis remarkably linear (Figure 2).

If the reduction in residual fuel oil production continues to decline at the samerate of about 4.6 percent every decade, then in approximately 30 years therewill be no residual fuel oil. Clearly at some point in time between now and 2035

the demand for residual fuel oil will outstrip the capability to supply it, causingresidual fuel oil to lose its current exclusive status as a cheap source of energy.

Refining processes

This section describes the basic refining processes, which are applicable tothe production of residual fuels. Typically the source of all fuels is derived from

crude oils. Some of the refinery processes are described below.

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5

T o n n a g e ( m i l l i o n t o n n e s )

Year

19890

200

400

600

800

1000

1200

1400

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

GasolinesMiddle distillates Fuel oil Others

Figure 1. Oil consumption worldwide (excluding the former Soviet Union).

Source: BP Statistical Review 2004 www.bp.com/centres/energy

% o

f f u e l o i l c o n s u m e r / T o t a l o i l p r o d u c t s

Year

198913

14

15

16

17

18

19

21

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Fuel oil Linear (fuel oil)

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Figure 2. Residual fuel oil as a percentage of all oil products (excluding the former Soviet Union).

Source: BP Statistical Review, 2004 www.bp.com/centres/energy


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Crude oil

Crude oil is a complex mixture of hydrocarbons that must be processed to

provide both the products and the quantity of each product required to fulfil

worldwide demand.

No two crude oils are alike; there are significant variations in density, viscosity,

sulphur content, vanadium content and in the yield of each product that can

be produced by a refinery.

Originally crude oil was distilled in an atmospheric distillation process to obtain

the required grades at the required quantity. The vacuum distillation process

was developed to enable further refinement of atmospheric residue.

Both atmospheric distillation and vacuum distillation are refinery processes

based on the physical separation of crude oil components into distillate fuels.

Because these processes do not chemically alter the structure of the oil, the

original crude oil could be reproduced by mixing together all of the output

streams and components in the appropriate proportions.

These two simple distillation processes however do not produce the amount

of distillate oil products that is consistent with the increasing worldwide

demand. Therefore subsequent and more complex refinery processes,

known as secondary conversion processes, are required.

Atmospheric distillation

The first step in the refining process is the separation of crude oil into various

fractions by distillation. Distillation is carried out as a continuous process in a

fractionating tower, which is fitted with perforated trays. The crude is heated

to approximately 350°C and pumped into a section near the base of the

tower, see Figure 3. The temperature is restricted so as not to induce thermal

decomposition of the crude oil.

As the crude oil enters the tower all except the heaviest hydrocarbons

evaporate and rise through the tower as vapours. The process takes advan-

tage of the fact that the crude contains a complex mixture of hydro-carbons,

which have different boiling points. When crude oil is heated, the lightest and

most volatile hydro-carbons boil off as vapours first and the heaviest and least

volatile last. If the vapours are cooled they condense back into liquids in the

reverse order, the heaviest first and the lightest last.

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7

Petroleum gas

(LPG)

• Camping gas

• Industrial gas

Gasoline

• Petrol

Naptha

• Chemical feedstock

Kerosene

• Aviation fuel

• Domestic heating oil

• Industrial heating oil

Gas oil

• Diesel oil

Lubricants

and waxes

• Lubricating oil

• Wax

Residue

• Fuel oil

• Bitumen

gas

Liquids fall

Furtherprocessing

Further

processing

Furtherprocessing

Further

processing

Furtherprocessing

Furtherprocessing

Furtherprocessing

Vapours rise

Preheated

crude oil

Figure 3. Atmospheric distillation of crude oil.


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As the vapours rise through the tower, some of the vapour condenses due to

the reduction of temperature caused by the cooler liquid in the trays. At the

same time the heat of the vapour causes some of the condensed liquid to

re-evaporate thus steadily enriching each fraction with its correct components.

At successively higher points in the tower, various fractions are drawn off for

further processing.

Those components with the lowest boiling points collect at the top of the

tower and those with the highest boiling points are collected at the base of

the tower. Boiling points vary from approximately 60°C at the top to over

300°C at the base.

All of the crude oil, however, is not fractionated into distillate products. Someof the vapours flow out of the top of the tower as gases. Fractions of crude

that have a boiling temperature above 300°C fall to the base of the tower; this

product is known as atmospheric residue which can become a component of

a residual fuel oil.

Atmospheric distillation is a simple physical process, where the fuels are

separated according to specific boiling ranges. The type of crude determines

the percentage of each product that can be obtained. Low-density, low-

viscosity crude oil yields more distillate fuel and a smaller proportion of the

atmospheric residue than high-density, high-viscosity crude.

As mentioned, the use of the atmospheric distillation process alone to refine

crude oil does not satisfy the worldwide demand for fuel. To satisfy demand,

additional refining processes, which reduce the amount of residual fuel and

increase the amount of distillate fuel, must be employed.

Vacuum distillation Vacuum distillation is another refinery process that is similar to atmospheric

distillation but takes place under vacuum conditions. Since liquids in a

vacuum boil at lower temperatures, the atmospheric residue evaporates

thus enabling more of the light distillate fractions to be drawn off without

exceeding the temperature at which thermal cracking will take place.

As in atmospheric distillation not all the liquid vaporizes; what is removed

from the base of the unit is called vacuum (or short) residue, which can

become a component of a marine residual fuel. Despite the addition of

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vacuum distillation as another means to refine crude oil by means of

physical separation, worldwide demand for light distillate fuel still outstrips

the supply.

Secondary conversion processes

In order to satisfy the demand for distillate fuel products, refineries use

secondary conversion processes, which use some of the product streams

from the distillation processes and alter the chemical structure of the oil.

This reduces the amount of residual fuel oil and increases the amount of

distillate fuel.

Secondary conversion processes include “cracking” processes that break

the long molecules of the heavier fuel fractions into shorter molecules thatcan more easily be processed into fuel oil products that are in demand.

There are two basic types of cracking processes: thermal cracking and

catalytic cracking.

Thermal cracking

Thermal cracking uses temperature, pressure and time to provoke a chemical

reaction that alters the structure of the oil. Thermal cracking can be carried

out on both distillate fuels and on residual fuels. Typical thermal cracking

processes include visbreaking which significantly lowers the viscosity of a

heavy residue to enable blending with other fuel oils and coking which

destroys the fuel oil, producing only distillate fuel and coke which is a form

of carbon.

Catalytic cracking

Catalytic cracking processes also alter the chemical composition of residual

fuel oil. They use a catalyst rather than high pressure to break down com-

plex hydrocarbons into simpler molecules. The catalyst is a substance that

assists the process of the chemical reaction, but does not change its

own properties.

The most common process is fluid catalytic cracking (FCC), which can con-

vert gas oil and residual oil into high-octane gasoline and diesel fuel. Here the

catalyst in the form of fine particles that are approximately 20 to 100 microns

in diameter circulates between a reactor and a regenerator in a fluidised bed

process (Figure 4). Large catalytic crackers will contain about 500 metric tons

of expensive catalyst.

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The hot catalyst from the regenerator mixes with the feedstock and then

enters the reactor. Upon contact with the catalyst the feedstock vaporizes

and the vapours in turn react, breaking the chemical bonds to achieve the

desired product quality. The reaction causes some carbon to be deposited

onto the catalyst. The catalyst and the vapours are separated in the reactor.

The vapours rise and flow into a fractionating tower for further processing.

The catalyst flows back into the regenerator, which heats the catalyst to burn

off the carbon prior to mixing it with the feedstock and passing it back into

the reactor.

The continual recycling of the catalyst causes the catalyst structure to break

up primarily due to attrition and some of these small particles are carried over

into the fractionator. Although refiners attempt to minimise the loss of catalystfrom the catalytic cracking process some carryover of the expensive catalytic

fines from the unit is inevitable.

The product drawn off the bottom of the fractionator is called slurry oil, which

is also known as decant oil or FCC bottoms. Highly aromatic, the slurry oil

has a high density, generally about 1000 kg/m3 at 15°C and a low viscosity

of approximately 30 to 60 cSt at 50°C. It is an ideal blending component for

residual fuels due to its high aromaticity, which confers stability to the finished

fuel. It is from this refinery source that catalytic fines enter residual fuel.

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Figure 4. Equipment for the catalytic cracking process.

Gasoline

Light cycle oil

Heavy cycle oil

Gases

Decant oil

Flue gas

Reactor

Regenerator

Origin of catalyticfines in HFO

Fractionator

Steam Air Feed

Catalyst

C o u r t e s y o f D N V P S


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Catalytic fines

The composition of catalytic fines varies depending on the type of feedstock and on whether the main unit of the cracker is intended to optimise gasoline(light) grades or diesel (heavier) grades. Exact catalyst compositions are never

quoted today but all of them contain some form of synthetic crystalline zeolitematerial.

The zeolite is dispersed in a matrix, which consists of an active matrix, clayand binder. Both the zeolite and the active matrix are forms of aluminiumoxide and silicon oxide. The ratio of silicon to aluminium in the catalyst variesconsiderably, ranging from 0.65:1 to 2:1.

In the mid 1990s, Det Norske Veritas (DNV) determined that the annual world-wide average ratio of silicon to aluminium was 1.64:1 based on analyses of fuel oil samples. Recent analyses indicate that the worldwide average ratio iscurrently about 1.2:1 and that this ratio is likely to fall because modern cata-lysts tend to contain a higher concentration of aluminium.

Catalytic fines are formed through the break up, primarily due to attrition, of the catalyst, as it is recycled through the cat cracker plant. Catalytic fines arevariable in size with a range from sub-micron to approximately 30 microns and

occasionally up to 100 microns. Whilst they are considered to be sphericalparticles this is not necessarily the case.

A typical particle size distribution of catalytic fines in heavy fuel oil is shown inFigure 5.

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Figure 5. Particle size distribution of catalytic fines.

BP investigation

Particle size By numberdistribution

(micron) (%)

5–10 5710–15 2715–35 1535–100 1

5–10

10–15

15–35

35–100


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ISO 8217 Fuel Standard

ISO 8217:1996 is the current international standard for specifications forpetroleum products for marine diesel engines and boilers. These specificationsdefine the limit for catalytic fines in fuel oils, expressed as Al + Si (aluminium

plus silicon), at 80 ppm. This limit will remain the same in the revised edition(third edition), which will soon be issued1.

It is generally accepted that onboard fuel cleaning systems, including settling,purification and filtration, will reduce the level of catalytic fines in bunker fuel atdelivery below the 15 ppm level that is acceptable in order to inject the fuelinto an engine.

In September 2004 DNV published statistics indicating that the percentage of bunker fuel deliveries that contained specific ranges of aluminium and silicon2.Nearly 80 percent of all bunker deliveries in the marine market contain lessthan 20 ppm of aluminium plus silicon.

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Al + Si (mg/kg) % of deliveries

>80 0.3>60, 20,


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Measuring catalytic fines content

Several methods for directly measuring catalytic fines in the fuel have beendeveloped, tried and discarded. The current and approved method, ISO10478, measures catalytic fines using an indirect method whereby a weighed

quantity of a homogenised fuel oil sample is heated in a platinum crucible andthe combustible material is removed by burning. The residue and crucible arethen transferred to a muffle furnace held at 550 ±25°C to reduce the carbonresidue and provide the ash. The ash is then fused with a lithium tetraborate/ lithium fluoride flux and the fused mixture is dissolved in a mixture of tartaricacid and hydrochloric acid and diluted with water to achieve the desiredvolume.

The solution is then aspirated in the plasma of an inductively coupled plasmaemission spectrometer or absorption spectrometer, and the aluminium andsilicon elements are detected by radiation emissions. The quantity of catalyticfines is then derived through the comparison of the emission results withstandard calibration solutions. It goes without saying that the contents of aluminium and silicon measured in this way are independent of particle sizedistribution.

Catalytic fines in residual fuel oil – future trends

As the world demand for distillate fuels continues to increase, the adoption of more advanced refining processes will slowly populate the globe. The naturalconsequence of this is that more marine residual fuels will contain some levelsof catalytic fines. However with the combined forces of marine fuel specifica-tions and properly designed and maintained centrifuges it is highly likely thatthe issue will pass unnoticed by the shipping community.

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Marine diesel engines

Two-stroke marine diesel engines today are able to operate

within the ISO 8217 fuel standard and the CIMAC HFO 55No. 21 recommendation.

The condition is that the fuel is properly cleaned in the centrifuge. BothISO 8217 and CIMAC fuel recommendation specify that the content of catalytic fines in the fuel oil delivered on board may not exceed a maximumof 80 ppm, an abbreviation for parts per million which is expressed as aweight-to-weight ratio.

To clean the fuel oil of catalytic fines and other impurities before injectioninto the engine, fuel cleaning systems are required on board. Engine buildersgenerally anticipate that the maximum level of 80 ppm catalytic fines will bereduced by the fuel cleaning system on board to a maximum of 15 ppmbefore the fuel oil is injected into the engine. As the level of catalyst fines inthe bunkered fuel is lowered, the engine builders expect a related reductionin the amount of catalyst fines in the fuel entering the engine.

The design of the fuel cleaning system to be used on board is the criticalfactor for optimal reduction of catalytic fines content in fuel oil from 80 ppmupon delivery to the ship to 15 ppm before injection into the engine. This isto ensure safe operation and optimal engine performance.

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Max. 15 ppmcatalytic fines

Max. 80 ppmcatalytic fines

Separator EngineFuel conditioning module

Figure 6. Reduction of the catalytic fines by the fuel cleaning system on board.


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Catalytic fines and engine performance

Catalytic fines are small particles of spent catalyst, that remain in the fuel afteremploying catalytic cracking processes to refine crude oil into more valuablefractions, leaving residual fuel oil as a bottom phase, enriched in contaminants.

These particles vary in size anywhere from submicron to tenths of microns,ranging from a speck of dust or pollen to the width of a strand of coarsehuman hair. Though virtually invisible to the human eye, catalytic fines arevery hard and capable of severely scratching, if not cutting, metal.

All catalytic fines, that remain in the fuel oil after centrifugal separation havethe potential to cause abrasive wear and damage to the engine, which in turncan lead to potentially unsafe operating conditions. That is why the level of

catalytic fines must be reduced as much as feasibly possible by the fuelcleaning system. Catalytic fines smaller than five microns are considered tobe less harmful than larger cat fines.

The higher the amount of catalytic fines is, the greater the risk for unsafeoperating conditions due to engine wear and breakdown. Under such oper-ating conditions, there is an increased risk for breakdown and it is likely thatthe engine will require more frequent maintenance than what has beenrecommended by the engine manufacturer.

If the amount of catalytic fines is removed at optimum efficiency by the fuelcleaning system, according to experience the engine has a controlled andacceptable wear, defined as mean time between overhaul, which is specifiedby engine suppliers.

The importance of fuel cleaning

Marine diesel engines are designed to be capable of accepting all commer-cially available fuel oils, provided they are adequately treated on board. Forthis purpose, a well-designed fuel cleaning system is a must. Centrifuges incombination with a settling tank are generally accepted within the marineindustry as the fuel cleaning system of choice. Filters are only consideredas a safety to pick up larger particles so that these particles do not reachthe engine, and do not as such “clean” the fuel.

To ensure safe operation, a heavy fuel oil that meets ISO 8217 and theCIMAC fuel recommendations should be cleaned by an onboard fuel cleaning

system that satisfies these conditions:

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• Preheating the fuel oil to the correct temperature before the centrifuges;

• Correct capacity/layout of the centrifuge (i.e., correct throughput of fuelthrough the centrifuges); and,

• Proper operation and maintenance of the centrifuges.

Because of continuous advances in centrifuge technology, it is virtuallyimpossible for any engine builder to specify the exact size requirements forthe individual centrifuges that are part of an onboard fuel cleaning system.

The correct sizing of the centrifuges depends on the daily fuel consumptionand on the design viscosity of the system. Engine builders can only provide

recommendations for optimal engine performance, which must then be con-firmed by centrifuge manufacturers. See “Sizing of centrifuges”.

Settling and service tanks

Heavy components of large sizes, such as large catalytic fines, in the fuel oilsettle on the tank bottom due to gravitational force. However, at high sea andrough sea conditions, these components can be hurled up and fed into thecentrifuges. The presence of such heavy components influences the purity of fuel after the fuel cleaning system. It is therefore important to drain the settling

and service tanks regularly.

Centrifuges

If properly operated, a centrifuge has the capability to remove nearly 100 per-cent of all catalytic fines that are larger than 10 microns. However, the majorityof catalytic fines smaller than five microns will not be removed due to smallsize and their relative light weight.

In order to check the efficiency of the centrifuges, it is recommended thatsamples be taken before and after the centrifuges at least every four monthsand sent to an established institution for analysis. Samples should be takenmore frequently whenever operating on fuel oil with more than 25 ppm of catalytic fines when the fuel is bunkered.

Homogenisers

Some fuel cleaning systems use homogenisers, which split any water presentin the fuel into small uniform droplets. Positioning homogenisers upstream

from the centrifuges, however, is not currently recommended. This is because

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catalytic fines are hydrophilic, which means that they are attracted to anywater present in the fuel oil. It is therefore expected that it will be difficult forthe centrifuge to remove the small homogenised water droplets, which includeany salt water and attached catalytic fines.

Filters

Filters in relation to fuel cleaning are to be considered as additional protection,guarding the engine from large particles.

Engine wear and damage from catalytic fines

Abrasive wear is mainly caused by the failure of the centrifuges to removecatalytic fines from the fuel oil. Rust, sand and dust are other components,

which are also removed by the centrifuges; however, they are normally lessharmful and are found in the fuel in much smaller quantities.

Fuel injection system

The fuel pump is the first component on the engine that is subject to theharmful effects of catalytic fines. The fuel pump pressurises the injectors,which enables the injectors to deliver the correct amount of fuel to the engineunder the different operating conditions.

The fuel pump consists of a plunger that slides back and forth within the fuelpump barrel. There are relatively small tolerances between the components.

Any catalytic fines that approximate the size of these clearances will be forcedbetween the plunger and barrel and may become embedded in the material of the plunger and/or the barrel (Figure 7).

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Figure 7. Fuel pump spindle guide with scuffing and excessive wear.


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Excessive wear of the plunger or the barrel affects the injection pressure andthereby performance. If abrasive wear to the fuel pump has occurred, it isimpossible to maintain the correct compression pressure for the individualcylinder units.

Catalytic fines that manage to pass through the fuel pump will reach the fuelinjectors, where excessive wear can change the size and shape of the injectorholes. Any change in the size and shape of the holes alters the injector patternof the fuel oil, which in turn can decrease combustion efficiency. Changes inthe fuel injection pattern also result in fouling of combustion chamber compo-nents and increase the amount of unburned hydrocarbons (HC) emissions inthe exhaust gas.

Combustion chamber

When the fuel is ignited, the catalytic fines become trapped in between thevarious working components of the combustion chamber – between the piston

ring and ring groove or between the piston ring and liner. This creates apotentially high-risk situation in the combustion chamber. Any catalytic finesthat are not taken out by the exhaust gas or drained to the bottom of thecylinder unit can become embedded in the softer material of the piston rings.

This quickly creates wear on both the liner and the chrome-plated piston ring

or piston ring grooves (Figure 8).

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Figure 8. Catalytic fines embedded in the surface of a piston ring.


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Case study: Land-based power plant

The quality of fuel delivered to a land-based power plant varied widely in thelevels of catalytic fines. At various times and with various fuels, the amount of catalytic fines contained in the fuels was measured at up to 125 ppm.

Moreover, the fuel quality after cleaning was not acceptable due to compli-cations relating to the operation of the fuel cleaning equipment. Good main-tenance practices are critical to the proper operation of auxiliary machinery; itis virtually impossible for engine designers to predict the mean time betweenoverhauls (MTBO) if the engines and fuel cleaning system are not properlymaintained and the fuel oil specifications are not followed.

The high level of catalytic fines resulted in heavy wear and scuffing on thecylinder liners and pistons. Catalytic fines became embedded in the pistonring surface, causing excessive wear on the cylinder (see Figures 8 and 9).

Before determining that catalytic fines were the root cause of engine mal-function, several possible reasons for excessive wear were investigated.

The situation for this plant could have been avoided if the fuel had been inaccordance with ISO 8217 and CIMAC recommendations and the fuel

cleaning system had been functioning properly.

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Figure 9. Wear on a cylinder liner.


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Fuel cleaning systems

As mentioned earlier, the efficient cleaning of heavy fuel oil is

mandatory for the reliable and cost-effective operation of marinediesel engines.

Cleaning is required to remove impurities, such as water and catalytic fines. As the level of catalytic fines increases in a fuel then it is important to ensurethat that fuel is handled with increasing levels of care. Fuels in excess of the80 ppm limit as specified in ISO 8217 and recommended by CIMAC shouldbe handled with great care.

Fuel from the storage tank is fed into a vessel’s settling tank where the forceof gravity causes contaminants (water and coarse solid particles) to sink to thebottom of the tank. However, in order to achieve complete separation of thecontaminants from fuel oil, fuel cleaning is required.

A fuel cleaning system that includes a settling tank and/or a service tank,centrifuges and protection filter(s) is the general practice of the marine industry.

Centrifugal separation is widely accepted as the most effective means to cleanfuel oils before injection into diesel engines. When the fuel cleaning system iscorrectly dimensioned this will ensure the reduction of catalytic fines to anacceptable level in order for the fuel oil to be injected into the engine, and forthe efficient operation of the engine.

The modern approach

Since 1980s, centrifuges without gravity discs have been the standardsolution for fuel cleaning. Until now, it has been virtually impossible tocompare centrifuge performance because the manufacturers’ maximumrecommended capacity (MRC) tables were the only way to gaugeperformance.

However, the new separation performance standard which was defined in August 2005 now enables buyers, on a given separation performance level,to compare the capacity and price of different centrifuges before makingtheir purchasing decision. This modern approach will be explained in the

“New Standard for Separation Performance”.

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Stokes’ Law

The operating principles of centrifugal separation are based on Stokes’ Law.

The general formula for Stokes’ Law is:

V = gd2 (ρp–ρ

l)/18µ

V = settling velocity (m/sec)

g = acceleration in centrifugal field (m/sec2 )

d = diameter of particle (m)

ρp

= density of particle (kg/m3 )

ρl

= density of medium (kg/m3 )

µ = viscosity of medium (kg/m, s).

The rate of settling (V) for a given capacity is determined by Stokes’ Law.

This expression takes into account the particle size, the difference between

density of the particles and the medium, which in this case is fuel, and the

viscosity of the medium.

Density and viscosity are important parameters for efficient separation.

The greater the difference in density between the particle and the medium

is, the higher the separation efficiency. The settling velocity increases in

inverse proportion to viscosity. However, because both density and viscosity

vary with temperature, separation temperature is the critical operating

parameter.

Particle size is another important factor. The settling velocity increases rapidly

with particle size. This means that the smaller the particle, the more challenging

the separation task. In a centrifuge, the term (g) represents the centrifugal

force, which is several thousand times greater than the acceleration due to

gravitational force. Centrifugal force enables the efficient separation of particles

that are only a few microns in size.

Operating parameters

Various operating parameters affect separation efficiency. These include

temperature, which controls both fuel viscosity and density, flow rate and

maintenance.

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Temperature of HFO before centrifuges

It is often seen that the HFO preheaters are too small, or that the steamsupply of the preheater is limited or that they have too low a set point intemperature. Often the heater surface is partly clogged by deposits. Thesefactors all lead to reducing separation temperature and hence the efficiencyof the centrifuge.

In order to ensure that the centrifugal forces separate the heavy contami-nants in the relatively limited time that they are present in the centrifuge, thecentrifuge should always be operated with an inlet temperature of 98°Cfor HFO.

Figure 10 shows the relationship between temperature and suitable through-put. For example, a centrifuge operating with an inlet temperature of 90°Cwould require a reduction in the throughput of at least 23 percent in order toobtain the same separation efficiency as a centrifuge operating with an inlet

temperature of 98°C.

22

C a p a c i t y f o r s a m e s e p a r a t i o n ( % )

cSt at 50°C

8870

80

90

100

90 92 94 96 98 100

180 cSt 300 cSt 700 cSt

Figure 10. Relationship of throughput and temperature.


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Because the throughput is normally controlled by a constant flow pump,it is not possible to reduce the flow of fuel to the centrifuges if the pre-heating temperature is reduced below 98°C. Maintaining an inlet tempera-ture of 98°C for fuels above 180 cSt (at 50°C) is therefore of critical

importance.

Flow rate

It is known that separation efficiency is a function of the centrifuge's flowrate. The higher the flow rate, the more particles are left in the oil and there-fore the lower the separation efficiency. The flow rate is usually constant andbased on the highest viscosity. As the flow rate is reduced, the efficiencywith which particles are removed increases and cleaning efficiency thus

improves. It is, however, essential to know at what capacity adequateseparation efficiency is reached in the typical case.

Up until now, there has not been any common practice among the suppliersof centrifuges for sizing equipment. The new separation performance stan-dard establishes a uniform method to find the capacity at which a particularlevel of performance is achieved. See the “New Separation PerformanceStandard”.

Maintenance

Proper maintenance is an important, but often overlooked, operating para-meter that is difficult to quantify. If the bowl is not cleaned in time, depositswill form on the bowl discs, the free channel height will be reduced, and flowvelocity increases. This in turn tends to drag particles with the liquid flowtowards the bowl’s centre. This decreases separation efficiency.

How fuel oil quality affects separation

Fuel oil quality varies considerably, despite the fact that the fuel oils may benominally classified as the same type. Because of wide variations in qualityit is impossible to predict the separability of a given batch of fuel oil based onits chemical analyses. This in turn makes it impossible to foresee the actualseparation result and therefore equally impossible to provide any processguarantees.

Characteristics that differ between fuel oils and affect separation efficiency

include:

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Polarity

This defines how the molecules of the oil are arranged according to the posi-

tive and negative charges of the molecules. Polarity is critical in determining

how susceptible the fuel oil is to the formation of hard-to-handle emulsions as

well as the degree of surface tension of water droplets, which is an important

factor for water splitting and emulsions.

Heavily cracked fuels are typically more difficult to treat than the straight-run

fuel oils from less sophisticated refining methods.

Stability

This is the ability of a fuel oil to remain in a constant state despite conditions

that may cause the structure of the fuel oil to break down. When a fuel oil isunstable, it contains precipitated asphaltenes that can be separated when the

particles created have amassed to a sufficient size.

However, these precipitated asphaltenes tend to produce asphaltenic sludge

as a function of time and/or temperature, or both. Mixing or blending two

incompatible fuels together may also cause the coagulation of precipitated

asphaltenes and the formation of a substantial amount of asphaltenic sludge.

Contaminants

Beside catalytic fines, contaminants include water and any insoluble residues,

such as sand, dirt and rust scale that taint fuel quality. These small particles

are not derived from processing fuel but come from other sources, such as

the storage tank or pipes used to transfer of the fuel oil, and may contribute to

the creation of stable emulsions, which make the separation process difficult.

Series or parallel operation?

Centrifuges can be operated in series or in parallel. Over the years, there has

been much discussion about which method of operation provides the best

separation efficiency in theory and in reality.

Fuel cleaning systems typically consist of centrifuges that are sufficiently large

enough to handle the flow of fuel that will be burned. Stand-by centrifuges are

also part of a typical installation.

More than 20 years ago when centrifuges with gravity discs were common,

the standard industry practice was to operate purifiers as stand-alone units.

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To achieve optimum separation results using purifiers with gravity discs, theinterphase level has to be positioned just outside the disc stack, close to theedge of the top disc. Any fluctuation in the operating parameters will causethe interphase level to shift.

In order to avoid alarms under normal operating conditions, the interphaselevel setting was therefore often positioned too close to the centre of the discstack. However, this effectively blocked the upward flow outside the discs anddirected an excessive portion of the flow through the lower part of the discstack, which quickly became overloaded and resulted in poor separation.

To compensate for this effect and to best utilize the centrifuges installed, the

recommendation in the 1980s was to start up the stand-by centrifuge andoperate it as a clarifier in series after the purifier.

However, with the introduction of modern centrifuges without gravity discs,the recommendation is now to operate all available centrifuges in parallel.

This is because modern centrifuges operate correctly without the need forconstant adjustments, which created problems as described above.Operating centrifuges in parallel instead of series enables the reduction ofthe throughput of each centrifuge in half. This ensures the longest possible

residence time in the centrifuges and thus increases separation efficiency.

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New standard for separation

performance

Purchasers of fuel cleaning systems will now have access toan industry standard for separation performance, which makesit possible to verify that a new type of centrifuge operates withan agreed efficiency at a flow rate, which is specific for thecentrifuge in question. This enables more accurate dimensioningand the fair comparison of different centrifuges from differentsuppliers.

The new standard makes it possible to relate operational experiences of bothship operators and centrifuge suppliers to well-defined separation efficiency.On the one hand, operators may refer to wear and maintenance data of theirdiesel engines. On the other, centrifuge suppliers may refer to their experi-ences using data from a large installed base in order to ensure satisfactoryseparation performance.

Up to now, manufacturers of centrifuges supply buyers with maximum

recommended capacity (MRC) tables. However, buyers cannot be sure thatthe centrifuges that they specify and buy using these tables actually removea sufficient amount of solids and abrasives particles, such as catalytic fines,from heavy fuel oils.

The separation performance standard is based on a new test method that

closely approximates normal operating conditions, but provides highly accurate

and highly reproducible results. It provides independent verification of separa-

tion performance according to separation efficiency rather than throughput.

The new standard enables manufacturers to establish a certified flow rate(CFR) for every centrifuge. Certified flow rate is defined as the throughput ratein litres per hour at which 85 percent of five-micron mono-dispersed artificialparticles, which simulate harmful catalytic fines, are removed from a syntheticfuel oil, which simulates a high viscosity fuel oil.

The separation performance standard will contribute to the safety of the ship

by safeguarding the engine from excessive wear. It is scheduled for distribu-

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tion by the European Committee for Standardization (CEN) in August 2005. The intention is to advance the new European standard into a full internationalstandard recognised by the International Organization for Standardization.

Model test method

In order to compare different centrifuges, Alfa Laval successfully developed amodel test method that simulates normal operation and provides highly accu-rate and highly reproducible results. Based on actual separation tests, this testmethod enables fair comparison of the performance of different centrifuges.

The test method is known as the Dyno Test Method due to the sphericalplastic dynosphere particles that are dispersed in a particle-free synthetic oil of

defined viscosity. Identically sized, these five-micron particles simulate harmfulcatalytic fines (Figure 11).

The five-micron particles also help discriminate between good separation andpoor separation at those capacities recommended by centrifuge suppliers.

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Figure 11. Catalytic fines in relation to dynospheres.


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All centrifuges are capable of cleaning particles that are larger than 10 micron,while particles less than 2.5 micron prove too difficult to separate at the

capacities of interest.

During laboratory tests principally shown in Figure 12, the mixture of syntheticoil and plastic particles is heated to a temperature that provides the sameviscosity as fuel oils (380 cSt and 700 cSt @ 50°C), when heated to thenormal separation temperature of 98°C. The mixture is then fed through thecentrifuge at different feed rates, and samples are taken at defined intervalsafter a discharge.

A 30-minute duration after discharge has been selected because separationefficiency generally decreases approximately 15 to 20 minutes after a dis-charge and then stabilises into steady-state condition. Separation efficiencycan only be correctly measured after this steady-state condition has beenachieved.

The certified flow rate of a centrifuge is defined as the capacity at which 85percent of plastic particles are removed 30 minutes after the start of the test.

Capacity and efficiency are measured accordingly.

28

Cin

Feedtank

Receptiontank

Cout

Separator

Figure 12. Simplified schematic sketch of the test rig for measuring certified flow rate.


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Correlation to actual operating conditions

It is impossible to create a standard based on the evaluation of tests usingreal catalytic fines in real heavy fuel oil. Fuel oils also vary in terms of chemicalcharacteristics and in terms of physical characteristics, such as density and

viscosity.

Due to these wide variations, it is impossible to obtain repeatable and com-parable results from tests using actual bunkers.

While the model test method has little to do with real bunkers and contami-nants, it is by far more realistic than any theoretical calculations and simulatesthe particles, including catalytic fines, found in heavy fuel oil.

Tests of centrifuges from three different suppliers were conducted in a modeltest rig using the synthetic oil as well as real heavy fuel oils. A comparisonof the results indicates that the model test method provides results that arein line with those of real heavy fuel oils. In other words, the centrifuges thatprovided the best performance when using real heavy fuel oils also providedthe best performance when using synthetic oil. In addition, the differencesnoted in performance between the different centrifuges were similar regard-less of the oil type tested.

Therefore ranking centrifuges according to the results using the model testmethod provides a good indication of how the centrifuge will perform underreal operating conditions and that any differences between centrifuges undertest conditions will most likely be the differences that will be experiencedunder real operating conditions.

The definition of CFR is based on well-established centrifuge capacities thathave been used in the marine industry for decades. This translates into reliableseparation performance when CFR capacities are employed.

Why a model test method?

Let us look at two different scenarios where fuel oil contains catalytic finesat the maximum allowable values, but where the distribution of particle sizediffers. In the first and best-case scenario, the fuel oil contains only largeparticles and the separation efficiency is 100 percent; in the second andworst-case scenario, it contains all submicron particles and the separation

efficiency is zero (Figure 13).

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In reality, the particle size distribution in heavy fuel oil falls somewherebetween these two extremes. Given the specified limit of 80 ppm of catalyticfines for fuel oil and the maximum value of 15 ppm that is generally acceptedby engine builders, fuel cleaning systems therefore must deliver a separation

efficiency of a minimum of 81 percent. This indicates why a reliable modeltest method that simulates actual operating conditions is required to comparethe performance of different centrifuges.

Type approval

So far, six classification societies have granted type approval for certified flowrate. Det Norske Veritas is one of these classification societies and has inaddition defined a new voluntary Class Notation Fuel, which includes Type

Approval (CFR). The other five classification societies are:

• American Bureau of Shipping• China Ship Classification Society• Germanischer Lloyd• Lloyd’s Register• Russian Maritime Register of Shipping.

Alfa Laval is the first supplier to receive type approval (CFR). This concerns its

complete range of new SPS-separators.

30

Best case

80 ppm

0 ppm

Large particlesall separated

Worst case

80 ppm

80 ppm

Small particlesnothing separated

Figure 13. Difference in separation performance due to particle size.


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Economical aspects

It is impossible to calculate what economic benefits may be realised by usingcertified flow rate (CFR) to dimension centrifuges for marine fuel cleaningsystems rather than manufacturers’ maximum recommended capacity (MRC)

tables. This is due both to the various process parameters that cannot becontrolled and to the variable quality of the fuel.

Because the capacities specified in MRC tables are higher than those of CFR tables, a centrifuge installation based on CFR tables will thereforetypically consist of larger and more expensive centrifuges than the sameinstallation based on MRC tables.

In return, however, the results include improved separation efficiency and areduction in the amount of catalytic fines present in the fuel that is injected intothe engine. This not only will lower maintenance costs since there will be lessabrasive wear, but more importantly, will improve the chances of avoidingengine component breakdown.

Currently, there is not any objective data to compare the actual impact of using CFR tables rather than MRC tables. However, rough calculations forpay-off time as a function of assumptions about the reduction in engine wear

are indicated below.

If on average a reduction in wear of five percent may be achieved, then theinstallation of a centrifuge based on CFR tables essentially pays for itself withinone to three years. If a reduction in wear of 10 percent is achieved, then thepayback time is considerably shorter – anywhere from between six monthsand two years.

The investment in a fuel cleaning system is typically between 0.5 and 1.5percent of the total costs for annual fuel consumption. This indicates that itwould be wise to invest in an amply dimensioned fuel cleaning system. Thisone-time investment would then enable cheaper fuel oil to be purchased overthe long run.

In other words, a major upfront investment in a fuel cleaning system basedon the new separation performance standard may deliver more savings inoperating costs in the long run without compromising ship safety.

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Dimensioning of fuel

cleaning systems

Sizing of centrifuges

Correct sizing of the centrifuges is of utmost importance. When specifyingthe total required flow rate of the fuel cleaning system, the fuel consumptionof auxiliary engines and boilers, if any, must be taken into account. Currently,the appropriate centrifuge is selected from the capacity tables issued by thecentrifuge suppliers and engine manufacturers.

To base oil consumption on the Maximum Continuous Rating (MCR) of the

engines, the following formula can be used.

Q = P · b · 24 (l/h)ρ · T

Q = Fuel oil consumption (l/h)

P = MCR (kW or HP)

b = Specific fuel oil consumption (kg/kWh or kg/HPh) specified by the engine supplier

ρ = Fuel oil density (Presumed to be 0.96 kg/l)

T = Daily net operating time (Number of operating hours per 24-hour day)

Tests for finding specific fuel oil consumption are normally conducted usingdistillate fuel, and the results may have to be adjusted by a factor for so-callednon-ISO conditions. It is assumed that the engine supplier includes this valuein the factor b above.

Certified Flow Rate vs. Maximum Recommended Capacity

Maximum Recommended Capacity

Suppliers of centrifuges determine the maximum recommended capacity(MRC) for each unit according to individual criteria, which are not commonlyknown and not absolutely comparable. Suppliers have been inclined torecommend higher capacity values for individual units as a simple way to

remain competitive.

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Certified Flow Rate

A centrifuge’s certified flow rate (CFR) is measured according to the separa-tion performance standard. Using CFR to specify centrifuges ensures theselection of the correct centrifuge size for the performance required and

thereby ensures safe engine operation.

Conventional capacity requirement

The capacity requirement takes into account several safety factors, whichtogether ensure that the selected centrifuge will perform well. The conventionalrequirement, which is based on centrifuge size, is not uniform and generallyemploys a number of unknown safety margins.

For example, dimensioning a centrifuge using the conditions below will resultin a far higher given requirement for separator size than is actually needed.

• Requirement for stand-by centrifuges• Dimensioning for 700 cSt HFO, whereas 380 cSt will typically be burned• Operation on 100% of MCR, whereas 85% is the typical power• A non-ISO factor of for instance 1.18, even if this factor is already

included in the Specific Fuel Oil Consumption (SFOC) provided by theengine supplier.

This will lead to considerable oversizing of the centrifuge installation, whichhas become a generally accepted means of countering functional problems.

This approach can to some extent be explained by the fact that certain instal-lations have historically provided insufficient performance.

Accurate capacity requirement

Using an accurate calculation of fuel consumption and eliminating “extra”safety margins provides a requirement that is considerably smaller than theoutput using conventional methods.

Matching size to requirement

Given the information above, precision in matching size to requirement wouldbe gained by eliminating the non-defined margins of both requirement andof centrifuge capacity. A recommendation therefore is to abandon the widemargins used to define size requirement and to use CFR to define the correctcentrifuge size.

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Conclusion

The levels of catalyst fines in bunkered fuels are controlled by the ISO 8217

marine fuel standards and CIMAC fuel recommendation. However it is impor-tant that bunkered fuels are cleaned on board all vessels to ensure that thefuel is suitable for use in the engine. High levels of catalyst fines entering theengine have catastrophic effects.

It is important that centrifuges are correctly operated on board the vessel,but equally important that the centrifuge is of a suitable size for its intendedpurpose. The new Separation Performance Standard, which uses the test

method that determines the Certified Flow Rate of each centrifuge, will ensurethat vessels are equipped with centrifuges that are correctly sized.

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Alfa Laval AB

SE-147 80 Tumba

Sweden

+46 (0) 8 530 650 00

www.alfalaval.com/marine

BP Marine Ltd

Chertsey Road

Sunbury-on-Thames

Middlesex

TW16 7LN

United Kingdom

+44 (0) 1932 762000

www.bpmarine.com

MAN B&W Diesel A/S

Teglholmsgade 41

DK-2450 Copenhagen SV

Denmark

+45 33 85 11 00www.manbw.com

0 5 1 2

w w w . f o t o s k r i f t . s e

MAN B&W Diesel

catalytic fines - safe operation

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