Kittiwake : A Fuel Sample - How it Works
A Fuel Sample - How it Works Taken literally, the value of a fuel sample is about $1.10. That is $1 for the bottle, label and seal plus $0.10 for the fuel. However, it is best not to take things too literally and in truth the value of the sample may rise to that of a claim for short delivery or even severe machinery damage. Taking a sample is therefore not a simple matter of filling the bottle as an afterthought at the end of a long bunkering. The care taken to obtain a representative sample should reflect its possible value. How is this achieved on the vast majority of bunker barges and ocean going ships who care to sample at all ? There are three broad techniques adopted: ● ● ●
Use an automatic sampler. Use a drip sampler. Do nothing or crack the flange right at the end of bunkering.
The first is frankly the most accurate option. However, they are heavy, expensive and very rarely found in practice. The third method is probably pretty common but useless. The second method is that most commonly found and can be implemented for a few hundred dollars.
Where ? First of all where to take the sample ? The point of custody transfer is the simple answer. In practice, this can either be at the barge or vessel end of the delivery hose. The vessel end is more accessible for the crew; the barge end provides a more reliable pressure head but access may be difficult. What with ? You will need a drip sampler of some design. Use one satisfying one or more standards such as ISO 3171 or PSA regulations. The construction is very straightforward and any marine engineer could fabricate a reliable sampler over a few sea watches. To comply with Port of Singapore regulations and also offer a fair sample to both the barge and vessel you will also need a device to lock the sample container and sampler valve. When you have the sample it should be transferred into the sample bottles, sealed and labelled correctly with both parties as witness. Do not use any old bottle to hand. Use a clean tamper evident container with some form of numbered locking device. How ? Fit the sampler between the hose and manifold valve or permanently behind the valve. Remember to put the joints in and watch your fingers when connecting the hose. A minority of vessels (e.g. some tankers) drop the fuel from the bunker manifold directly into a large bunker tank. This can produce a partial vacuum on the line at the bunker manifold (It makes taking a sample a bit difficult !) . For this reason, fitting the sampler at the barge end of the hose is
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Kittiwake : A Fuel Sample - How it Works
sometimes preferred. Flush the sampler with fuel as the delivery begins then fit the sample container. This is usually a 5 or 10 litre expandable plastic bottle or "cubitainer". Adjust the flow rate to give a SLOW steady drip and time the fill over a ten minute period as the delivery begins. Adjust as necessary then, if required, lock the valve and cubitainer with a suitable seal. If the cubitainer fills before delivery is completed, witness removal of the seal and fitting of a new cubitainer. Seal the full cubitainer until delivery is complete. When delivery is complete (and preferably before air purging of the lines) close the sampler valve, remove and seal the cubitainer. What then ? When all urgent tasks are completed, return to the cubitainers, shake well to mix the contents then fill into the sample containers. Filling should be done in 3 or more passes to provide a more reliable split of the sample. Seal the sample containers, label and witness. It is as easy as that.
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Kittiwake : Acidity
Acidity Marine lubricating oils are generally composed of base oils and additives. The purpose of these additives is to enhance the characteristic of the base oil or to give a specific characteristic. The base oils used in quality lubricants are carefully refined and may have a slight acidity because of the presence of small amounts of organic acid but this has no corrosive effects on machinery.
Lubricants in service frequently come into contact with air at high temperatures and become partly oxidised in the process. The rate of oxidation is minimised by the inclusion of a suitable anti-oxidant. Products of oxidation consist partly of organic acid bodies that may cause the lubricant to become corrosive to metals. Hence the degree to which acidity increases in service is a measure of the deterioration of the oil through oxidation or contamination. In many applications such as diesel engines, turbines and hydraulics, resistance of the lubricant to oxidation is important.
The acidity of a lubricant is expressed by the neutralisation number, known as the Total Acid Number (TAN). The test for mineral based lubricants is defined in ASTM - D664 and determines the number of milligrams of potassium hydroxide required (KOH) required to neutralise the acid content of 1 gram of oil. The units are mgKOH/g. All oils have a TAN value and what is important is not the initial value, but the change over a period of time.
It should be noted that diesel engine oils which have a level of alkalinity (expressed as Base Number or BN) may also have a TAN value. In the case of a trunk piston engine oil in service the alkalinity reserve is reduced because of the neutralising effect it has on the acidic products of combustion. The predominant one is sulphuric acid as a result of sulphur in the fuel. All trunk piston engine manufacturers recommend a minimum alkalinity value. If the engine was operated at lower values there would be an increasing possibility of the presence of a Strong Acid Number (SAN) (again determined by ASTM-D664). One might expect that this could not happen until the alkalinity of the oil had fallen to 0 mgKOH/g. Because the used oil charge is not totally homogenous, it is possible for an SAN value to exist even though there is a low level of alkalinity expressed as BN.
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Kittiwake : Acidity
Strong acids will initiate very rapid corrosion. At no time should a lubricating oil have a Strong Acid Number (SAN). If this is found in service the lubrication system should be flushed through and the oil charge renewed.
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Kittiwake : Directory of Bearing Failures
Directory of Bearing Failures Water
Acidic Attack
Insolubles & Debris
Viscosity
Overload
Misalignment
Fretting
Electrical
The illustrations below represent a compilation of damaged bearings from a wide variety of rotating machinery. Whilst the operation of most bearings is reliable and hence their operation un-noticed, some fail for often strange and unpredictable reasons. The listing below may provide guidance and illumination on a few of the possible causes. WATER
87 - general view of corrosion damage due to water 72 - severe black scab damage to a thrust pad, contamination during storage. also shows the effects of resultant seizure
93 - severe cavitation damage of an alternator bearing. Attributed to chronic water contamination
93a - severe cavitation damage of an alternator bearing. Attributed to chronic water contamination
70 - black scab damage (tin oxide) to the face of a thrust bearing. ACIDIC ATTACK
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Kittiwake : Directory of Bearing Failures
78b - corrosion of the lead phase of a copper lead bearing. Lead is particularly susceptible to acid attack, happily this type of bearing is now less common.
92- acid attack of bearing materials. The bearing material has been completely dissolved showing the overlay, nickel interlayer, underlay and steel backing. In consecutive order!
INSOLUBLES & DEBRIS 73 - scoring and impact damage from severe debris entrainment in the oil. Look at the debris tracks and also where the oil film was at a minimum. Close up machinery before shot blasting anything nearby!
75 - severe abrasive wear, look how the whole bearing overlay has been worn through.
76 -fine abrasive wear - similar to 74 but less pronounced
96 - circumferential scores due to a few large particles.
97 - multiple fine scoring from entrained debris (overloaded filter has caused the oil flow to bypass the filter hence debris removal was minimal. VISCOSITY
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Kittiwake : Directory of Bearing Failures
88 - thin film wiping due to engine over speed
79 - generalised bearing wear due to insufficient oil film thickness. Probably the result of oil dilution with a distillate fuel. OVERLOAD
68 - long term overload and resultant axial fatigue damage of a small end bush.
71 - thermal facetting of a white metal thrust pad (caused by severe alternate heating and cooling resulting in grain growth)
82 - fatigue damage due to interference between the bearing tang and the housing. The bearing was held away from the housing thus badly supported allowing it to flex.
89 - fatigue of the bearing overlay
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Kittiwake : Directory of Bearing Failures
98 - localised fatigue on a poorly supported bearing (design fault).
95 - central bore wiping caused by a differentially worn bearing journal. Typically only happens when new bearings are fitted as old bearings have worn with the journal.
101 - severely overloaded ball bearing pin showing plastic deformation, fatigue and scoring.
102 - fatigue damage to a roller bearing pin, less severe than 101.
MISALIGNMENT
77 - cavitation damage within the oil film. Perhaps not true misalignment but still caused by geometry. See also water.
94 - severe axial shaft misalignment overloading one edge of the bearing.
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Kittiwake : Directory of Bearing Failures
80 - cavitation erosion of the bearing groove edge
90 - hard rubbing in bearing bore due to interference between housing and tang. (see also 82)
91 - generalised overlay erosion attributed to cavitation at exit of bearing groove. FRETTING ALWAYS tighten bearing caps to the correct settings, use a well-maintained torque wrench/ hydraulics and lubricate threads according to the instruction manual.
74 - fretting damage to the edge of a bearing shell
78a - static fretting due to machinery exposed to external vibrations
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Kittiwake : Directory of Bearing Failures
84 - fretting again, this time it is severe in way of 83 - fretting on the rear of a shell bearing. This is the joint edge and cannot be allowed to continue. common but generally not threatening. Probably damage has also occurred to the bearing housing.
86 - fretting at two localised spots has resulted in high spots on the bearing face itself. Could eventually result in localised wiping, hopefully this would be self healing. ELECTRICAL
69 - electrical discharge damage to an alternator bearing, estimated at the 50mV level.
81 - scoring and pitting from electrical discharge on an operating high-speed alternator.
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Kittiwake : Black Paint
Black Paint
Medium speed diesel engines operating on residual fuel are prone to the formation of deposits within the engine, often called black paint or black sludge. This sludge may also affect auxiliary engine equipment including the lubricating oil centrifuge, filters and fuel pumps. The result is a requirement for more maintenance and a poor cosmetic effect. The deposits may be seen in the rocker box and generally in the sump. Deposits may also affect the piston ring pack to such an extent that they interfere with the correct lubrication of the ring / liner interface.
There are various theories why black paint exists. Some consider it to be a relatively recent problem that has arisen because of the increase in brake mean effective pressure and fuel injection pressure that causes the fuel to leak into the engine crankcase. Once there, the asphaltenes in the residual fuel come out of solution and form the characteristic deposits. Others believe that it is a fuel related problem because fuels have become more viscous and closer to the specification limit. Another theory is that the black paint problem is a fuel combustion problem and can be overcome by the use of a fuel additive, thus providing an economical solution when better fuel quality is not an option. In some engines and some lubricants black paint has been observed. However, it is interesting to note that other engines and lubricants have operated without any appearance of black paint. Even though these engines have been burning the same type of residual fuel, which on a world-wide basis gets closer to the specification limits. This would suggest that whilst fuel may be a contributing factor it is not necessarily the dominant one. It is well known that the modern medium speed engines stress the lubricant more than in the past. Indeed some designs are such that the specific lubricant consumption is low and the sump charge is small. Clearly the stresses on the lubricant is a factor that becomes more dominant as engine designs operate at higher and higher mean effective pressures with smaller sumps. This would suggest that the solution must lie with the lubricant. Trunk piston engine oil has various functions, which include provision of: ● ●
● ● ●
A gas seal and cleaning action for the ring pack. A transport medium for the alkaline additive system that neutralises the corrosive acids generated during the combustion process. Effective lubrication at the ring pack / liner interface. A coolant for the pistons. An oil film for the bearings and running gear.
Suitable base oil and additive selection meet these various functions. One of the aspects that are considered is the detergency of the oil. The lubricating oil works to limit black paint by agglomerating the asphaltene deposits so that the centrifuge can remove them. A consequence of this is that the centrifuge has to discharge more frequently but at least the deposits are removed from the engine.
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Kittiwake : Cool and Clean
Cool and Clean
How would you like to slide 300,000 km. a year, most of it at a boring 40 km.h? Fatigued? Yes, you would be. But that's about the distance the surface of the main bearing of a marine engine has to travel, irrespective of engine size, and do it for each year of its life, say 3 to 4 years.
That ride, although cushioned by the oil film, is not very smooth, clearing the bearing surface by a minimum of 2 micron in a 200mm bore engine to a maximum of 0.5mm in a 600mm bore engine. Take, for instance, an engine with a bore of 350mm having a main bearing diameter of say 300mm. The combined forces from the rotating, reciprocating and cylinder gas loads give the crankshaft journals a bumpy ride over the bearing surface. In the worst bearing case the clearances can vary between 3 micron to 0.25mm. Maintaining these minimum clearances throughout the life of a bearing without incurring premature wear or damage caused by dirt, overheating, fatigue, cavitation etc. requires not only good initial engine design and operation within its performance parameters, but that the lube oil is kept in good condition. The engineering crew must adhere to the filtration and cooling regimes laid down by engine and equipment manufacturers. Forget to change the filters and the pressure drop increases across them. Higher pressure drop - lower oil flow and increased oil temperature! A rise in oil temperature has the biggest influence on oil film thickness in the bearings, far greater than a reduction in oil pressure or changing oil grades. Forget to clean the coolers and up goes the oil temperature (Yes, there are thermostats, but they have a limit). Changing up an SAE grade or increasing oil pressure by 0.5 bar may increase oil films by 5% (don't operate outside the design spec.), but an increase in oil temperature by 10 deg. C may decrease oil film thickness by 30%, that's down to about 1.5 micron on a generator engine and 5 micron on a 600mm bore engine. Filter mesh size is about 10 to 15 micron. How does the dirt go through the bearing? How do the bearings work? They do, and you have to help by keeping the oil cool and clean. Oh, and don't forget the big end bearings. They can run safely with smaller oil films, but that's another cool clean story.
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Kittiwake : Water in Lube Oil
Emulsifiable sterntube lubricants Benjamin R Vickers & Sons Ltd is an independent company specialising in the development, manufacture and world-wide supply of a range of specialised lubricants for the marine industry. Areas of application include thrusters, stabilisers, CPP’s and primarily sterntubes where the benefits of an emulsifiable oil can be fully realised. Water contamination of the sterntube system is a frequent occurrence as a result of small, persistent water ingress or, in more dramatic cases, resulting from damage to the sterntube seal. Water contamination of the sterntube can lead to : · Corrosion within the sterntube and of the sternshaft. · Loss of lubrication with the potential for increased wear down, increased running temperatures and even wiping of white metal bearings. The Vickers Hydrox sterntube oils are designed and formulated specifically to meet the requirements of oil lubricated sterntube bearings, providing optimum lubrication conditions. The Hydrox oils absorb any water entering the sterntube, reducing the potential for problems associated with free water, and forming a ‘water in oil’ emulsion that maintains a very high level of corrosion protection and lubrication. The emulsions formed have excellent stability therefore free water is not released even after prolonged standing. The Hydrox oils will absorb very high amounts of water whilst maintaining the corrosion protection and lubrication properties though a maximum recommended water content of 20% is advised for normal sterntube applications. In very many cases the properties of the Hydrox oils have enabled vessels to maintain their normal operating schedule and to avoid the high costs of carrying out emergency repairs or even dry dockings. To assist Hydrox users in determining and controlling the amount of water present in the sterntube oil Vickers offer a free water in oil analysis service and alternatively have tested and are pleased to approve the Kittiwake water content test kit for immediate onboard analysis Further information regarding the properties, use and availability of the oils is available on our website: www.vickers-oil.com.
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Kittiwake : Grease - the Cinderella of Lubrication
Grease - the Cinderella of Lubrication
Ask anyone to describe a lubricant and they will probably think of the liquid product used in an engine. There is however a large family of lubricants that are not liquid at all and often get overlooked for this reason. These Cinderella's of the oil industry are the greases used to lubricate bearings or slide ways and protect wires throughout the ship.
Greases are usually made from mineral oil stock of quite wide range in viscosity but still similar to those used for more liquid lubricants . Synthetic oil stocks are sometimes used for extreme temperature applications. Having selected a base stock it is then necessary to add thickeners to solidify the lubricant, to improve the temperature stability, retention and load carrying properties. In conventional greases these are metallic soaps of calcium, lithium, sodium etc. Sometimes other compounds such as clay, silica or more exotic synthetic compounds are used The next products to be included into the grease are the additives. These perform similar functions to those in more conventional lubricants and are used to impart specific beneficial properties to the grease. Oxidation and corrosion inhibitors, pour point depressants, extreme pressure additives, pigments, water repellents are all typically found to some extent. Solid lubricants such as molybdenum disulphide, graphite, polyethylene even PTFE for heavily loaded applications. A few more greasy facts that may be of interest include : ●
●
●
● ●
Over greasing a bearing can be just as damaging as under greasing. Correct application and flushing are the key to long bearing life. Greases of the same thickener are usually compatible; those of different thickeners may be incompatible. Greases are very good at absorbing water and can take between 40 and 100% of their own weight in water without failure. Heavier greases have better water wash resistance than lighter ones. The re-lubrication interval of a bearing should be halved for every 15 - 20oC increase in operating temperature.
So it appears that the humble grease can contain quite a complex product even if it is often regarded otherwise.
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Kittiwake : Water in Lube Oil
Hydrox Sterntube Oils What are the Hydrox sterntube oils? · Designed and formulated specifically to meet the requirements of oil lubricated sterntube bearings, providing excellent anti-wear properties under boundary and hydrodynamic conditions and excellent corrosion protection properties. · The oils are fluid and designed to be suitable for use in modern circulatory oilfeed systems and are formulated to have good compatibility with rubber lip seals, being approved for use by the major seal manufacturers. · The Hydrox oils absorb any water entering the sterntube, reducing the potential for problems associated with free water in the sterntube. The oils form an emulsion with water that maintains a very high level of lubrication and corrosion protection. The emulsions formed have excellent stability therefore free water is not released, ensuring corrosion protection and lubrication even after prolonged standing. · The oils will absorb very high amounts of water. The maximum water content recommended for normal sterntube lubrication is 20% although the oil will actually absorb much higher amounts than this. When a water content of 20% is reached we recommend draining some of the emulsion and replenishing with fresh oil. Hydrox 550 A lower viscosity grade, approximately SAE30, designed for normal/regular running in the sterntube where its use will provide insurance against the effects of any slight or persistent water ingress, but will also protect the sterntube in the event of any sudden water contamination caused by damage to the seal. Hydrox 550 emulsions remain fluid even with high water content in the oil, ensuring free circulation of the oil. Hydrox 550 is suitable for use as a combined sterntube/hydraulic oil and therefore is suitable for use with many controllable pitch propeller systems.
Hydrox 21 A higher viscosity version, approximately SAE 50, with the same water absorbing, lubrication and corrosion properties as Hydrox 550. Proven in practice to be very effective in reducing the rate of sterntube oil leakage from a damaged or worn aft seal. A leakage reduction of 70% or greater is commonly reported where Hydrox 21 has been introduced.
How should the Hydrox oils be used? Both Hydrox grades are designed for use as sterntube lubricants in their own right though Hydrox 21 in particular is frequently used mixed with existing sterntube oils due to its ability to help in emergency situations. The Hydrox grades are generally compatible with the oils commonly used for sterntube lubrication and whilst compatibility cannot be agreed without specific laboratory testing, problems have never been encountered in practice over many years world-wide use. The oils can therefore be readily introduced by addition to the existing oil. If it is not possible to completely change the sterntube system to the Hydrox oils then as much of the Hydrox should be added to the system as possible to obtain maximum benefits. A minimum of
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Kittiwake : Water in Lube Oil
50% oil change over is normally recommended. To introduce Hydrox into a sterntube we recommended draining the main header tank of the existing oil, fill the header tank with Hydrox and then circulate the oil through the system. This process can be repeated as necessary and the header tank topped up with Hydrox as required. Where a separate oilfeed to the aft seal is present, this should also be filled with Hydrox. Further information Stocks of Hydrox 550 and Hydrox 21 are available world-wide. The ability of Hydrox emulsions to protect the stern shaft and bearings in the presence of water is recognised by some of the major classification societies, e.g. Lloyds, DNV. Further information regarding the properties, use and availability of the Hydrox oils is available on our website: www.vickers-oil.com.
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Kittiwake : Ignition Quality
Ignition Quality
It is only a split second between injection of a fuel and the start of combustion. That small but finite time has a great significance for the life of an engine. Have you ever heard a knocking noise that you suspected was not the cross head bearing or does the generator sound unusually noisy this watch? The ignition quality of a fuel is a measure of the relative ease by which it will ignite. Distillate fuels measure this by the cetane number using a special test engine. The higher the number, the more easily will the fuel ignite in the engine. Unfortunately the test is expensive and the test engine will not operate on residual fuel grades. It is therefore of little direct use for the majority of marine fuels.
frequently quoted.
Ignition quality of residual fuel is calculated using two empirical equations, both based on the density and viscosity of the fuel. These are the Calculated Carbon Aromaticity Index (CCAI) and Calculated Ignition Index (CII). The CCAI typically gives numbers in the range 800-870 and the CII gives values in the same order as the cetane index for distillate fuels. Of the two equations, CCAI values are more
What happens after the start of injection ? Fuel takes a finite time from injection to the start of combustion, this is called the ignition delay. During this time the fuel is intimately mixed with the hot compressed air in the cylinder. When combustion begins the initial or "pre-mixed" phase of combustion is very rapid. The longer the ignition delay, the more fuel that will have been injected before ignition and therefore the more extensive is this phase of combustion. An important point to remember is that ignition delay is time dependent whereas injection rate is crank speed dependent. It is for this reason that slow speed, large bore engines are less sensitive to fuel quality than medium speed engines as proportionally less fuel is ever injected during the delay period.. The second or "diffusion burning" phase of combustion is controlled by how rapidly the oxygen and remaining vaporised fuel can be mixed. This is because the initial supply of oxygen near the fuel droplets has already been used during the pre-mixed phase. Rapid, pre-mixed combustion causes very rapid rates of pressure rise in the cylinder resulting in shock waves, broken piston rings and severe overheating of metal surfaces. Diesel engines are designed to withstand a certain rate of pressure rise within the cylinder although the figure will vary between different engine designs.
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Kittiwake : Ignition Quality
Rate of pressure rise Comments (bar/ƒcrank) ● ● ● ●
Below 10 No problems 10-12 Acceptable 12-16 May cause problems Over 16 Probably damaging
CCAI is an empirical attempt to estimate how long the fuel will take from injection to ignition and by implication the likelihood of engine damage. Knowing the CCAI, the operator must then judge the acceptability of that fuel for effective operation in the engine. Variations of engine load, rated speed and design alter the likelihood of poor combustion hence it is impossible to give precise figures that apply to all engines. The illustration shows typical values for a range of medium speed diesel engines. Be aware that these predictions are most accurate under full load engine conditions. Fuel performance at light loads can be very different.
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Kittiwake : Major Advance for Bunker Fuel Testing
Major Advance for Bunker Fuel Testing Kittiwake have grown by understanding the needs of the operator. The results from a recent market survey indicated that existing designs of on board fuel test equipment was not user friendly and needed modernisation. What was required was firstly accurate and user friendly equipment and secondly, a co-ordinated range of equipment that would handle everything from sampling and sample storage through to testing, dispute procedures and finally crew training. A new range of test equipment was designed in close co-operation with UNITOR who were able to offer direct feedback from a very extensive customer base. UNITOR Product Manager, Erwin Gorlitz was quoted as saying: "Fuel is often the largest operating expense for a shipowner. Some 70 to 80% of all fuels bunkered are not subjected to a laboratory analysis and this leaves the shipowner in a very vulnerable position, totally dependent upon the integrity of the fuel supplier. On board test equipment has existed for many years but our customers increasingly regard the existing choice as dated and in need of improvement. We therefore set out to design rugged, reliable and user-friendly sampling equipment employing the very best in modern electronic technology". Residual fuel oils contain large proportions of "left overs" from secondary refining, i.e. visbreaking and catalytic cracking etc. A particular consequence for marine fuels has been the increase in use of higher viscosity and density grades that sell for the lowest prices. Also as the various impurities carried in the crude stock are not extracted with the more valuable hydrocarbon fractions they remain and are concentrated in the residual fuel grades. Today the engine designer has to develop machines capable of operating on the worst grades of fuel available. This is not an easy task as the properties of these fuels are constantly varying. Once the fuel is bunkered, it is the chief engineer's responsibility to see that it is both acceptable and provided with the correct treatment to render the fuel suitable for use in the engines. Fuel has to be settled, purified, preheated and filtered etc, in order to render it fit for injection systems. During handling and treatment on board, a number of problems can occur. These problems differ in scope and severity from fuel to fuel and ship to ship but it is correct to say that every engineer has experienced them as a matter of daily routine. The problems related to the use of residual fuel can roughly be divided into three groups, pre combustion, combustion and post combustion related. Careful handling and pre-treatment of the fuel can solve or alleviate most potential problems and the engineer should have good information to hand about each fuel on board (e.g. compatibility or stability rating). Some pitfalls cannot be easily solved by physical means alone and it is in this area that fuel treatment chemicals prove extremely cost effective. Additives such as stabilizers, emulsion breakers, combustion catalysts and improves, ash modifiers fuel biocides etc all have a useful role to play provided that they are applied with the backing of good technical advice. Unitor are recognised for their expertise in fuel treatment and fuel chemical technology. Kittiwake test equipment forms an integral part of this process in providing accurate information on fuel properties and potential problems before they become serious ones. Together the combination is unbeatable. Microchip technology has been used throughout the range of Kittiwake fuel test equipment to provide a number of key advantages including; fast and accurate results, automatic self calibration, correction of measured results to standard reference conditions and estimation of derived parameters such as CCAI. The range of new equipment includes: Density Meter Fuel oil density is perhaps the most important parameter to measure. Underestimating the density by 0.5% will cost the owner of a typical bulk carrier the equivalent of 45 mt/year of fuel oil. Heated Viscometer Knowing the viscosity of the fuel is crucial for reliable operation of fuel handling and storage equipment. Viscosity is also a key indicator of combustion performance. Low
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Kittiwake : Major Advance for Bunker Fuel Testing
viscosity, high density fuels can cause severe engine damage if handled incorrectly. Fuel oil compatibility/stability test Mix two incompatible fuels and you will need to clean the fuel tanks of copious amounts of sludge by hand ! The test follows ASTM procedure D 4740 to quantify potential stability and compatibility problems. Water in Oil Test Water is the most common fuel contaminant and for which no one likes to pay up to $100 mt. Understating water content by 1% costs the average shipowner a further 90 mt of fuel per year. Detailed Technical Manual Test results are only of use if the ships engineer has a sound understanding of what they mean. The equipment comes complete with a clearly written and well illustrated, 80 page technical manual detailing standard terms, bunkering procedures, sampling and testing of fuel oils, dispute procedures and international standards for marine fuel oils. The new equipment from Kittiwake and UNITOR is designed to test the complete range of commercially available bunker fuels from, ISO fuel grades DMB through to RML 55. A full suite of tests can be completed within 10 minutes with a further 3 minutes to completely clean and store all equipment. It is available as individual items or as a complete Oil Test Cabinet specifically aimed at the new build or refit market.
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Kittiwake : All That Glitters May Not Be Gold
All That Glitters May Not Be Gold
Figure shows scuffing marks on a fuel injector guide. Recent events have paid much attention to the damage caused by engine and boiler exhaust emissions. The argument is that these are inherently bad, damage the earth and anything that can be done to remove them is by definition good. One of the prime targets for legislation in this respect is the sulphur content of marine fuel oils. Now it is a common misconception that residual fuel oils are low quality fuels that can only be used by virtue of the engine designers ingenuity. Anyone concerned with providing the highest quality fuel would specify a distillate blend if only he could afford to pay the price. This misconception was popular when I was at sea where clear, bright gas oil was treated with reverence. I believe that the same holds true today. Examine the photograph above. This is part of a fuel injector removed from a small 2 stroke marine diesel engine. The injector is uncooled as the engine was operated on distillate fuel. A close examination will reveal longitudinal polished marks on the guide faces. These marks are also present in the bore of the injector body. What do you think caused these marks on all the fuel injection equipment in the engine ? The answer is quite involved. Over the last 20 years, diesel fuel used in road haulage has been subjected to increasingly stringent legislation with regard to the sulphur content. The latest legislation to apply has been grouped under Euro II emissions limits, part of which specifies a maximum sulphur content of 0.05%. Petroleum refineries are large and expensive capital plant. Once an investment is made, the facilities are used to their capacity. European refiners have invested heavily in such plant capable of producing very low sulphur product streams as have those in the USA. This investment is necessary in order to satisfy the demands of automotive and land based power generation industries that are subject to stringent environmental legislation. Now, very low sulphur diesel is often treated in a a similar manner to the "winter diesel" used in trucks which must operate in cold climates. Kerosene is often added to winter fuels as a cloud point depressant. However, the lubricity of kerosene is poor and the fuel must be improved by the use of additives to prevent wear of the fuel injection equipment. Not so the fuel stream entering the marine bunkering market. Here the only requirement is for a maximum sulphur content of 1 - 2% (DMX to DMC of ISO 8217) . Very low sulphur fuels (e.g. 0.1%) are present in the market and perfectly acceptable according to the standard. Sulphur produces acidic exhaust gasses that cause environmental damage. Compounds based upon sulphur are also used in lubricants to prevent scuffing of metal surfaces under extreme pressure conditions (EP additives). The sulphur found naturally in fuel performs a similar function preventing scuffing on the sliding surfaces of fuel pumps and injectors. Take the sulphur out and this beneficial effect is removed. Very low sulphur fuels that meet the existing ISO specifications can unfortunately cause the fuel injection equipment to wear and sometimes completely seize. The injector shown used fuel that satisfied the ISO specification. It was a distillate and therefore considered to be good quality but was basically unfit for purpose as the crew later discovered. Caveat Emptor - let the buyer beware.
file:///D|/wwwroot/knowledge_base/technical_library/merrev.html7/13/2007 7:23:15 AM
Kittiwake : O-Rings
O-Rings O-Rings are strange little things preferring a pressurised existence than a life of idleness. I find it quite mystifying how that simple bit of rubber can seal reliably against quite immense pressures and yet leak when the system is not used. O-rings are a form of seal made from varying grades of flexible materials which display chemical resistance and elastomeric behaviour over a wide range of temperatures. They are often grouped under a general heading of rubber. They are usually toroidal in shape and circular in cross section although this can vary, particularly on sliding or rotating seals. A typical example for routine to arduous applications would be a low compression set flourocarbon elastomer such as Viton. This would be made from a blend of the base polymer, filler, plasticers and curing agents. During manufacture, the o-ring is first moulded then subjected to a curing cycle that will, in conjunction with the base materials and additives, determine the final properties. In use the O ring is fitted in to a groove typically rather larger in cross section one one axis and smaller on the other than that of the O ring itself. This allows for the O ring to compress and distort freely during assembly and for high-pressure fluid to act directly on the side of the ring. The mechanical compression between components and just as importantly the compression due to fluid pressure on the ring results in a sealing force between the elastomer and metal surfaces. This compression coupled with thermal cycling causes the O ring to develop a permanent set which increases with operating hours and greatly reduces the sealing force. The result would be leakage but luckily, this effect is balanced by seal swelling with operating temperature and by the swelling action of the lubricant as fractions become absorbed into the elastomer. The O ring eventually reaches an equilibrium condition between compression set and seal swelling, typically after about 1000 hours operation on a single lubricant. This last point has importance when an oil grade or manufacturer is changed as different oil formulations can change the amount of swelling in the elastomer. This swelling action is primarily influenced by the lubricant base stock and by the degree of lubricant degradation in proximity to the seal (especially on high temperature applications such as gas turbines). In this respect the elastomer acts like a molecular sieve allowing lower molecular weight materials to be preferentially absorbed. If swelling becomes excessive then the O ring may be extruded from the groove resulting in almost certain leakage. Another common cause of failure is gradual reaction with the lubricant causing hardening (polymer cross linking) of the elastomer surfaces. The effect is often visible as hard-flattened surfaces, often having a crazed appearance or cracking when flexed. So, elastomers need fluid pressure for an effective seal. This is why they may leak when idle but not when in use. The higher the pressure of the working fluid the better the seal. They will distort with time and swell in contact with a lubricant. Swelling rates can change and the surfaces can become hardened with time, temperature and incompatible lubricants. What is the best policy when assembling equipment after overhaul? - Fit NEW O rings!
file:///D|/wwwroot/knowledge_base/technical_library/o_rings.html7/13/2007 7:23:49 AM
Kittiwake : The Price of Sampling Oil
The Price of Sampling Oil Used Oil Analysis has been widely adopted by all lubricant suppliers in the perennial pursuit of customer service and as a clear means of adding value to a product that has become (perhaps unjustly) viewed as a commodity. The first companies to offer a "used oil analysis programme" (UOAP) gained a considerable marketing advantage. Others were not slow in responding and the net result was a reduction of this marketing edge whilst the cost of servicing each customer correspondingly increased. Certainly there are differences and options in levels of service between the market players, some now charge for the service in full or in part but the position is basically unaltered. The service represents a direct cost to an industry where margins are under heavy pressure.
Why have a sampling programme? - The value for the customer will be dictated by the sampling frequency, speed of reporting and quality of interpretation. The value for the supplier is customer loyalty and a fertile opportunity to increase the level of customer interaction from machinery diagnosis reports to Web based inventory and logistics control all undertaken directly by the lube supplier. The cost of it all - Two major cost items are the sampling kits themselves and the logistics of getting those kits to the customer on time. The authors' company supplies UOAP systems and logistics to over half of the worlds deep sea fleet and a number of large European and US based industrial services. This article attempts to illustrate where savings can be made without compromise to quality or service. So what is the cost of a kit of 12 sample bottles? A 100ml HDPE bottle should come in at about $0.25 so 12 of them makes it $3.00 each - wrong! The cost is for the whole system; the kits themselves represent only a small and fairly constant proportion of this. For an indication of where other costs lie, read on. These costs can be logically split in to a number of groups: Hidden costs Equipment costs Shipping costs Hidden costs - what does not get billed to your cost centre does not exist - wrong. An example of this is where trained and expensive sales engineers are used as a cheap distribution system taking equipment from stores, collating labels, even packing bottles into kits. A $60,000 employee in the US may cost the company $120,000 a year with overheads making the price for packing kits $60 an hour! Another good example is where programmes are organised at the affiliate level, common with industrial oil analysis programmes but still a few examples exist in marine spheres. Here advantages for bulk buying, common image and systems are lost and the attendant costs get hidden in a large number of smaller units; too small to address at the corporate level but none the less real for that. Time can also be called upon on receipt of the sample. Poor label systems will confuse customers and it will require time to correct the information (remember $60 an hour). Hidden costs include wastage. From experience a typical marine oil analysis programme of 100,000 samples per year would consume anything between 130 and 180,000 sample bottles. The remainder disappear and do not re enter the system at some later date. Presumably they form a handy source of bottles for other purposes. It is notable that some companies are more successful than others in minimising the wastage, typically they are also the ones who focus on co-ordination and reporting to provide accountability and clarity to the process. Hidden costs can also be in-built with outmoded and ostensibly unchangeable procedures. For example what is the necessity to constantly confirm that a sample kit order has been shipped?
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Kittiwake : The Price of Sampling Oil
Shipment should be contractually guaranteed the same day as a request; delivery details from courier and some air freight services are widely available over the internet, shipping charges confirmed at the month end. Ordering and reporting structures should be simple, clear, accountable and their logic regularly challenged. Equipment costs - Logic would indicate that the design of the kit would have a large effect upon the price. Surprisingly this is not reflected by reality and the spread of costs across a wide range of differing kit designs is relatively little. In the final analysis, plastic bottles, cardboard boxes and paper labels are cheap items. Some trimming of costs can however be made. By effective design of the labelling system, the number of separate labels can be reduced to a single multi part sheet, the new system from Elf is a good example of this concept. Pre numbering the labels can save confusion and labour in the laboratory and is being increasingly used to streamline this final stage, FAMM, Esso/ Exxon - Exxcare and again the Diagomar-plus programme from Elf have all adopted this to varying extents. A logical step would be to add bar code system but unfortunately bar codes and black carbon loaded oil stains do not mix well. Order quantities can be used to reduce costs, a 3 year supply contract allows stocking of a much larger production quantity and fixed pricing over the duration for major cost items. An alternative approach to labelling is the system adopted in various forms at least one marine lube oil supplier, Fuchs Lubricants and most recently SKF for their new Oil Analysis Service (the latter two are industrial programmes). The sample kits are supplied without labels. The labels are generated independently, using the customer database system to pre print details of machinery, sample point and lube grades. Several systems are now based on the Web or similar servers allowing the generation of customer specific sample labels. The up front costs of such a system are higher but the clarity of sample information is greatly improved. The number of final laboratories can complicate an analysis service, more laboratories means greater complication in shipping instructions, more address labels and general confusion. A world-wide marine oil analysis programme can be effectively serviced from just 3 strategically placed laboratories. Industrial programmes are more difficult to service in this way. They generally need more laboratories and languages but this need not unduly complicate matters in a well designed system (e.g. SKF provide a common kit design with 10 language options). You get what you pay for is an old and well used adage but fails in its simplicity. Attention to production processes and order quantities can mean an improved product for the same price. The sample kit is the most visible part of the service, influencing the users perception of how they may or may not benefit. Printing of boxes is a good illustration. Brown card boxes are still sometimes used, they are the cheapest of all but the image they portray is not good. Flexo printed (rubber print roller) is almost the same price and looks better but only allows for simple artwork and at most 2 colours. Screen printing allows more flexibility but is labour intensive. Litho printing (4 colour on to paper or directly on to the board) provides an excellent image and flexibility on what information is presented. The cost is around $0.30 per kit more provided the total number of kits is above 3-5000 over the duration of a contract. Shipping Costs - This is perhaps the most fertile area for cost reductions, especially in marine oil analysis services. Two systems are used to varying degrees. Firstly sample kits are sent directly to the ship or end user. Secondly kits are sent in bulk to local affiliates for delivery with oil supplies. To place this in perspective, the charges for courier deliveries are typically equal to or in excess of the cost of the kits themselves! The empty sample kits contain mostly fresh air. Shipping charges will be calculated on an equivalent to volumetric weight basis (LxWxD in cm / 6000). Anything that can be achieved in reducing the volume of the kit will result in a direct and proportional saving in shipping costs. Reducing the sample volume is a good starting point but is also dependent upon the final analysis technique. FTIR methods use very small sample volumes but are best suited to industrial oil samples as the reference oil sample is fixed. A 50 or 60ml bottle is about the minimum volume used for marine analysis services. They save little in production costs when compared to the more common 100 or 120ml volume but produce a 25-40% reduction in delivery charges. Similarly, the kits should be designed to pack efficiently. Shipping charges are made from 2 components, fixed costs per shipment and variable costs based on the size of each shipment. For deliveries directly to the customer, the number of bottles in each kit will affect the frequency of deliveries and the fixed cost element of the delivery charges. The smallest numbers used appear to be 6, 12 -14 is more typical with some containing up to 24 per kit. file:///D|/wwwroot/knowledge_base/technical_library/petrolium.html (2 of 3)7/13/2007 7:24:16 AM
Kittiwake : The Price of Sampling Oil
This must be balanced against the risk of wastage, higher numbers encouraging their use for alternative purposes. A neglected area of shipping is the delivery service used. Standard postal services are sometimes used but their reliability is not good when sending to remote locations. Courier services are more expensive but the tariff rates are only a part of the story and should be viewed in conjunction with discount structure on offer, service and efficiency. Damaged goods are useless, late deliveries miss ships and go to waste effectively doubling the costs and causing customer dissatisfaction. Delivery problems in more than 0.25% of the total shipments need not be tolerated. In Summary - the three cost areas of any sampling kit programme are: ●
● ●
The kits themselves where the costs are relatively firm. Good designs should not be expensive, commonality across services and longer contract periods will help reduce prices. The shipping costs which have a major impact and can be profitably examined. The hidden costs which are often poorly understood but have a real and sizeable effect on the year end profitability. By outsourcing the entire programme, hidden costs can be largely eliminated.
file:///D|/wwwroot/knowledge_base/technical_library/petrolium.html (3 of 3)7/13/2007 7:24:16 AM
Kittiwake : The Practical Implication of Test Results
The Practical Implication of Test Results The extracts below are taken from "A Practical Guide to Marine Fuel Oil Handling" by Chris LeighJones Index Ash
Burnability
Carbon residue
Catalyst fines
Centrifuges
Density
Filters
Flash Point
Fuel treatment system
Pour Point
Settling tank
Stability
Sulphur
Vanadium and Sodium
Viscosity
Water
Introduction The great majority of the residual fuel supplied to merchant ships could be burnt without any pre-treatment, without causing any operational problems. However, almost without exception all merchant ships burning residual fuel have some form of fuel treatment system. The extent of this plant depends firstly on the design of the system and secondly on the vigilance of the ship's staff in its operation. When considering the onboard fuel treatment plant, the general standard practice is to have a settling tank, centrifuges and filters. Some vessels are additionally fitted with a homogeniser, but the inclusion of such a piece of equipment is very much the exception rather than the rule. back to top Settling tank The effectiveness of a settling tank depends on a number of factors. at the design stage, the number of tanks, size and height will affect the effectiveness for the removal of either water of solids. many ships today only have one settling tank fitted and this factor reduces the residence time of the fuel within the tank. To an extent the residence time can be increased by increasing the size. Traditionally settling tanks were sized to cover a days consumption. However, increasing the size probably results in an increase in height. unfortunately increasing the height reduces the effectiveness of the settling tank, because of the increase in time for the water or solid particles to settle out. Ideally the settling tanks should be as low in height as possible taking due regard of the tank capacity. in practice the height of the tank is dictated by other parameters within the overall design of the ship. The actual height may vary from as low as 5m to 15m. also, whilst a sloping bottom of the tank is technically desirable for ease of drain off, this is rarely found in practice. Operationally the effectiveness is determined by the viscosity of the fuel bunkered and the temperature at which the tank is maintained. It can be shown from Stokes' Law that the greater the viscosity the slower the rate of settling. By heating the tank the viscosity is reduced and it is usual to maintain a temperature of 50oC. back to top Centrifuges The correct sizing of the fuel centrifuges depends on the daily consumption and on the design viscosity of the system. As a matter of prudence many centrifuges are selected on a fuel of 700 cSt at 50oC. This fuel being the maximum specified in accepted marine fuel standards, such as ISO 8217. In actual service the fuel used is probably IF180 of IF380. The reality of this is that under these conditions is that it is oversized and theoretically operates more effectively. Basically there are two different types of centrifuge which are installed and found in merchant ships today. There are those of what may be described as the "traditional" design with a density limit of 991 kg/m3 (0.991 kg/m3) at 15oC. In such instances it is usual to fit three machines; two in use and one as a stand by unit. Over the years the consistent recommendation has been that in normal circumstances, two machines are run in series, with one as a purifier and the other as a clarifier. The purifier, for the removal of water and solids, is situated before the clarifier which primarily removes solids. The other type of machine is able to operate effectively on fuels up to 1010 kg/m3 at 15oC. In this case, normally only two machines are fitted, one in operation with the other on standby.
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Kittiwake : The Practical Implication of Test Results
back to top Filters In all residual fuel systems filters are fitted. these range from simple suction strainers on the transfer pump suction, through to the various designs of cold and hot filters. on board ship's staff can only maintain and clean them according to the manufacturer's recommendation. back to top Fuel treatment system It is recommended that ship staff are fully conversant with the design parameters and the manufacturer's recommendations for operation of the various units in the system. It is against this background that the practical implications of the analysis results can be assessed. This is particularly the case when the results approach or exceed the limits given in the procurement specification. It cannot be emphasised too strongly that the results are worse than meaningless unless they are from a representative sample. The various characteristics of the fuel as usually tested by routine laboratory analysis are discussed separately. back to top Density From the commercial point of view this is an essential parameter because residual fuel is ordered by weight but supplied by volume. If the actual value is less than that stated where will be a shortfall in the quantity of product supplied. from the technical aspect the incorrect gravity disc may by selected for setting the traditional purifier. Figure $$ (cumulative distribution to be plotted from a recent paper by Wanda given at the CIMAc day in Hamburg - I have the data) shows the variation in density for residual fuel on a world-wide basis. What is of particular interest is that some 40% of deliveries have a density in the range 985-991 kg/m3 at 15oC and only some 2% are a density overshoot of 991 kg/m3. Considering first the significant proportion close to the density limit. This emphasises the importance of operating two traditional centrifuges in series, further that they are operated under a steady state condition so as to maintain the interface in the correct position in the disc stack. In the event of the fuel being marginally over the 991 kg/m3 limit it is possible to operate both centrifuges as clarifiers as long as the water level in the fuel is low. If the purifier is fitted with a gravity disc for a greater density than the actual value the effectiveness of the purifier is reduced because the interface moves towards the Centre. back to top Water Usually the level of water in fuel is very low, typically 0.1-0.2 % by volume. Water can be introduced from a number of sources including tank condensation, tank leakage or deliberate contamination. Where steam is used for tank heating purposes, heating coil leakage is another potential source of water. A further potential source is the purifier itself, if the incorrect gravity disc is fitted. In practice, the nature of the water present may be fresh, brackish or salt depending on the level of sodium as determined by elemental analysis. the world-wide salt content of sea water varies, however, in first order 100 mg/kg (ppm) is associated with 1% water. Excessive water represents a double loss. Firstly there is the loss of specific energy in the fuel which will affect the fuel consumption and secondly there is the cost of disposal of the water removed by on board treatment. Such water is unlikely to pass through a 15 mg/kg (ppm) oily water separator, so it has to retained for disposal to a shore reception facility, with a cost to the ship operator. The water distribution for residual fuel is shown in figure $$ (fig10 MEP p24). From this it may concluded that the vast majority of fuels contain less than 0.2% water, in excess of 90% contain less than 0.5% and about 1% exceeds the present ISO 8217 of 1%. It is anticipated that the next revision of ISO 8217 will include a water limit of 0.5%. The settling tank will remove gross water as long as it is not emulsified. However the effectiveness will be less for very viscous fuels. It should be noted that the centrifuge is unable to remove water if it is emulsified. back to top Sulphur
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Kittiwake : The Practical Implication of Test Results
Sulphur is a naturally occurring element in crude oil, concentrated in the residual components of the crude oil distillation process. The amount of sulphur depends mainly on the source of the crude oil and to a lesser extent on the refining process. It is the principal non hydrocarbon in all residual marine fuels. From figure $$ (Fig11 MEP p25) it may be seen that on a world-wide basis is typically between 2-4 % . The level of sulphur has a marginal effect on the specific energy of the fuel and this is discussed in a later section. During the combustion process in a diesel engine the presence of sulphur in the fuel can give rise to corrosive wear. This can be minimised by using suitable operating conditions and lubrication, with an alkaline lubricant for the cylinder liner. Considerable work has been done by the various engine manufacturers to ensure the cylinder liner surfaces do not approach the dew point, which is the temperature at which gases condense to liquid. In a diesel engine the sulphur in the fuel burns to SO2 first, then combines with excess oxygen to form SO3 . In the presence of water vapour the SO3 is converted to sulphuric acid, which then forms on the cylinder walls if the temperature is below the dew point for the condensation of acid at the prevailing pressure. This dew point is a function of the fuel's sulphur content and the pressure in the cylinder. Only a relatively small proportion of sulphur is normally converted in this way and the remaining sulphur oxides pass out of the cylinder with the exhaust gases. In a crosshead engine a suitably formulated lubricant is used for cylinder lubrication. The generally accepted alkalinity level for crosshead engine trading world-wide is 70 mg KOH/g. It should be noted that besides the alkalinity level the rate of neutralisation and the feed rate are also important factors. Variation in the sulphur level will affect the rate of corrosive wear. However, it is standard practice to accept this variation. further it should be appreciated that the percentage number of fuels with a sulphur level greater than 4% is very low, to the extent of being negligible. If the ship is on a trading pattern where it continuously bunkers low (say 1.5%) sulphur fuel the use of a lower alkaline cylinder may be advantageous. Conversely if high (say above 4%) sulphur fuel is bunkered the effect of this as far as corrosive wear can be reduced by increasing the cylinder feed rate. For a trunk piston engine where the same lubricant is used throughout the engine there is nothing which can be done operationally to the variable levels of corrosive wear caused by a changing sulphur level. If the trading pattern is such that fuels of low sulphur are bunkered there may be some economic advantage in changing the lubricant to one with a lower alkalinity level and the economics of this action would have to be studied for each case. In the medium term, for both crosshead and trunk piston engines, high maintenance costs may arise if the lubricant / lubrication is not matched to the sulphur content back to top Catalyst fines The accepted way of establishing the presence of catalyst fines in the fuel is by elemental analysis for aluminium and silicon. For residual fuel ISO 8217 has a limit of combined aluminium and silicon of 80 mg/kg. These fines are particles of spent catalyst that arise from the catalytic cracking process in the refinery. The fines are in a form of complex alumino-silicates and, depending on the catalyst used, vary both in size and hardness. If not reduced by suitable treatment the abrasive nature of these fines will damage the engine, particularly fuel pumps, injectors, piston rings and liners. Figure $$ (fig13 MEP p27) shows the distribution of combined aluminium and silicon in residual fuel world-wide. It may be seen that over 90% of the samples have a combined aluminium value less than 40 mg/kg. Further it should be noted that the incidence of fuels greater than 80 mg/kg is less than 0.5%. Reduction of catalyst fines to an acceptable level for inlet to the engine takes place in the settling tank and the centrifuge. The extent of this reduction depends on the water content of the fuel as catalyst fines are "hydrophilic", in that they attract water and become contained in a water shell. The rate settling is determined by Stokes' Law which takes account of the particle size, difference in density of the catalyst fine and the fuel, and the viscosity of the fuel. Various values are quoted for the density of catalyst fines, but in reality they may be likened honey combed structures, which retards the rate of separation. This is further hindered by the outer shell of water by virtue of the close proximity of the density of water to that of the fuel. The extent of the removal also depends on the height of the tank which is fixed and the size of the particles which are variable. As far as the centrifuge is concerned critical factor is the relationship between the actual viscosity of the fuel and that for which the centrifuge was sized. If there is a difference in viscosity the residence time of the fuel in the centrifuge will be greater than the design value, hence directionally the centrifuge should be able to remove fines of a smaller size. Whilst this approach is theoretically correct, the operational result is totally on the size distribution of the fines. If is known that a high level of fines are present but there is negligible water two centrifuges could be run in parallel each on reduced output so that the combined output treated equalled the consumption. file:///D|/wwwroot/knowledge_base/technical_library/practical_implication_of.html (3 of 6)7/13/2007 7:24:57 AM
Kittiwake : The Practical Implication of Test Results
back to top Ash The ash value is related to inorganic material in the fuel. The actual value depends on three factors, firstly the inorganic material naturally present in the crude oil, secondly the refinery processes employed, and thirdly on possible subsequent contamination due to sand, dirt and rust scale. Residual fuels have more ash forming constituents as they are concentrated from the residue of crude oil refining processes. Vanadium and other materials such as silicon, aluminium, nickel sodium and iron are the main contributing components. Typically the ash value is in the range 0.03-0.07 % m/m as shown in figure $$ (Fig9 MEP p23). During onboard treatment as a result of the settling tank and the centrifuges the ash level may be reduced. This is because of the partial removal of catalyst fines (aluminium and silicon), sodium if in the form of salt water and iron if in the form of debris. Less than 1% of residual fuels contain an ash level greater than 0.1 %. Excessive ash levels are invariably caused by the inclusion of some waste material in the fuel which will increase the tendency for engine fouling. back to top Carbon residue The carbon residue of a fuel is the tendency of carbon deposits to form under high temperature conditions in an inert atmosphere. It is well known that the correlation between carbon residue is poor. However, in the absence of any other parameter this property is included in fuel specifications and is generally considered to give an approximate indication of the carbonaceous deposit forming tendencies of the fuel. Historically for residual fuels the Conradson Carbon Residue (CCR) test method was used and this has been superseded by the Micro Carbon Residue (MCR) method which has the same numerical value as CCR. Numerous factors can effect the combustion process in diesel engines, including engine loading, engine tuning and the ignition qualities of the fuel. All these factors have an effect on the deposit tendencies of a particular fuel. Some older engines, typically of the 1970s, are such that difficulties may be experienced burning fuels with an MCR greater that 12 % m/m, especially at low loads. Above this level there is likely to be increased carbonaceous deposit which will effect the performance of the prime mover. Operational experience has shown that the present generation of marine engines designed for residual fuel can tolerate a wide range of MCR values without any adverse effect. The carbon residue value of a fuel depends on the refinery processes employed in its manufacture. for straight run fuels the value is typically 10-12 % m/m, while for fuels from secondary refining processing, the value depends on the severity of the processes applied. On a global basis this value is typically 14-15 %, however, in some areas it can be as high as 20 % m/m. back to top Flash Point The flash point of a fuel is the only parameter which is governed by international legislation. It is the temperature at which vapour is given off which will ignite when an external flame is applied under standardised conditions. A flash point is defined to minimise fire risk during normal storage and handling. The minimum flash point for fuel in the machinery space of a merchant ship is 60oC. Even when residual fuels are at a temperature below their measured flash point that are capable of producing light hydrocarbons in the tank headspace causing the vapour composition to be near to, or within, the flammable range. Testing agencies frequently quote the flash point of a fuel sample as greater than 70oC. When the temperature is below this value an actual value is quoted. A low flash point can be indicative of fuel contamination by a more volatile product. The incidence of a flash point contravening the legal requirement is negligible, but it must be appreciated that it does happen from time to time. back to top Vanadium and Sodium
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Kittiwake : The Practical Implication of Test Results
Vanadium is a metal present in all crude oils in an oil soluble form. The levels found in residual fuels depend mainly on the crude oil source, with those form Venezuela and Mexico having the highest levels. The actual level is also related to the concentrating effect of the refinery processes used in the production of the residual. Most residual fuels have vanadium levels of less than 150 mg/kg, however, some fuels have a vanadium level greater than 400 mg/kg. No economical process exists for removing vanadium from either crude oil or residual fuel. In general, fuel when delivered contains a small amount of sodium which is typically below 50 mg/ kg. The presence of sea water increases this value by approximately 100 mg/kg for each per cent sea water. If not removed in the fuel treatment process, a high level of sodium will give rise to postcombustion deposits in the turbo charger. Although potentially harmful, these can normally be removed by water washing. High temperature corrosion and fouling can be attributed to vanadium and sodium in the fuel. During combustion these elements oxidise and form semi-liquid and low melting salts which adhere to exhaust valves and turbochargers. in practice the extent of hot corrosion and fouling are generally maintained at an acceptable level by employing the correct design and operation of the diesel engine. Temperature control and material selection are the principal means of minimising hot corrosion.. It is essential to ensure exhaust valve temperatures are maintained the below the temperatures at which liquid sodium and vanadium complexes are formed and for this reason valve face and seat temperatures are usually limited to below 450oC When because of operation constraints a fuel is bunkered with a vanadium level greater than that recommended by the engine designer, there is a risk that hot corrosion and fouling may occur in some engines. One operational solution is by the use of a fuel additive and numerous ash-modifying compounds are available. They should be used with care as situations can arise when the effect of the ash-modifier, by incorrect application, can cause further problems in the down-stream postcombustion phase. back to top Viscosity The viscosity usually quoted for a residual fuel is the kinematic viscosity expressed in centistokes (cSt) at some reference temperature. Although ISO 8217 has a reference temperature of 100oC, it is anticipated that this will be changed to 50oC at the next revision. this is a value widely used throughout the industry. Knowledge of the viscosity is important for several reasons, as it determines the temperature for handling, the size of the centrifuges and the temperature at which the fuel is injected into the engine. It is well known that as the temperature of the fuel is increased the viscosity reduces. Some oil suppliers publish temperature/viscosity charts which are based on the average data of a large number of samples. However, estimations from the charts cannot be regarded as precise as the exact relationship depends on the source and composition of the fuel.. Although fuel may have been ordered to one of the grades in ISO 8217, frequently on delivery only the viscosity grade is stated. For example IF 180 - this means that the viscosity is a maximum of 180 cSt at 50oC. Figure $$(fig 8 MEP p21) shows a typical temperature/viscosity chart of residual fuel. From this it may be seen that if the delivered viscosity is marginally above IF 180 the practical effect on injection temperature is only a few degrees. There are various ways this small increase in temperature may be achieved. One is by applying more heat to the fuel oil heater. this may out be possible especially if fouling has occurred in the heater. Alternatively the temperature of the clean heavy tank can be raised. If the centrifuge is only sized the slight increase in viscosity will reduce its performance. This may be over come, if the piping configuration permits, by operating two purifiers in parallel with a clarifier in series. The operational effect of operating with a viscosity lower than ordered is discussed separately under the heading of ignition quality. back to top Pour Point
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Kittiwake : The Practical Implication of Test Results
The pour point is the lowest temperature at which a residual fuel can be handled without excessive amounts of wax crystals forming out of solution. If a fuel is below the pour point, wax will begin to separate out which will block filters. Wax will also build up on tank bottoms and on heating coils. when heat is reapplied it can be difficult to get the wax to redissolve because of its insulating nature. In extreme cases manual cleaning of tanks becomes necessary. To avoid the operational difficulties just described it is necessary to store the fuel at least 5oC above the pour point. the transfer pumps in the fuel system are usually designed to operate at a maximum viscosity of 800-1000 cSt, therefore, for efficient transfer the fuel should be heated to the right viscosity. For less viscous fuels, i.e. below IF 100, the important parameter is the pour point. For residual fuels the typical pour point is less than -3oC, However there are occasional instances of +24-30oC. The pour point is a characteristic of the crude processed and can also be affected by the manner in which the fuel is manufactured. The pour point can simply determined on board although the result cannot be considered to be absolute, it will at least establish if the fuel has a high pour point. (you may wish to add how it is done) back to top Stability The stability of a residual fuel may be defined as the ability of the fuel to remain in an unchanged condition despite circumstances which may tend to cause change: or, more simply as the resistance of an oil to breakdown Conversely, instability would be the tendency of a residual fuel to produce a deposit of asphaltenic or carbonaceous matter as a function of time and/or temperature. There are various filtration methods which define sediment levels under defined conditions. The one quoted in ISO 8217 is the Total Potential Sediment method. In the event of a limited stability reserve of a fuel it is likely that filter blockage will occur. Should there be difficulty in identifying the nature of this material onboard, a small portion should be placed in an open container at a temperature of 60-70oC. A waxy material will melt but an asphaltenic sludge will not. With respect to stability less than 1% of fuels exceed the ISO specification limit of 0.10 %m/m. Once the fuel has chemically broken down there is no way of satisfactorily reversing the process. back to top Burnability Burnability of a residual fuel is a multi-stage process of which one part in the ignition quality of the fuel. The most common method of assessing this aspect is by an empirical equation involving density and viscosity, known as the Calculated Carbon Aromaticity Index (CCAI). Of the two parameters density has the major effect. The incidence of fuels with a CCAI exceeding 870 is in the order of 0.2% , whist those in the range are less than 3%. Ignition performance requirements of residual fuels in marine diesel engines are primarily determined by engine type and more significantly engine operating conditions. Fuel factors influence ignition characteristics to a much lesser extent. It is for this reason that no general limits for ignition quality can be applied, since a value which may be problematical to one engine under adverse conditions may perform quite satisfactorily in many other circumstances. On a world-wide basis there are some residual fuels with unusual burning characteristics which impose an additional load on the ring/liner interface. If this is not controlled, there is a danger that excessive maintenance will be incurred.
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Kittiwake : Total Base Number
Total Base Number It is a widely held belief that the Total Base Number (TBN also called Base Number) of an oil is required to prevent acidic corrosion within the combustion chamber of a running engine. This implies that it protects the piston rings, cylinder liner and piston crown from damage by sulphur or nitrogen containing acids. If you believe that then you are quite right, full marks, but what else does it do? Oils within an engine tend to deteriorate due to reasons of temperature, fuel ingress or other sources of contaminants forming harmful deposits. These deposits can build up behind piston rings, on ring lands, under the piston crown or on sliding surfaces. The net result is sticking and increased wear. Lubricating oils contain detergents and dispersants to delay the formation of these deposits and reduce the rate at which they form. Detergents are generally considered as those compounds that neutralise the deposit precursors that form under high temperature and pressure conditions or as the results of using high sulphur fuel. Dispersants on the other hand are those compounds that disperse or suspend the deposit forming contaminants. However in reality there is no sharp line of demarcation between the two. The principal detergents are soaps and salts of alkaline metals, usually calcium in the case of marine oils; they are often referred to as matallo-organic compounds such as sulphates, phenates and salicylates. These compounds are usually "over based". They contain more alkaline metal than is required to neutralise the acidic components used in manufacture of the additive. These are usually ash forming and spent additive will contribute to the insolubles loading in a used oil. Typical dispersants are usually "ashless". They do not form ash on combustion. They have fairly unpronounceable names such as polymeric succinimides, polyesters and benzylamides. The molecules have a polar or charged end that will attract and hold on to potential deposit forming compounds keeping them in suspension in the oil. The TBN additive will neutralise acidic combustion products, they will also neutralise deposit forming compounds within the oil, hold them in suspension preventing deposition and with luck also remove any deposits that are formed. Have you ever seen the results of a lubricant trial described in the technical press where you have a before and after picture of a nice clean piston? That is a direct benefit of these TBN additives. Conversely what would happen without this additive? The illustration below shows acidic corrosion of a lead bronze shell bearing. This type of bearing is now uncommon, partially because of its susceptibility to corrosion. However it does serve to illustrate just how useful is the protection offered by TBN additives. The lead phase of the bearing is very susceptible to acid attack resulting in the formation of large pits on the bearing surface, breakdown of the oil film and the beginnings of a bearing seizure.
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Kittiwake : Total Base Number
Total Base Number
Last month discussed why TBN additives are used but what typical TBN levels are found in marine engines? Starting with system oils used in the crankcase of a cross head engine, 6-9 TBN is common. The crankcase is protected from combustion products so there is little need to neutralise acids. Oil is required to keep the crankcase clean and to remove any deposits formed under the piston crown on engines with oil cooled pistons but the overall operating conditions are not arduous. This is also the only application where the TBN can increase. Hence scraper box drains containing very high TBN cylinder oil waste and acid contaminants should not be returned to the crankcase.
Moving up the TBN range are medium and high-speed diesel engines run on distillate fuels. Typical applications are ferries, fishing boats and military vessels. These oils will have a TBN of 8 - 15 or thereabouts. More engine cleanliness is required and there are some combustion products entering the crankcase. Fuel sulphur levels are low (<0.5%) thus the requirement to counter acidic corrosion is also low. These oils are sometimes also used to lubricate the auxiliary engines. Moving up the TBN scale again there are medium speed diesel engines operated on residual fuel oils. Here the typical TBN could be between 20 and 40 depending on the engine type, load regime and fuel sulphur levels. These engines can be operated on fuel with up to 5% sulphur although 3.5% would be more typical. They are often run at high load factors, producing relatively large amounts of deposits for the oil to retain in suspension.
The traditional upper limit for TBN in medium speed engines was about 40 but most oil suppliers now offer a TBN up to around 55. The reasoning is simple; some modern engine designs (e.g. Wartsila) have very low oil consumption (< 0.5g/kW.h). Relatively little fresh top up oil is used and therefore the TBN can deplete rapidly as it does not benefit from replenishment by fresh oil. The life of the oil charge becomes limited by the TBN depletion rate and using a higher initial TBN will help to extend this. A typical depletion limit is when the TBN level in the sump reaches about 50% of that for the new oil. This is very easy to test for, either on board the vessel or by using an oil analysis service. The figure below illustrates the relationship between TBN depletion and oil consumption for realistic range of oil consumption rates. Last on the list are the cylinder lubricants used in cross head engines. These oils are used once then either burned off or rejected through the scraper box and scavenge drains. High TBN (and also fast reaction rates for the additives) are required to efficiently neutralise acidic combustion compounds and protect the large exposed areas of the cylinder walls. Typical TBN levels are between 60 and 100 again dependent on the fuel sulphur level, engine design and load regime. SIZE="4">
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Kittiwake : Total Base Number
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Kittiwake : Testing Times, For Used Lubricants
Testing Times, For Used Lubricants
The lubricating oil used in a large marine engine is an expensive asset. Filling a modern crosshead diesel engine with sufficient reserves typically costs around $5 - $6.00 per hp for system oils. The oil in a 20,000 hp engine would therefore be worth over $100,000.
What can the ships engineers do to preserve the value of this expensive asset? There are 3 quite simple answers. Firstly, good housekeeping. Ensuring that oil and fuel leaks are detected, the oil is centrifuged, filtered and generally cared for in a professional manner. Secondly, by using a laboratory based analysis service. All the major oil companies offer this service usually branded with a catching name such as PFA (Mobil), Texlube (Texaco), Exxcare (Exxon/Esso). These are sometimes free, sometimes not. They are invariably accurate and provide the most complete snapshot of the oil's condition. By examining the wear metal content and particle morphology, the results can be used to infer the condition of the machinery as well as the oil. The word snapshot is used advisedly; the process works best if used regularly. In this manner it is possible to build a picture of what is happening. More often this picture tells as much of a story in the changes to various parameters as it does by examining the absolute levels as these can vary significantly from ship to ship. They are usually used sparingly with typical analysis intervals from 3 to 6 months depending on how critical the system is. Highly critical systems such as slewing crane bearings on North Sea oil production platforms are often analysed monthly. The third method is to analyse the oil using equipment on board the vessel. Again most major oil companies can supply suitable equipment. The standard varies from simple pass/fail type tests through to sophisticated electronics providing laboratory grade results. Most offerings lie somewhere in between these two extremes. It is possible to measure many physical properties of the oils, Viscosity, Water content, TBN, Insolubles, TAN, Salt content etc but not for wear metals. This equipment can provide a useful and often accurate indication of the oil condition but not much information on the condition of the machinery itself. The advantage is that the results are available immediately and testing can be undertaken more frequently, for example weekly or after maintenance. It is for this reason that on board testing is often undertaken in conjunction with a laboratory based analysis service. With an early and quick indication of oil degradation it is often possible to save the oil charge and the machinery at little or no unnecessary cost. So, you have 3 ways to care for your oil: ● ● ●
Good house keeping Testing in a laboratory Testing on board.
The oil charge is an expensive asset and so worth a few minutes a week to keep it in prime condition. Care for your oil and it will care for your machinery.
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Kittiwake : Acidity
overdue Marine lubricating oils are generally composed of base oils and additives. The purpose of these additives is to enhance the characteristic of the base oil or to give a specific characteristic. The base oils used in quality lubricants are carefully refined and may have a slight acidity because of the presence of small amounts of organic acid but this has no corrosive effects on machinery. Marine lubricating oils are generally composed of base oils and additives. The purpose of these additives is to enhance the characteristic of the base oil or to give a specific characteristic. The base oils used in quality lubricants are carefully refined and may have a slight acidity because of the presence of small amounts of organic acid but this has no corrosive effects on machinery.
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Kittiwake : Testing Fuels
Testing Fuels Last months article covered options available to the engineer for monitoring the quality of the lubricants. There is of course another expensive liquid asset on every vessel, namely the fuel used to propel the vessel. What option is available for quality control in this instance? Firstly, there are several fundamental differences in the requirements for monitoring fuel oil quality. Lubricating oil is a highly technical product containing additives to perform many functions. Conversely residual fuel oil basically a waste product, unwanted by the refinery. The aim in monitoring fuel oil quality is not to detect subtle trends but merely to confirm that the product conforms to a set of minimum standards such as those detailed in ISO 8217. The second difference is that as the fuel is used and consumed in the process, only a single set of tests are necessary on any one delivery. Just as with lubricants there are 3 options available. Do nothing, do something or use a laboratory, all have advantages or otherwise. The do nothing option implies that the source of the fuel is very reliable and probably the vessel is on a dedicated run taking fuel from the same point on every occasion. It is not the best policy for avoiding problems, especially when residual fuel is being bought. Do something generally consists of good practice during the bunkering operation to ensure that a representative sample of the fuel is obtained. This can then be stored for future reference in case of problems and/or tested on board the vessel for a number of key parameters. Modern test equipment is quick to operate and will provide very accurate results for water, density, viscosity, salt, compatibility or stability. An advantage of testing on board the vessel is that results are available immediately and before the fuel has to be used. In the event of problems it is therefore possible to mitigate the eventual cost, a very good position in instances of legal actions. The third option is to use a laboratory and this provides the best protection in the event of problems. The main fuel testing services such as FOBAS and DNV offer a fast and reliable service providing both test results and a view on the significance of these results. Should problems arise they are on hand to provide detailed technical support that is often beyond the capabilities of a hard-pressed marine superintendent. So, you have 3 ways to check fuel quality: ● ● ●
Do nothing and rely on a good supplier relationship Test on board the vessel to obtain immediate indications of potential problems Use a laboratory service for a comprehensive analysis and interpretation of the results
In many instances both options two and three are used for immediate results and a thorough verification as further insurance against potential problems.
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Kittiwake : The Real World For Fuel Deliveries
The Real World For Fuel Deliveries Theory and advice are excellent bedfellows, they can be proffered freely without the slightest reference to reality. Our industry however must operate in the real world where practicality, cost and time constraints are all subservient to the great motivator of profit, or at least staying in business. Now if we were to develop the ultimate car what would be the requirements. Maximum space in minimum size, maximum acceleration but miserly fuel consumption, must fit into zero space but carry everything. Clearly not a practical vehicle. The shipping and bunkering industries are subject to similar constraints and unfortunately are apt to publish theory and advice without recourse to what is actually happening. Let us take for example the perennial issue of sampling ships fuel. Theory. All parties reach amicable agreement before delivery begins on how, when, at what rate, temperature etc.. Fuel samples are taken at the point of custody transfer using a type approved flow proportional automatic sampler, meeting the requirements of ISO 3171. Samples are sealed and witnessed by all parties and retained; one each plus one for an analysis service. Faultless communication is maintained throughout the process, meeting the specified quality standard, fuel is delivered into empty bunker tanks. All parties depart on good terms. Several recent articles have gone to great lengths and detail to describe exactly this process and to explain why anything else is simply not good enough. Reality. The ship operator does not correctly specify the grade and neither does he like spending money as the business is barely profitable. He realises that fuel is a major cost but his thinking is purely short term, cashflow is king. The fuel barge captain speaks Latvian, the ships crew are Cape Verde speaking Portuguese and broken English. The Chief Engineer has a broad Scottish accent and in any event is down in the engine room with a disabled engine. The ship is at anchorage, the engine should not be disabled but is because this is the only chance to do some much needed maintenance. There is a 6 foot swell running, the mobile radio sets were broken last week when the third mate dropped them in the hold. The fourth engineer is more concerned with the forthcoming trip ashore (if he is lucky). The ship is in ballast waiting to load 60,000 t of iron ore. The barge delivery pipe is coming on board near vertically and is very difficult to align. No one wants to loose any fingers. Fortunately loading is only into two cross bunkers but the fuel is going to get mixed with existing stock whatever happens. So what can be done to protect the owner (and crew) during bunkering that is within the capability of the crew ? Fortunately the answers are quite simple. A nothing and risk all B, spend $300 - $1000 and risk little C, spend $10,000 - $30,000 and risk very little The argument can be basically reduced to options B and C. This is also where good theoretical advice and the practicalities of marine operations diverge. Firstly, training We hear a lot about practices and procedures for bunkering processes. What is invariably missed is any reference to crew training. Any good sampling system will provide training on handling,
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Kittiwake : The Real World For Fuel Deliveries
bunkering and sampling the fuels, what to look out for and what to do when anything (or everything) goes wrong. Most samplers on the market whatever the price do not provide this training. The cost is very little and a good practical manual can be obtained for less than $20, for example the manual from Kittiwake provides 90 pages of guidance in clear English. Training video's such as those produced by DNV cost a little more but are very effective. Secondly sampling This is the real Achilles heel of the bunkering industry. It is estimated that some 30 to 40% of bunkers are regularly tested and the testing relies on a representative sample. Many more ships may take a sample but how many is not known, however the actual number of samples taken will run into tens of thousands. In reality, the shipowners will not spend unless forced to and the crew will not use anything heavy or complicated. There are then four basic requirements. Low cost - High priced samplers do not sell in significant numbers as shipowner will not spend the money - fact. If any design of sampler does not sell, then it cannot be used. Ease of use - well designed manual and automatic samplers are both easy to use. Type approval - Good samplers of any design will carry Classification Society approval International standards - If a sampler meets an existing international standard for example ISO 3170 (manual) or 3171 (automatic) then the sample cannot be rejected as invalid - period. Now to slay the last Dragon; that of performance. If all the theory was to be believed then samples drawn from low cost, easy to use manual drip samplers would be wholly unrepresentative, a complete travesty. Below is a table taken from a paper by Caleb Brett. A drip sample is compared against two designs of automatic samplers. There are only two commonly available designs of automatic sampler on the marine market. The paper concluded that in back to back tests there was no difference between the performance of either manual or automatic sampler, both were "very satisfactory" . It should also be noted that both major fuel testing services, FOBAS and DNV recommend the use of drip samplers. So please do not give us any more theoretically good but practically bad advice, we can do without it. QED. COMPARISON OF SAMPLES TAKEN BY MANUAL & AUTOMATIC SAMPLING EQUIPMENT , 1983 VISCOSITY AT 80ƒC CST SPECIFIC GRAVITY 15/15ƒC ASPHALTENES % WT WATER % V ASH % WT CONRADSON CARBON RESIDUE % WT SULPHUR % WT POUR POINT ƒC ELEMENTS SILICON PPM ALUMINIUM PPM VANADIUM PPM SODIUM PPM IRON PPM COMPARISON OF SAMPLES TAKEN BY MANUAL AND AUTOMATIC SAMPLING EQUIPMENT , 1993 KINEMATIC VISCOSITY AT 100ƒC CST DENSITY AT 15ƒC KG/L
RESULTS Automatic 73.76 0.9937 8.24 0.15 0.038 15.9 2.92 (+) 9
Drip 72.79 0.9937 5.05 0.15 0.042 14.78 2.89 (+) 9
16 13 167 12 17
15 14 154 10 17 RESULTS
Automatic 24.39 0.9885
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Drip 23.08 0.9875
Kittiwake : The Real World For Fuel Deliveries
WATER CONTENT % (V/V) ASH CONTENT AT 550ƒC % (M/M) MICRO CARBON RESIDUE % (M/M) SULPHUR CONTENT % (M/M) POUR POINT ƒC SILICON MG/KG ALUMINIUM MG/KG VANADIUM MG/KG SODIUM MG/KG IRON MG/KG PHOSPHOROUS MG/KG LEAD MG/KG CALCIUM MG/KG NICKEL MG/KG ZINC MG/KG
0.05 0.02 15.63 1.99 6 6 4 73 9 20 1 <1 2 27 1
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0.05 0.024 14.47 1.89 -3 6 4 66 9 16 1 <1 2 24 1
Kittiwake : To Pass or Fail ?
To Pass or Fail ? A large proportion of the shipping industry sends samples of the fuel bunkered for routine analysis. This analysis generally follows the tests as laid down in ISO 8217: 1996, the relevant international standard. No surprises so far? The results are sent back to the shipping office often accompanied by notes providing guidance and interpretation. It is at this point that problems begin to occur. Interpreting test results is not a precise science as we are dealing with human actions and equipment variability. The testing services such as FOBAS and DNV are well aware of this fact; the layman is often not so informed. Results are never "precise" merely they are accurate to the best that can be achieved in a practical situation. Send an identical sample to a number of good laboratories and it is reasonable to expect the results to be similar but not necessarily identical. This variability is quantifiable and the effects are well documented in another standard ISO 4259 that provides rules for interpretation of ISO 8217. Still awake? In essence, "Repeatability" and "Reproducibility" quantify variability. To use a simile, Repeatability quantifies the scatter of arrows fired by a single archer at a target. All arrows near the target is very repeatable, arrows all over the place is not very repeatable. Reproducibility quantifies the scatter of two archers using the same equipment and arrows firing at two identical targets. Both sets of arrows clustered on top of each other is very reproducible, two sets of arrows clustered around different points is not very reproducible. ISO 4259 is based on detailed statistical analysis of the performance of test laboratories. A band of acceptable results is provided for each test parameter such that if 100 laboratories reported on the sample using the same test method then 95 of the results would be within the band. This is called a 95% confidence level i.e. you can be 95% certain that the result is acceptable. So what does this mean for the ships engineer? ISO 8217 provides limits on various parameters of marine fuel oil. For example a maximum density for a given grade or a minimum flash point. Testing fuel provides data to compare the fuel against these limits. ISO 4259 provides instruction on how to make this comparison by considering the "accuracy" of a test method as well as the individual test result. As a practical example a lower limit on flash point is 60oC for most marine fuel grades. It would be tempting to reject a fuel with a reported flash point of 58.5oC as below the limit. However ISO 4259 says that if the real value were in fact an acceptable 60oC, 95 out of 100 laboratories that tested this fuel would report results above 58 and below 62oC. Therefore 58.5oC is acceptable according to these rules. Of course it can get a little more complicated than this but that's another story.
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Kittiwake : Water in Lube Oil
Water in Lube Oil
All modern engine lubricants are designed to remain stable in the presence of small amounts of water. Water contamination cannot realistically be avoided although its effects can be limited by good maintenance policies. It can enter the oil from condensation, leaking coolers and deck vents, blow-by gasses and even via poorly operated centrifuges. Water contamination and even salt contamination without the presence of water are very easy to detect either on board the vessel or in a laboratory. Some months ago I was discussing the possible effects of water contamination with a colleague from Glacier who showed me the following rather dramatic example of just what can happen. His example was taken from a main engine thrust bearing. It is the partial remains of a large tilting thrust pad. We have all seen spare examples of these pads, greased up and bolted to racks on the bottom flat of many a ships engine room. They are not highly loaded and never seem to cause problems from one inspection to the next. This one did: and with a vengeance! The lubricant in the thrust block had become contaminated with seawater. If fresh water is damaging, the addition of salts to the mixture greatly magnifies the effect. The type of corrosion in this example is not generally associated with fresh water contamination as it is electrochemical in nature. i.e. it requires an electrolyte and fresh water does not conduct very well, however, salt water fills this requirement admirably. The bearing overlay material is a tin based white metal alloy containing tin, copper and antimony in varying proportions. The characteristic feature of salt-water corrosion in tin rich bearing materials is the formation of a hard black scale. This scale is not a deposit; rather it is formed from corrosion of the bearing metal itself. The scale has a lower density (greater volume) than the parent metal causing it to expand, fill bearing clearances, flake and spall off into the oil flow only to cause further damage downstream. The blackened and scaled appearance of this bearing illustrates the symptoms admirably. There are no prizes for guessing that the steel thrust collar of this bearing was in a similar condition and that the failure was terminal.
Does anyone wish to buy a ship - good condition, one owner, "minor" engine problems?
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Kittiwake : Well Traveled
Well Traveled
Thoughtful design practices can be found in nearly every part of a modern ships engine room and no more so than in the engines themselves. These have to turn, pump, cool, inject, breath, exhaust and power in a very reliable manner. During this endeavour they are corroded, eroded, worn, burnt but otherwise generally treated tenderly by the ships staff.
I personally find some of the feats performed in their everyday function to be quite remarkable. Take for example the piston rings on two typical engines : The first is a generator engine, 200mm bore, 220mm stroke 1000 rpm. Now this engine runs at constant speed, 80% MCR for (say) 300 days per year. How far do the piston rings travel in that time ? Distance per: Stroke = 0.22 x 2 = 0.44m Minute = 0.44 * 1000 = 440 m Day = 440 * 60 * 24 = 633600 m Year = 633600 * 300 = 190,000 km That is in 1 year each piston ring will travel 190,000 kilometres or about 5 times round the world (probably about as far as the ship itself) Now let us consider the main engine, 850m bore, 2500mm stroke 100 RPM, at sea for 300 days per year. Distance per: Stroke = 2.4 * 2 = 5m Year = 216, 000 km Or in 1 year the rings travel about the same distance as those for the generator. This is not a coincidence as the maximum piston speed of most engines is selected for durability reasons to around 10 m/s irrespective of engine size.
Now following on from this, ponder upon the following :
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Kittiwake : Well Traveled
The oil that lubricates the rings and liners is scraped into a very thin film. At mid stroke this film is at its thickest, being about 10 micron or 10 * 10-6 meters. That is less than the thickness of a human hair. At the extremities of the stroke this thickness reduces to nearly zero. So in the engine you have a large number of piston rings travelling constantly for very great distances. They are separated from imminent destruction by an oil film thinner than a human hair. They perform this magical feat constantly and reliably, often without even a thought from the engineers walking past. The only reason this can happen is because of the oil film itself. What lesson can be drawn from this ? I would suggest that it pays to look after the oil.
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