24th International Bunker Conference Hilton Hotel in Rotterdam

7-9 May 2003

CIMAC Heavy Fuel Oil Working Group, and

Experience from Operation on Today’s Fuels

and Low-sulphur Fuels

by

Kjeld Aabo Senior Manager

Chairman of CIMAC HFO WG, and member of ISO 8217

MAN B&W Diesel A/S Copenhagen, Denmark

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CIMAC Heavy Fuel Oil Working Group, and Experience from Operation on Today’s Fuels and Low-sulphur Fuels

Introduction

CIMAC

− The history of CIMAC

− Description of tasks and recommendations for the Working Group

− Connections between CIMAC groups

− New edition of the Recommendations regarding Requirements for Heavy Fuels for Diesel Engines

− New edition of the Recommendations on the

Design of Heavy Fuel Treatment Plants

Experience from Operation on Today’s Fuels

− A brief introduction to MAN B&W two-stroke MC engine programme

− Off-spec. fuels

− Cat fines

− Organic waste in fuel oil

− Homogenisers in HFO systems

− Change-over from high to low sulphur fuels and vice versa

− Use of fuel additives

− Fuel ignition quality

Low-sulphur Fuel Operation

− Sulphur trioxide corrosion on liner surface

− Lube oils on the market

− Experience and case stories

− Relationship between cylinder oil feed rate, BN, and liner and piston ring wear

− How does the industry prepare for low-sulphur fuel operation?

Summary

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Introduction This paper describes the CIMAC Heavy Fuel Oil (HFO) Working Group (WG), the membership structure, how it works, and the future goals for the group in order to continue making tools for the industry. This is followed by a description of fuel qualities and experience in the marine and stationary markets for MAN B&W two-stroke engines. CIMAC The history of CIMAC CIMAC, founded in1950 on a French initiative, is an international organisation promoting technical and scientific knowledge in the field of internal combustion engines (piston engines and gas turbines). This is achieved by organising congresses and working groups. It is supported by engine manufacturers, ship operators, technical universities, research institutes, component suppliers, fuel and lubricating oil suppliers and a number of other interested parties. The first edition of the CIMAC fuel recommendation was published in January 1982, and the fourth edition will be published this year. The fuel recommendation issued by the CIMAC HFO WG was used as a guide for both the BI (BS-MA 100) and the ISO 8217 standards, both of which were introduced after the first CIMAC recommendation. Today, the ISO and CIMAC fuel groups work closely together to ensure consistency in the standards and recommendations introduced to the market and to unite the workforce. This is done by a certain overlap of members. As a chairman you are automatically invited to become a member of the other groups. Several members of the CIMAC HFO WG are also appointed as being their national representative for ISO 8217. Experience shows that as the two groups make standards/recommendations overlapping each other, the optimum use of resources in the groups is to coordinate their work in order to avoid duplication. The most obvious differences between the two groups are the following:

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Table 1: CIMAC HFO WG and ISO 8217 comparison chart

CIMAC HFO Group (Marine and stationary plants)

ISO 8217 (Marine plants)

A) Recommendation A) Standard

B) Short lead time B) Long lead time

C) High flexibility C) Limited flexibility The representation of various companies and industries is within in the working groups. To be able to make tools for the industry, the groups must have representatives from all parties involved in the industry. This representation includes manufacturers, suppliers, users, and not least the fuel analysis institutes who have large databases of analyses of the fuels being used on the market. New fuel samples are continuously being sent to these organisations, so operators can be sure that only in-spec. fuel oil is used on their engines. With such analyses, trends and predictions can be made which, in turn, are good foundations for the HFO WG when deciding on new recommendations for fuel specs.

Fig. 1: Representation of various companies and institutes

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Fourteen different countries are represented in the working groups. Most members come from European countries, but the USA and Japan are also represented.

Fig. 2: Representation from 14 countries

The difficulty in making decisions in such a working group consists in the different interests from different parts of the industry, and in securing that single industry wishes are not placed in a higher priority than the best solution for the whole industry. If, on a case story basis, we look at the subject of “Used lube oils in fuels”, the following illustrates the main economic interests of different industry groups:

Fuel companies Low process costs Lube oil companies Easy disposal of ULO Shipowners Clean fuel at low cost Engine builders Our engines can operate on cheap, but

not too dirty, fuel. Fuel analysis companies The more analyses, the more earnings

Table 2: Economic priorities of WG interests In spite of this, the group’s task is to make tools for the whole industry.

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Description of tasks and recommendations for the Working Group Description of tasks and recommendations:

• HFO recommendations • HFO system installation • HFO measuring methods • HFO and emissions.

Group activities – papers:

• Recommendations regarding fuel requirements for diesel engines • Recommendations concerning the design of heavy fuel oil treatment

plants.

Paper distribution:

• Members • Technical organisations • Press.

So far, the above two categories of papers have been published in paper form only. This is being changed now, as we are able to introduce the papers on the Internet, from where it will be very easy for the Working Group to make revisions in order to ensure that the latest updates are available for the users when new observations and changes in the fuel oil market are seen.

Fig. 3: CIMAC recommendations for HFO fuels for diesel engines

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Connections between CIMAC groups Besides the ISO 8217 working group, the CIMAC HFO WG also works closely together with other CIMAC groups. The relationship between the Lube Oil and the HFO WGs has always been there, but in the last ten years the Exhaust Emissions WG has taken a more and more important role in the design of fuel oil and lube oil. As exhaust emissions originate from the combustion process, including the use of fuel and lube oil, cooperation between the groups is essential. The diagram shows some of the data shared between the groups during the course of their cooperation.

Fig. 4: Connections between the CIMAC groups New edition of the Recommendations regarding Fuel Requirements for Diesel Engines This edition of the Fuel Requirements introduces several important changes. Some of them are:

• Reduction of the number of residual fuel grades from thirteen to ten

• Change of temperature for viscosity measurement from 100°C to 50°C and, therefore, revision of grade nominations

• Incorporation of the future lower global limits for sulphur in emission control areas

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• Reduction of maximum water content to 0.5% V/V

• Revision of minimum viscosity limit for A 30 fuel grade

• A prohibition on the inclusion of used lubricating oil in marine fuel, controlled by means of inclusion of the limits for elements fingerprinting the presence of used lubricating oil.

This new edition of the Recommendations regarding Fuel Requirements for Heavy Fuels for Diesel Engines is available on the CIMAC homepage (www.cimac.com). New edition of the Recommendations on the Design of Heavy Fuel Treatment Plants The Recommendations regarding the Design of Heavy Fuel Oil Treatment Plants for Diesel Engines is also going through some large-scale changes, and the main changes are listed below:

• Definition of the Fuel Treatment System Fuel Properties • Layout of the Total Treatment and System Tanks • Fuel Cleaning System • Fuel Conditioning System • Sludge Treatment System • Fuel Treatment (additives) • Sampling

This new revision of the Recommendations is expected to be ready for inclusion on the Internet in 2004.

Fig. 5: Proposal for new revision

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Experience from Operation on Today’s Fuels A brief introduction to MAN B&W two-stroke MC engine programme Since 1982, over 7,000 MAN B&W two-stroke MC engines have been sold worldwide. As can be seen from the engine programme, the power range is from 2,000 bhp to 132,000 bhp per engine

Fig. 6: The two-stroke engine programme 2003

The MC engine is both for marine and stationary applications and is now also available in an electronically controlled version for engines of 50 cm bore upwards.

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Fig. 7: 10K98MC-C and 6S35MC on the same testbed Even though two-stroke engines have so far proved not to be sensitive, in general, to the fuels on the marine market today, the standard limits recommended by the ISO 8217 and CIMAC are important in order to ensure high reliability and long time between overhauls of the engine, to the satisfaction of the shipowners. The quality of the various fuels in the market is changing, one reason being the increasing demand for low-sulphur fuel, generated by environmental protection restrictions, which increasingly limit the use of normal fuel types. In addition, the engine design is also changing to meet the demand for higher power outputs, and thus the thermal condition in the combustion chamber changes as well. It is therefore important that we, as engine designer, follow fuel development, thereby enabling us to support our licensees and engine operators. In addition to the joint development of the tools for the industry through our memberships of the ISO 8217 and the CIMAC HFO WG, MAN B&W Diesel also participates in projects within the EU to investigate how low-sulphur fuel legislation can be introduced and what will be the possible effect of such legislation. The most frequently discussed issues on the marine fuel market in the last five years have been the dumping of polypropylene and the influence of the large amounts of waste organics which have been added to fuels in some unique cases.

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In both situations, the issue concerns the dumping of waste in the fuel oil and, with regard to the polypropylene, fuel filter blocking was the main problem whereas organic waste can have an impact on the actual performance of the fuel pumps and, thereby, influence the cylinder liner and piston ring condition. Both as chairman for CIMAC HFO WG and as an engineer in MAN B&W Diesel, it is very frightening to see the difficulties encountered by the industry, as it has been in the most recent case from Singapore, when trying to trace the source of organic waste contaminants and ensuring that they will not occur again. Without the possibility to analyse the pollution in the fuel oil the contaminants will probably reoccur. Off-spec. fuels Several selected off-spec. fuels (i.e. beyond ISO 8217) have, inter alia, been tested on the MAN B&W two-stroke research engine in Copenhagen:

• Natural gas (stationary 12K80MC-GI-S plant operating in Japan) • Bitumen • Orimulsion • Bio fuel (Holeby engines).

The stationary power generating market is somewhat different with regard to the quality of fuels. The supplier is often local, and the standards and recommen-dations from ISO 8217 and CIMAC are to be taken more as guidelines than as quality requirements. As such we have seen both sulphur content and viscosity outside ISO 8217 and CIMAC limits. Therefore, extra consideration should be given to the handling of the fuel and to the choice of lube oil used. A power plant engine was also the first to show the need for a change in the cylinder lube oil type (the BN number) because of the use of a relatively low-sulphur fuel. Cat fines Case story. Another plant, also with a local fuel supplier, showed a changing and, in some periods, extreme level of the cat fines (Al + Si) in the fuel. On this actual plant, the level of cat fines was measured at between 38 ppm and 125 ppm, whereas the limit according to ISO 8217 is 80 ppm (and not without reason). Moreover, the quality of the fuel, after cleaning, was reduced due to complications relating to the operation of the fuel treatment equipment.

Proper household practices of auxiliary machinery are important! The high level of cat fines led to a critical condition for the cylinder liners and pistons, resulting in a high number of cases of heavy wear or scuffing.

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Fig. 8 shows cat fines embedded in the piston ring surface, and the resulting influence on the cylinder condition. It is an almost impossible task to design engines and predict the MTBO if the engines are not properly maintained and the fuel and oil specifications are not followed.

Fig. 8: Catalyst particle pressed into piston ring surface

Before the real reason for the excessive wear could be determined, other possible causes were investigated:

• Scuffing caused by the ring pack. The scuffing cases had occurred for new liners as well as for honed liners with different ring packs. For the same reason, any deviations from the honing procedure could be excluded as the cause.

• Malfunctioning of the water mist catcher drain and the possibility of a water leakage from the air coolers was investigated and excluded.

• The piston ring grooves were checked and measured, and found to have little wear. Peeling-off of the chromium layer was observed at the end of the period. This appeared to be due to the poor cylinder condition.

• The cylinder lubricating oil quality had not been changed, and a high feed rate had been maintained.

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Fig. 9: The influence of catalytic fines on the cylinder condition These investigations were necessary in order to find the reason for the malfunctioning of an engine, but the situation could have been avoided if the fuel had been in accordance with the specification and the fuel cleaning system had been working properly.

Organic waste in fuel oil Case Story. Quality problems with bunker fuel supplied in Singapore in October and November 2001. It has been reported that a special bunker fuel supply affected some ten vessels and caused operational problems such as filter blockage, fuel pump sticking, liner wear and carbonaceous deposits in the engine. There has been quite a debate in the industry and in fuel analysing companies about what it is in the fuel that has caused the low quality of fuel. Chemical solvents such as xylene, toluene methyl benzene and alkyl benzene esters are some of the components that have been suggested as being the cause. However, the main question which still remains unanswered is the burden of proving the link between the contaminant and the engine damage. So far, no one has provided an explanation of how these esters could cause filter blockage or excessive liner wear. Of course, these solvents (also used in paints, lacquers, varnishes and paint strippers) might well be only one part of the whole story. The solvents might have been introduced into the bunkers with other industrial wastes, which could well hold a key to the better understanding of the mechanisms which cause damage. It seems, however, that this is still “guess work” in the industry

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However, we can conclude that:

1) Chemical waste can be difficult to trace and discover in fuel oil. 2) Chemical waste in fuel can have a very damaging effect on the engine. 3) No chemical waste should be dumped into heavy fuel oil for diesel

engine operation. Homogenisers in HFO systems In the design of the (auxiliary) machinery systems for engines, MAN B&W Diesel has safety, simplicity, environmental friendliness, easy installation and low operation cost as design criteria. We have therefore closely followed the debate about the use of homogenisers in the HFO system, and have had several meetings with suppliers such as Martec, Drew and SIT as well as meetings with the three HFO separator manufacturers. Through our member/chairmanship in the ISO 8217 and CIMAC Working Groups, the issue has also been raised, and the opinions expressed in these forums are very sceptical to the use of homogenisers. Strong reservations were raised with regard to the use of homogenisers before separators and, especially, to the use of homogenisers instead of fuel oil separators. The question is whether the homogenisers protect the engine from metals, including aluminium and silicon (i.e. catalytic fines), as well as sodium from salt water, and what are the consequences of their presence? A large amount of Al + Si undoubtedly has an impact on abrasive wear on injection equipment, piston rings and liners, but it is not certain what amount of Al + Si will cause damage, and what size of Al + Si particles is critical. In ISO 8217 and the CIMAC HFO WG, we work with fuel specifications to a max. of 80 mg Al + Si, which is seen in some fuels. We have experienced that when a centrifuge is out of order or is not being maintained properly, and the Al + Si level is relatively high, the result is increased wear, which can result in unscheduled maintenance and exchange of components. We can sum up as follows on our technical argument:

1. The homogenisation of the fuel will lead to a low separation efficiency of both water and particles:

• Both freshwater and seawater will be emulsified, i.e. the water

droplets will be too small to be removed by centrifugal separation. If they are not removed, there will be excessive amounts of seawater, containing chlorides, in the fuel fed to the engine.

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• Some contamination particles, like catalytic fines, are hydrophilic. The particles will then create non-separable aggregates together with emulsified water. This state is also valid for low water levels.

2. Samples that have been received from installations using homogen-

isers support the above statements. Water, sodium, aluminium, silicon and inorganic particle levels are generally higher with the homogeniser in operation than without. The intended effect of the settling tank is lost. The contamination level in terms of water, aluminium and particles seems actually, in some cases, to be higher after the settling tank than before. This is probably due to back-mixing of previously settled contaminants.

In tested cases, the treatment system is not able to bring down the contamination level. This is due to the homogenising effect. As a reference, the untreated bunker has been tested with a spin test in order to simulate the effect as if the oil had been treated in a separator without homogenising.

3. The samples that have been received show that the loss of separation

efficiency cannot be compensated for by the use of 10-micron fine filters.

On the other hand, if the fuel oil contains very little cat fines or other abrasive components and very little salt water, it might work with a homogeniser. However, if the fuel bunker specification is suddenly changed, the operator can end up with severe complications and costs. Regarding bunker fuels with a limited content of Al + Si, this will, in our experience, limit the choice of bunker sites, which can again influence on the fuel oil prices. Another risk is the accidental spill over of Al + Si into bunkers even at bunker stations with good reputations. In the eighties, some ship operators experimented with the use of very fine filters instead of HFO separators, but had to acknowledge that the risk of Al + Si damage to components was far too high. The question is whether it is worthwhile to take the risk. One argument from the homogeniser manufacturer is that the amount of sludge will be considerably reduced, and the use of HFO separators gives a loss of calorific value of some 2%. This would, of course, lead to a very high cost for use of an HFO separator, but are the figures correct? Not in our experience. In practice, HFO separators operate with max. 0.005% for the latest design of separators. In our experience with MAN B&W Diesel two-stroke engines, a homogeniser is not, in general, necessary in the HFO system to operate on HFO in accordance with ISO 8217 standards and CIMAC recommendations. There are, however, a

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few exceptions in the supply line downstream of the separator if water is added to reduce the NOx level in the exhaust gas. Other locations have not been neces-sary, not even for the so-called Asphaltenes. If Asphaltenes one day would become a problem, a much simpler solution would be a relocation of the fine filter to the cold supply line (see enclosure), but only if this should be necessary, which is still to be seen. In very special cases concerning some off-spec. fuels, e.g. fuel outside the ISO and CIMAC specifications, a homogeniser can be an advantage if centrifuges cannot remove the water. The homogeniser can blend the non-removable water into the fuel so that the fuel becomes less damaging to the engine. Such fuel is mostly used in stationary power plant engines. At this time, Marintek in Norway, in cooperation with several other companies, is making a test on homogenising before the centrifuge. This will give more informa-tion to work on. However, based on the above, we cannot today recommend the use of homogenisers before HFO separators, nor the use of a homogeniser instead of an HFO separator, where the last is considered the worse case. We are, however, very open to continuing the debate about the issue to find optimal solutions for the MAN B&W two-stroke engines. Change-over from high to low sulphur fuels and vice versa A very important part of the discussion of marine exhaust gas emission is concentrated on SOx. As engine designer, we have been and still are investiga-ting the possibility of doing SOx cleaning onboard the ship, but we have not yet found a solution which is economically and practically justifiable to utilise. Therefore, the only option available, so far, is that the ship should change between high and low sulphur fuels, depending on which area and related restrictions a ship operates under. We have already made such a change-over procedure possible several years ago, but the new feature is an MAN B&W Diesel patented system which will automatically take care of the change-over. This ensures that there will not be any danger to fuel pumps, etc. due to a fast change-over to high viscosity fuels and thus influence the performance of the fuel valve injection nozzles and risk of sticking fuel pumps due to changing temperatures. Use of fuel additives During the last six years, we have tested four fuel additives, and some operators find that the exhaust gasways, including boilers, are cleaner. Others find that there is no difference when using the additive. MAN B&W Diesel’s official opinion is that an additive is not needed for a normal fuel oil type that meets ISO 8217. We have no experience with the use of fuel additives in fuels with specifications outside ISO 8217. Therefore, MAN B&W

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Diesel does not recommend the use of any fuel additives in the engine under the guarantee period. Further, as licensor and designer of the MC engines, it is our responsibility to give the engine operator technical recommendations, so for us to give a “No Objection Letter” for the use of a fuel additive, the product has to be tested, and the tested engine to be inspected by our engineers. As for all other sub-suppliers of components, the additive supplier has to prove acceptable performance, and prove that the use of his product will not harm the engine. An engine operator needs to agree on a test, having MAN B&W Diesel to participate in the test and, if the positive results are proved, MAN B&W Diesel will give a Letter of No Objection for the additive in question. The duration of the test is a minimum of 4,000 hours, and one of our engineers will inspect the engine before start and after the 4,000 hours of testing. Liners, pistons, piston rings and gasways are all inspected to check the condition of the engine and engine components. Today, four fuel additives have successfully been through this test and been given a no-objection letter. Whether it is worthwhile for the operator to use a fuel additive is their own judgment. Fuel ignition quality Normally-applied analytical data for fuel oil contains no direct indication of ignition quality, neither do current specifications and standards. Although not an impor-tant parameter for engines with high compression ratios, a high density in combination with low viscosity could, as mentioned, be an indication of poor ignition quality. In a few cases (less than five), we have observed that the fuel had such poor ignition quality that the engines could not operate properly. Having said this, only off-spec. fuel oils mixed with non-fuel products are expected to have properties that can result in an ignition delay, which can affect the performance of MAN B&W two-stroke and four-stroke engines. Ignition quality can, to some extent, be predicted by calculations based on viscosity and density, using formulas issued by the oil industry (CCAI by Shell or CII by BP). However, tests carried out on the MAN B&W research engine in Copenhagen, Denmark, have shown that the CCAI and CII do not, in all situations, give a correct picture of a fuel’s ignition quality. Better methods are now available. Tests performed together with fuel analysing institutes show better indications of the ignition qualities of the different fuels. Test instruments utilising a constant volume combustion technology have been developed, and these are currently

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being used for marine fuel testing at a number of fuel laboratories and builders of marine diesel engines worldwide. By the use of calibration fuels, a recorded ignition delay can be converted into an instrument-related Cetane Number. In addition, the Rate of Heat Release is pre-sented, reflecting the actual heat release process and, thus, the combustion quality of the fuel tested. The test results reflect the differences in ignition and combustion properties of diesel engine fuels due to variations in the chemical composition of the fuels being tested. The difficult part is to correctly apply the data. What could be a low quality fuel, with regard to ignition properties, for one engine might not necessarily give any problems for a different engine design. Therefore, methods like FIA are consider-ed merely as a tool from which an operator, by experience, learns what FIA cetane number is acceptable for his engine. Low ignition properties will, all other conditions being equal, be more difficult to burn in a medium or high-speed engine, as the time from which ignition can happen without complication is shorter but, as mentioned above, very rarely happens. In the illustrations below, we have shown a list of different fuels, including the calculated CCAI value. All fuels have been working perfectly on our two-stroke engines, and still some have a CCAI value higher than recommended (around 850 max.).

Table 3: Different HFOs tested on MAN B&W two-stroke engines

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In Fig. 10, we have illustrated two FIA curves from two different fuels with very different ignition qualities.

Fig. 10: Fuel oil ignition quality According to the calculated CCAI value, both fuels are acceptable, however, according to FIA, Heavy Fuel 2 could give complications. Our point is that there is a difference between the calculated and measured values and the performance of the actual engine that a fuel is used on. Low-sulphur Fuel Operation Sulphur trioxide corrosion on liner surface The acid corrosion, which is by far the most influencing cause of wear seen in cylinder liners, is basically the result of the condensation of the HFO sulphur compound. The combination of the water that is present during the combustion process and a thermodynamic condition where temperature and pressure are below the dew point curve of the sulphur trioxide will lead to corrosion. Even though the water mist catcher of the scavenge air cooler removes water droplets, the scavenge air is saturated with water vapour when entering the cylinder. It is not clearly mapped how much sulphur trioxide is formed and what is the necessary time frame before the acid corrodes the surface of the liner wall, and before which new cylinder oil must be fed to the liner surface to neutralise the sulphur.

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Stronger Acid

Mo

re A

cid

Temperature,°C Pressure, Bar190

180

170

160

150

140

130

120

110

1000 1 2

Fuel Sulphur Content, %m3 4 5

200

100150

806040

20

10

1

Shell Acid Dew Point Curves (1978) Fig. 11: Shell acid dew point curves (1978) With reference to Ref. [2], the most recent dew point curves are listed in Fig. 12. These curves indicate that the old dew point curves from Belcher Ref. [3] (see Fig. 11) are not accurate enough for new two-stoke engine designs with higher outputs, new combustion chamber design, and a changed thermodynamic situation.

100

125

150

175

200

225

250

275

300

0 1 2 3 4 5

300Temperature,°C Pressure, Bar

Fuel Sulphur Content, %m

275

200

250

225

200

175

150

125

100

150175

12510075

50

25

10

1

Shell Acid Dew Point Curves (2000) Fig. 12: Shell acid dew point curves (2000) When the sulphur trioxide corrodes the cast iron, the chemical element iron will oxidise, and thus not contribute to abrasive wear, which would have accelerated wear in the liners. This means that, theoretically, it should be possible to control the corrosive wear when knowing the sulphur content, temperature, and pressure in the combustion chamber, as well as the dew point and the reaction time before sulphur dioxide corrosion occurs. Unfortunately, the relationship between all

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these factors is not that easy to establish in practice. Therefore, we have to deal with a few of the parameters at a time. Corrosion is among the mentioned factors that are influenced by the liner wall temperature and thus by the engine design with regard to the cooling of liners. This means that by keeping the liner temperature at a relatively high level, it is possible to reduce corrosion. However, high liner temperatures also place heavy demands on the thermal stability of the lube oil. This, in turn, calls for a choice of engine design that provides for both considerations. In this respect, the dew point curve is an important tool. If low-sulphur fuel is used, a lower liner wall tempera-ture can, in principle, have the same effect on liner wear as the reduction in BN, i.e. ensure the controlled corrosion that leads to proper lubricating conditions. Lube oils on the market For many years, BN 70-80 cylinder lube oils have been used because this level of BN, and the resulting neutralisation of the sulphur oxides present, provided the operators with a wide enough safety margin against corrosion, and also good detergency of the oil was achieved. Calcium compounds perform the major part of the neutralisation, but they also have a large impact on the detergency level of the cylinder oil. Calcium is good for the sulphur neutralisation and cylinder oil detergency with regard to the short-term cleaning of the combustion chamber. However, with regard to long-term deposit formation, we have often seen a large build-up on piston crowns and in piston ring grooves. Such deposit build-up can easily result in malfunctioning of the combustion chamber because of sticking rings or contact between deposits and the liner wall. As a countermeasure, we introduced the piston cleaning ring to prevent deposits from accumulating on the piston crown and touching the liner wall and, thereby, scraping off the oil film and exposing the liner wall to scuffing. However, this is only considered a solution that partly covers a technical problem. Thus, in spite of low-sulphur HFO operation for environmental reasons there is, in general, a need to lower the calcium content in the oil. However, this does not exclude the need for continuous short-term cleaning, which is still vital in order for the components to stay clean during engine operation. Experience and case stories of low-sulphur fuel oil operation We have only rather limited experience of low-sulphur fuel oil operation on two-stroke engines, and our experience mainly comes from engines used in power plants. However, the engine design is the same and, if all other conditions are equal, the relationship between the sulphur content and BN neutralisation of the cylinder lube oil is identical.

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For engines operating on gas or diesel oil, cylinder oils with a viscosity of SAE 50 and a TBN (alkalinity expressed as Total Base Number) of 10 to 20 should be sufficient, theoretically. This is the theory, because with the low sulphur level, only low TBN levels, and thus a low content of active calcium compounds, are necessary to neutralise the acid level but, in practice, only a few oils prove capable of this today. However, more oils are being tested now, and will be commercially available in the coming years. Correspondingly, long time use of lower-than-average sulphur fuels will, contrary to normal marine applications, call for the use of lower BN lube oils in order not to overdose the combustion chamber with deposit-generating additivated oils. It has been established that a certain level of controlled corrosion enhances lubrication, in that the corrosion (removal of iron) generates small “pockets” in the cylinder liner running surface from which hydrodynamic lubrication from the oil in the pocket is created. The alternative, no corrosion, could lead to bore-polish and, subsequently, hamper the creation of the necessary oil film on the liner surface resulting, eventually, in accelerated wear. This phenomenon also occurs on trunk piston engines, where a bore-polished cylinder liner surface hampers the functioning of oil scraper rings and leads to accelerated lube oil consumption due to the open access to the crankcase oil. Corrosion control – not avoiding corrosion – is therefore crucial, and adjusting the BN to the fuel oil sulphur content is essential. It should be considered that, irrespective of the sulphur content being high or low, the fuels used in low-speed engines are usually low quality heavy fuels. Therefore, the cylinder oils must have full capacity in respect of detergency and dispersancy, irrespective of the BN specified. This is a newly developed tech-nology now mastered by the well-reputed lube oil suppliers, who can individually tailor a cylinder lube oil to the relevant fuel. On some MAN B&W two-stroke power plant engines, we have experienced the occurrence of scuffing in several cylinders after changing to low-sulphur fuel. The wear rates measured on the cylinder liners that were not scuffed were extremely low. There had been no corrosion at all, and the liner surface was mirror-like without small pockets giving a proper tribology and hydrodynamic situation in the combustion chamber.

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TOO LITTLE CORROSION CAN RESULT IN TOO LITTLE WEAR AND IN DAMAGING POLISHING OF THE LINER SURFACE.

Fig. 13: Cylinder liner surface

Please also refer to Case Story 3. Basically, this means that if a BN 70 oil is used with a 1% sulphur HFO, the wear rate could become so small that there is a risk of polishing of the liner surface when the original wave cuts are worn away, and this would create a situation close to scuffing. Figs. 13 and 14 illustrate an anticipated optimum wear situation for BN 40 and BN 70 lube oil, respectively, with a controlled wear rate. Using an HFO with a sulphur content higher than 3% together with a BN 40 cylinder oil can lead to an uncontrolled situation with excessive wear. However, a sulphur content of up to 2.5% is considered within the controlled wear situation.

0

0,01

0,02

0,03

0,04

0 1 2 3 4 5 6 7

Sulphur %

Cylinder wear mm/1000h

BN40 BN70

Fig. 14: Comparison of sulphur content and lube oil TBN with respect

to cylinder wear, with equal cylinder oil feed rates

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Curve 2 shows that a sulphur content of 1% and a BN 70 cylinder oil can result in very little wear but, as described above, the liner and piston rings are more vulnerable to malfunction in that bore polishing could occur. In the following, three case stories are listed where MC engines have been operated on low-sulphur fuels and where low-BN lube oil has proved beneficial. In all cases, after discussion with the respective oil suppliers, the oil was given a No Objection letter for use in MAN B&W two-stroke engines operating on fuel with a sulphur content lower than 1.5%. Case Story 1 Ship operation Engine type: 8S60MC Fuel type: MDO max. 1% sulphur Cyl. oil feed rate: 0.95 g/bhph The engine has run on Marine Diesel Oil (max. 1% sulphur) for 14,537 hours. Cylinder unit Nos. 1 to 4 have been lubricated with Taro Special 50 in the same period. Unit Nos. 5 to 8 have continuously been lubricated with Taro Special 70. The lubricating oil feed rate was approx. 0.95 g/bhph at MCR, and normal service load is approx. 85-90% of MCR at 97 rpm. MCR power is 20,000 bhp at 101.3 rpm. With the intention of reaching a conclusion regarding the performance of TBN 50 cylinder oil in combination with MDO, compared with TBN 70 oil, two units Nos. 3 and 5 were used as reference, each representing the units having logged the most hours since the introduction of MDO and TBN 50 cylinder oil. Unit No. 3 (TBN 50) had run for 14,537 hours. Unit No. 5 (TBN 70) had initially run on HFO for a mere 843 hours before being switched to MDO, and had run for a total of 15,380 hours. The Taro special 50 oil has a less efficient cleaning ability, but a better matched sulphur acid neutralisation effect. The latter is obvious from the increased cold corrosion in the foremost four units (TBN 50) compared with the aftmost four units (TBN 70). Consequently, the most stable situation with regard to micro-seizures was found on the TBN 50 lubricated units. The overall cylinder condition was satisfactory. There was good gas sealing and mostly smooth and round rings. Some liners still have light remains of machining marks on the running surface. Considering the generally long overhaul intervals, combined with the still acceptable and low level of piston deposits, an advantage in terms of better balance between the corrosion/seizure conditions and an improved and satisfactory overall result was gained by the use of the BN 50.

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Below is shown the measured liner profiles/diameters.

Unit No. 3 (TBN50) Liner Diameters

600,00

600,10

600,20

600,30

600,40

600,50

600,60

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Stroke

Dia

met

er

F-A

M-E

Unit No. 5 (TBN70) Liner Diameters

600,00

600,10

600,20

600,30

600,40

600,50

600,60

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Stroke

Dia

met

er

F-A

M-E

*) F-A : Fore-Aft M-E : Manoeuvring-Exhaust gas side Fig. 15: Wear profile, TBN 50 and TBN 70 cylinder oils Wear profile TBN 50 and TBN 70 Max. total wear rates are calculated to be 0.014 and 0.011 mm/1000 hours, respectively. The higher total wear rate for unit No. 3 is related to a slightly increased wear rate due to increased corrosion during the test period. Case Story 2 Testbed running-in Engine type: Various S35MC engines Fuel type: MDO sulphur content below 0.2% Cyl. oil feed rate: Over-lubrication in accordance with the breaking-in programme

*)

*)

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In order to develop a running-in oil that provides good running-in properties and a clean piston during the test period, one of the major oil companies and MAN B&W Diesel carried out some tests at our Alpha Diesel works in Frederikshavn. The testbed tests were all carried out on the basis of low-sulphur diesel oil operation (i.e. sulphur content below 0.11%) because of the inland environmental requirements and the available fuel. The cylinder oil used by most engine manufacturers has been the standard BN 70-80 cylinder lube oil used in the market. Because of over-lubrication, this oil often causes very dirty pistons, which can be seen when the engine is inspected after the testbed trials, and this gives an inaccurate picture of the condition after normal operation. During the initial testbed running, when the so-called 'breaking-in' of the cylinder occurs, the cylinder oil feed rate is very high compared with the normal condition. This higher feed rate gives a lot of excess calcium carbonate (and less calcium sulphate due to the lack of sulphur), which is deposited on the piston crown. The endeavours to find a good running-in oil called for different tests and adjustments of the oil before the oil company provided an oil which showed controlled breaking-in of liners and rings and reduced deposit formation, also a precondition for the successful low-sulphur HFO operation. The photos below show the condition when using a BN 70 oil in combination with low-sulphur fuel oil.

Fig. 16: Condition after 17 hours on low-sulphur distillate fuel with BN70 commercial lube oil The detergency of typical cylinder oils is often linked to the BN additive. In the traditional understanding, higher BN means higher detergency. In trying to formulate a low-BN oil, the optimum solution, therefore, is not just simply lowering the BN, but to specifically design a low-BN oil with superior detergency to ensure excellent engine cleanliness performance. Below are examples from an exten-sive cooperation performed with an oil company, involving many engine inspections.

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Severe running-in with special NOx calibration - on 70BN Oil

Normal running-in. 70BN Oil

Running-in test. Failed 40BN Candidate

Running-in Test. Failed 40BN Candidate

Running-in Test Promising Candidate

Fig. 17: Experience with tested oils It is important, having established the cleanliness performance, that the oil is also reliable in other performance aspects, such as high temperature performance and anti-scuffing control, Ref. [4]. Case Story 3 Stationary plant Engine type: 12K90MC-S Fuel type: High viscosity HFO with a sulphur content of 0.2-1% Cyl. oil Feed rate: Currently at 0.85 g/bhph The plant entered operation in November 1998 and suffered from cylinder liner scuffing shortly after starting. The sulphur content varied, and the number of scuffing incidents increased when the sulphur content decreased. At the same time, the cylinder liner wear was extremely low on the engines that had not suffered scuffing.

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Fuel Sulphur Level

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,903

-05-

00

03-1

9-00

04-0

2-00

04-1

6-00

04-3

0-00

05-1

4-00

05-2

8-00

06-1

1-00

06-2

5-00

07-0

9-00

07-2

3-00

08-0

6-00

08-2

0-00

09-0

3-00

Sampling Date

Fu

el S

ulp

hu

r L

evel

, % W

t.

Fig. 18: Sulphur content This combination eliminated almost all corrosion on the liner wall and created a polishing effect of the liner. The oil itself has difficulties in adhering to a mirror-like surface, and metal-to-metal contact occurs, often resulting in scuffing which leads to extremely high liner and piston ring wear. The scuffing is self-increasing as metal-to-metal contact creates friction, and thereby heat hardens the liner surface and makes it difficult for the liner and piston rings to work together.

Fig. 19: Scuffed liner and piston ring surfaces In order to achieve controlled corrosion of the liner surface, and thereby normalise the tribological situation, a dialogue was started with the oil company, who was able to deliver a BN 40 cylinder lube oil to the plant within six months.

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Already during the first month of operation with the new BN 40, it became obvious that the cylinder condition had improved considerably, and in the cases where the liner had light scuffing, the surface was recovering. The BN 40 oil is now successfully used on all four engines, thanks to the extensive cooperation between the customer, the engine builder, the oil company and MAN B&W Diesel. Relationship between cylinder oil feed rate, BN, and liner and piston ring wear The above chapters describe the relation between the HFO sulphur content and the BN without mentioning the possibilities for adjusting the cylinder oil feed rate. Tests have shown that if the BN is low relative to the actual sulphur content in a fuel, a higher cylinder oil feed rate can reduce the wear because, in spite of a small concentration, the amount of active calcium compounds will increase and corrosion as such will be lower. Fig. 20: Cylinder lube oil feed rate and BN However, this is not the goal for engine operators, whose order of priority is firstly to achieve proper engine performance, and secondly to keep down the cylinder oil consumption and, thus, the engine operating costs.

Goal:

To minimise deposits

Uni-cylinder lubrication, to be able to change the BN

level and feed rate.

Intelligent lubrication

The calcium level is reduced by reducing the BN level in lube oil

Will reduce the neutralisation effect

The calcium level is reduced with a lower feed rate

Will reduce the neutralisation effect

Route preferred for Alpha Lubricator

Route preferred for traditional mechanical lubricator

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Consequently, the task is to find the optimum feed rate and BN based on the engine type and fuel oil used. On the basis of our statistical data of generally good engine performance experience, we have decided to lower our general lube oil guidelines, and a service letter has been issued to that effect.

Fig. 21: The Alpha lubrication principle – HMI panel

The introduction of the Alpha Lubricator, see Fig. 21, makes it possible to inject cylinder lube oil on the piston/piston ring pack when and where lubrication is found optimal. As can be seen in Fig. 22, the high pressure for the Alpha Lubricator provides this flexibility, unlike the traditional mechanical low-pressure lubricator, with which the lube oil cannot – due to the low pressure – be injected before the piston has passed. At the same time, we have found it necessary to introduce a max. recommendable feed rate of 50% above our guiding basic feed rate, to prevent over-lubrication.

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0

5

10

15

20

25

10 20 30 40 50 60 70 80 90 ms Time

Pressurebar

0

Injection pressureAlpha lubricator

Injection pressuremechanical lubricator

Oil quill pressure

1.2.3.4. Piston ring

Fig. 22: The Alpha lubrication principle – timing of cylinder oil injection For many years, it has been the general opinion that a cylinder cannot have too much cylinder lube oil and that, if an unstable situation occurs, more oil will normally redress the problem. This is not the natural cure on the engines of today. Experience shows that too much cylinder lube oil creates deposits which, in turn, complicates the ring movement and the topland cleanliness and, finally, can result in scuffing. To prevent this from happening, we have introduced the max. recommendable service feed rate.

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

0 600 1200 1800 2400 3000 3600 4200 4800 5400

Running hours

Cyl

ind

er o

il d

osa

ge

in p

erce

nt

of

bas

ic s

etti

ng

- Running-in feed rate- Basic feed rate- Example of service feed rate- Max. recommendable service feed rate- Minimum feed rate

Fig. 23: Cylinder oil feed rate during running-in

32

Fig. 23 shows MAN B&W's recommended range of cylinder lube oil feed rates based on our service experience from more than 5,100 MC engines in operation. Some operators work for economical reasons with a feed rate close to the minimum feed rate line. Experience shows that the wear in this area is much more sensitive to changes in the HFO sulphur content. In practice, this means that for the purpose of avoiding too much calcium or deposit build-up and polish-ing of the liner surface, it is not advantageous to operate with a limited BN level in the cylinder oil if the ship operates on HFOs with changing sulphur content, e.g. above 1.5% sulphur. Alternatively, correction for a possible high sulphur content will have to be made by increasing the cylinder oil feed rate. How does the industry prepare for low-sulphur fuel operation? Low-sulphur HFO operation will be increasingly common, especially in coastal areas. This means that when ships are in international waters, an up to 4.5% sulphur HFO (IMO cap) and a BN 70 cylinder oil might be applied. When entering restric-ted areas for periods of some length, a change could be made to a BN 40-50 lube oil, the HFO sulphur level being changed to max. 1.5% sulphur. This is a further complication for the operator, but it seems to be what the world is aiming at, see Fig. 24.

Fig. 24: Dual lube solution As such, ships need to be equipped with more segregated tank capacities. This, of course, also includes the fuel tanks for which the normal practice has been a very small DO tank for engine overhaul situations. However, with the use of low-sulphur HFO, extra heated tanks for this type of fuel will have to be considered during the ship's construction. On the other hand this means that, if

33

the ship's service pattern later on calls for operation for extended periods in emission-restricted areas, the ship is already equipped for this situation. On the lube oil design side, we will see low-BN cylinder lube oil with high general performance, tailor made for low-sulphur fuels. MAN B&W is cooperating with all the major oil companies to support their development of the cylinder lube oil of the future, and within the next year or so, the major oil companies will have low-BN oil for the latest two-stroke MC engine design in their programme. Summary

− Proper household on board is essential for good performance.

− No waste products should be added to the fuel oil.

− Fia and CCAI tests and calculations should only be used as a guideline.

− Homogenisers should not be installed before or instead of centrifuges.

− Engine design changes and lube oil development have to be synchronised.

− Sulphur caps are here to stay and will become stricter in the near future.

− The oil companies are designing low-BN lube oils in cooperation with the engine builders.

− Many new developments of low-BN oils are expected within the next year.

− One of the tasks for additive and lube oil companies is to design lube oil with less calcium dependency.

− Segregated storage facilities for different fuels and cylinder lube oils need to be considered in the design of new ships.

− MAN B&W Diesel feel comfortable with ISO and CIMAC fuel standards and recommendations

References [1] Egeberg, C.- E., “The Modern Two-Stroke Diesel Engine”,

ISME, Tokyo 2000 [2] C. Schenk, J. Hengeveld and K. Aabo: “The Role of

Temperature and Pressure in Wear Processes in Low Speed Diesel Engines”, ISME 2000

[3] P. R. Belcher, as cited in Elf paper “New method of measurement ….” for CIMAC Congress, 1998

[4] J. P. Liddy and K. C. Lim: “Cylinder Oils – Positioning for the Future”, Motorship Conference, Amsterdam, 29 March 2000