Fuel oils lubricating oils and their treatment

Crude oil is, at the present time, the source of most fuel oils for marine use. Synthetic fuels are being developed but will probably be too expensive for ship propulsion. Solid fuel, such as coal, is returning in a small way for certain specialised trade runs. The various refined products of crude oil seem likely to remain as the major forms of marine fuel.

The refining process for crude oil separates by heating and distillation the various fractions of the oil. Paraffin fuel would be used in gas turbine plants, gas oil in high- and medium-speed diesel engines and crude oils in slow-speed and some medium-speed diesels. Paraffin and gas oil are known as 'distillates', which are free flowing, easily stored and can be used without further treatment. Residual fuels, however, are very viscous or thick at normal temperatures, and require heating before use. Additional treatment to remove harmful chemicals or sulphur may be required for all or some of the refined products, depending upon their application. Finally blending or mixing of the various oils is done to provide a range of commercial fuels for different duties.

Fuel oils

Fuel oils have various properties which determine their performance and are quoted in specifications. The specific gravity or relative density is the weight of a given volume of fuel compared to the weight of the same volume of water expressed as a ratio, and measured at a fixed temperature. Viscosity is a resistance to flow. A highly viscous fuel will therefore require heating in order to make it flow. Measurement of viscosity is by Redwood, Saybolt or Engler instrument flow times for a given volume of fuel.

The ignition quality of a fuel is measured by the time delay between injection and combustion, which should be short for good controlled burning. Ignition quality is indicated as cetane number, diesel index and calculated cetane index; the higher the value the better the ignition quality of the fuel.

The flash point is a figure obtained and used mainly to indicate the maximum safe storage temperature. The test determines the temperature at which the fuel will give off sufficient vapours to ignite when a flame is applied. Two values are possible: an open flash point for atmospheric heating, and a closed flash point when the fuel is covered while heating.

Low-temperature properties are measured in terms of pour point and cloud point. The pour point is slightly above the temperature at which the fuel just flows under its own weight. It is the lowest temperature at which the fuel can be easily handled. At the cloud point waxes will form in the fuel. Below the cloud point temperature, pipe or filter blocking may occur.

The carbon residue forming property of a fuel is usually measured by the Conradson method. Controlled burning of a fuel sample gives a measure of the residual carbon and other remains.

Sulphur content is of importance since it is considered a cause of engine wear. A maximum limit, expressed as a percentage by weight, is usually included in specifications.

The calorific value of a fuel is the heat energy released during combustion. Two values are used, the more common being the Higher Calorific Value, which is the heat energy resulting from combustion. The Lower Calorific Value is a measure of the heat energy available and does not include the heat energy contained in steam produced during combustion but passing away as exhaust. The measurement is obtained from a bomb calorimeter test where a small fuel quantity is burnt under controlled conditions.

The various fuel properties have different effects on performance of the engine and the storage and handling requirements of the system. Blending and the use of various additives will also influence both the engine and the system.

Viscosity will affect jerk-type injector pumps and injector operation since the liquid fuel is the operating medium. The pump mechanism is lubricated by the fuel which, if it is of low viscosity, will cause wear.

Cloud point and pour point values are important when considering the lowest system operating temperatures. Wax deposited in filters and fuel lines will cause blockages and may restrict fuel flow to the engine.

The cetane number or diesel index will determine injection timing and also influences the combustion noise and production of black smoke. The temperature in a fuel system should be progressively increased in order to deliver fuel at the correct viscosity to the injectors or burners. System cleanliness is also very important to reduce wear on the many finely machined parts in the fuel injection equipment. Regular attention to filters and general system cleanliness is essential. Various additives are used to, for instance, remove lacquer from metal surfaces, reduce wear and prevent rust.

Lubricating oils

Lubricating oils are a product of the crude oil refining process. The various properties required of the oil are obtained as a result of blending and the introduction of additives. The physical and chemical properties of an oil are changed by additives which may act as oxidation inhibitors, wear reducers, dispersants, detergents, etc. The important lubricant properties will now be examined.

Viscosity has already been mentioned with respect to fuel oils, but it is also an important property of lubricating oils. Viscosity index is also used, which is the rate of change of viscosity with temperature.

The Total Base Number (TBN) is an indication of the quantity of alkali, i.e. base, which is available in a lubricating oil to neutralise acids.

The acidity of an oil must be monitored to avoid machinery damage and neutralisation number is used as the unit of measurement.

The oxidation resistance of a lubricant can also be measured by neutralisation number. When excessively oxidised an oil must be discarded.

The carbon-forming tendency of a lubricating oil must be known, particularly for oils exposed to heat. A carbon residue test is usually performed to obtain a percentage value.

The demulsibUity of an oil refers to its ability to mix with water and then release the water in a centrifuge. This property is also related to the tendency to form sludge.

Corrosion inhibition relates to the oil's ability to protect a surface when water is present in the oil. This is important where oils can be contaminated by fresh or salt water leaks.

The modern lubricant must be capable of performing numerous duties. This is achieved through blending and additives. It must prevent metal-to-metal contact and reduce friction and wear at moving parts. The oil must be stable and not break down or form carbon when exposed to high temperatures, such as where oil cooling is used. Any contaminants, such as acidic products of combustion, must be neutralised by alkaline additives; any carbon build up on surfaces must be washed away by detergent additives and held in suspension by a dispersant additive. The oil must also be able to absorb water and then release it during purification, but meanwhile still protect the metal parts from corrosion.

The various types of engine and other equipment will have oils developed to meet their particular duties.

Trunk piston engine lubricating oil must lubricate the cylinders as well as the crankcase: some contamination from the products of combustion will therefore occur, resulting in acidity and carbon deposits. The oil must, in addition to lubricating, neutralise the acids and absorb the deposits.

Turbine oil, while lubricating the moving parts, must also carry away considerable quantities of heat from the bearings. This calls for a stable oil which will not break down at high temperatures or form deposits. Where gearbox lubrication is also required certain extreme pressure (EP) additives will be needed to assist the lubricating film. Contact with water in the form of steam will be inevitable so good demulsifying properties will be essential.

Slow-speed diesel engines will have separate cylinder and crankcase lubrication systems. The cylinder oil will have to neutralise the acidic products of combustion and also have good detergent properties to keep the metal, surfaces clean. Crankcase oils are either detergent type, multi-purpose oils or rust and oxidation inhibited. Good demulsification and anti-corrosive properties are required together with oxidation resistance which is provided by the inhibited crankcase oil. The detergent or multi-purpose oil is particularly useful where oil cooling of pistons occurs or where contamination by combustion products is possible.

Oil treatment

Both fuel oils and lubricating oils require treatment before passing to the engine. This will involve storage and heating to allow separation of water present, coarse and fine filtering to remove solid particles and also centrifuging.

The centrifugal separator is used to separate two liquids, for example oil and water, or a liquid and solids as in contaminated oil. Separation is speeded up by the use of a centrifuge and can be arranged as a continuous process. Where a centrifuge is arranged to separate two liquids, it is known as a 'purifier'. Where a centrifuge is arranged to separate impurities and small amounts of water from oil it is known as a 'clarifier'.

The separation of impurities and water from fuel oil is essential for good combustion. The removal of contaminating impurities from lubricating oil will reduce engine wear and possible breakdowns. The centrifuging of all but the most pure clean oils is therefore an absolute necessity.

Centrifuging

A centrifuge consists of an electric motor drive to a vertical shaft on the top of which is mounted the bowl assembly. An outer framework surrounds the assembly and carries the various feed and discharge connections. The bowl can be a solid assembly which retains the separated sludge and operates non-continuously, or the bowl can be

Figure 8.1 Purifying bowl arrangement arranged so that the upper and lower parts separate and the sludge can be discharged while the centrifuge operates continuously. The dirty oil is admitted into the centre of the bowl, passes up through a stack of discs and out through the top (Figure 8.1).

The purifying process

The centrifugal separation of two liquids, such as oil and water, results in the formation of a cylindrical interface between the two. The positioning of this interface within the centrifuge is very important for correct operation. The setting or positioning of the interface is achieved by the use of dam rings or gravity discs at the outlet of the centrifuge. Various diameter rings are available for each machine when different densities of oil are used. As a general rule, the largest diameter ring which does not break the 'seal' should be used.

The clarifying process

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

Diagram Lube Oil Clarifier

Figure 8.1 Purifying bowl arrangement

1-feed

2-purified

3-separated water

4-sludge

clarifier bowl has only one outlet (Figure 8.2). No gravity disc is necessary since no interface is formed; the bowl therefore operates at maximum separating efficiency since the oil is subjected to the maximum centrifugal force.

The bowl discs

Purifier and clarifier bowls each contain a stack of conical discs. The discs may number up to 150 and are separated from one another by a small gap. Separation of impurities and water from the oil takes place between these discs. A series of aligned holes near the outside edge permits entry of the dirty oil. The action of centrifugal force causes the lighter components (the clean oil) to flow inwards and the water and impurities flow outwards. The water and impurities form a sludge which moves outwards along the undersides of the discs to the periphery of the bowl.

Nm-continuous operation

Certain designs of centrifuges are arranged for a short period of operation and are then shut down for cleaning. After cleaning and removal of the sludge from the bowl, the machine is returned to service. Two different designs are used for this method of operation: a long narrow bowl and a short wide bowl. The narrow-bowl machine has to be cleaned after a shorter running period and requires dismantling in order to clean the bowl. Cleaning of the bowl is, however, much simpler since it does not contain a stack of discs. The wide-bowl machine can be cleaned in place, although there is the added complication of the stack of conical discs which must be cleaned.

Continuous operation

Modern wide-bowl centrifuge designs enable continuous operation over a considerable period of time. This is achieved by an ejection process which is timed to discharge the sludge at regular intervals. The sludge deposits build up on the bowl periphery as separation continues, and the ejection process is timed to clear these deposits before they begin to affect the separation process. To commence the ejection process the oil feed to the centrifuge is first shut off and the oil remaining in the bowl is removed by admitting flushing water. Water is then fed into the hydraulic system in the bottom of the bowl to open a number of spring-loaded valves. This 'operating' water causes the sliding bowl bottom to move downwards and open discharge ports in the bowl periphery. The sludge is discharged through these ports by centrifugal force (Figure 8.3). Closing 'operating' water is now fed in to raise the sliding bowl up again and close the discharge ports. Water is fed into the bowl to remake the liquid seal required for the separation process, the oil feed reopened, and separation continues.

The complete ejection cycle takes only a few seconds and the centrifuge is in continuous operation throughout. Different bowl designs exist for various forms of sludge discharge, e.g. total discharge, controlled partial discharge, and so on. With controlled partial discharge the oil supply is not shut off and not all of the sludge is discharged, in this way the separation process is not stopped. Whatever method is adopted the centrifuge can be arranged so that the discharge process is performed manually or by an automatic timer.

Maintenance

The bowl and the disc stack will require periodical cleaning whether or not an ejection process is in operation. Care should be taken in stripping

Sludge discharge

Figure 8.3 Sludge discharge

Sludge discharge

Figure 8.3 Sludge discharge down the bowl, using only the special tools provided and noting that some left-hand threads are used. The centrifuge is a perfectly balanced piece of equipment, rotating at high speeds: all parts should therefore be handled and treated with care.

Heavy fuel oil separation

Changes in refinery techniques are resulting in heavy fuel oils with increased density and usually contaminated with catalytic fines. These are small particles of the catalysts used in the refining process. They are extremely abrasive and must be removed from the fuel before it enters the engine. T he generally accepted maximum density limit for purifier operation is 991 kg/m3 at 15°C.

In the ALCAP separation system the separator has no gravity disc and operates, to some extent, as a clarifier. Clean oil is discharged from the oil outlet and separated sludge and water collect at the periphery of the bowl. When the separated water reaches the disc stack, some water will escape with the cleaned oil. The increase in water content is sensed by a water-detecting transducer in the outlet (Figure 8.4). The water

Figure 8.4 Fuel oil separation control

transducer signal is fed to the MARST 1 microprocessor which will discharge the water when a predetermined level is reached. The water will be discharged from sludge ports in the bowl or, if the amount is large, from a water drain valve.

The ALCAP system has also proved effective in the removal of catalytic fines from fuel oil.

Lubricating oil centrifuging

Diesel engines

Lubricating oil in its passage through a diesel engine will become contaminated by wear particles, combustion products and water. The centrifuge, arranged as a purifier, is used to continuously remove these impurities.

The large quantity of oil flowing through a system means that full flow lubrication would be too costly. A bypass system, drawing dirty oil from low down in the oil sump remote from the pump suction and returning clean oil close to the pump suction, is therefore used. Since this is a bypass system the aim should be to give the lowest degree of impurity for the complete system, which may mean running the centrifuge somewhat below the maximum throughput.

Water-washing during centrifuging can be adopted if the oil manufacturer or supplier is in agreement; but some oils contain water-soluble additives, which would of course be lost if water-washed. The advantages of water-washing include the dissolving and removal of acids, improved separation by wetting solid impurities, and the constant renewal of the bowl liquid seal. The washing water is usually heated to a slighdy higher temperature than the oil.

Detergent-type oils are used for cleaning as well as lubricating and find a particular application in trunk-type engines and some slow-speed engines. Detergent-type oil additives are usually soluble and must not therefore be water-washed.

Steam turbines

The lubricating oil in a steam turbine will become contaminated by system impurities and water from condensed steam, so bypass separation is used to clean the oil. The dirty oil is drawn from the bottom of the sump and clean oil returned near the pump suction. Preheating of the oil before centrifuging assists the separation process. Water washing of the oil can be done where the manufacturer or supplier of the oil permits it.

Homogenisers

A homogeniser is used to create a stable oil and water emulsion which can be burnt in a boiler or diesel engine. Such an emulsion is considered to bring about more efficient combustion and also reduce solid emissions in the exhaust gas.

Various designs utilise an impact or rolling action to break down the fuel particles and mix them with the water. It is also considered that agglomerates of asphaltenes and bituminous matter are broken down and can therefore be burnt. The manufacturers contend that a homogeniser can render a sludge burnable whereas a centrifuge would remove such material. Homogenisers are able to reduce catalytic fines into finely ground particles which will do no harm.

Shipboard experience with homogenisers is limited and generally not favourable. Most authorities consider it better to remove water and solid contaminants than simply grind them down.

Blenders

Blending is the mixing of two fuels, usually a heavy fuel and marine diesel oil. The intention is to produce an intermediate-viscosity fuel suitable for use in auxiliary diesels. The fuel cost savings for intermediate fuel grades are sufficient to justify the cost of the blending plant. Furthermore no supply problems exist since the appropriate mixture can be produced by the blender from available heavy and marine diesel oils.

The blending unit thoroughly mixes the two fuels in the appropriate proportions before supplying it to a blended fuel supply tank.

Compatibility can be a problem and tests should be conducted on any new fuel before it is used. Incompatible fuels may produce sludge or sediment. The cracked residues presently supplied from many refineries are very prone to incompatibility problems when blended with marine diesel oil.

Filters and strainers

Mechanical separation of solid contaminants from oil systems (fuel and lubricating) is achieved by the use of filters and strainers. A strainer is usually a coarse filter to remove the larger contaminating particles. Both are arranged as full flow units, usually mounted in pairs (duplex) with one as a standby.

The strainer usually employs a mesh screen, an assembly of closely packed metal plates or wire coils which effectively block all but the smallest particles. It is usually fitted on the suction side of a pump and must be cleaned regularly or when the pressure differential across it become unacceptable. Where suction conditions are critical the strainer will be fitted on the discharge side of the pump. When cleaning is undertaken the other unit will be connected into the system by changeover valves or levers and oil circulation will continue. The particles of dirt collect on the outside of the strainer element or basket and can be removed by compressed air or brushing. A strainer should be cleaned as soon as it is taken out of the system, then reassembled and left ready for use.

Magnetic strainers are often used in lubricating oil systems, where a large permanent magnet collects any ferrous particles which are circulating in the system. The magnet is surrounded by a cage or baskei to simplify cleaning.

Fine filters, again in pairs, are used to remove the smallest particles oi dirt from oil before the oil enters the finely machined engine parts in either the fuel injection system or the bearings of the rotating machinery. Fine filters are full-flow units which clean all the oil supplied to the engine. The filtering substance may be a natural or synthetic fibrous woollen felt or paper. A felt-type fine filter is shown m Figure 8.5. A steel division plate divides the steel pressure vessel into an upper and a lower chamber. Dirty oil passes into the upper chamber and through the filter element, then the filtered oil passes down the central tube to the lower chamber and out of the unit A magnetic filter can be

Figure 8.5 Fine filter

positioned as shown in the central tube. A spring-loaded bypass is shown in the diagram, for lubricating oil filters only, to ensure a flow of oil should the filter become blocked. The cartridge in the design shown is disposable although designs exist to enable back-flushing with compressed air to clean the filter element as required. The filter unit shown will be one of a pair which can be alternately in service.

In full-flow filtration systems all the oil passes through the filter on its way to the engine. In a by-pass system most of the oil goes to the lubrication system and a part is by-passed to a filter. A higher pressure drop across the filter can then be used and a slower filtration rate. A centrifugal filter can be used in a by-pass system where the oil passes through a rotor and spins it at high speed (Figure 8.6). Dirt particles in the oil are then deposited on the walls of the rotor and the clean oil returns to the sump. This type of filter cannot block or clog and requires no replaceable elements. It must be dismantled for cleaning of the rotor unit at regular intervals.

Microbiological infestation

Minute microorganisms, i.e. bacteria, can exist in lubricating oils and fuel oils. Under suitable conditions they can grow and multiply at phenomenal rates. Their presence leads to the formation of acids and sludge, metal staining, deposits and serious corrosion. The presence of

Body

Figure 8.6 Centrifugal filter

Body

Hollow spindle -

Drive chamber

Figure 8.6 Centrifugal filter slime and the smell of rotten eggs (hydrogen sulphide) indicates a contaminated system.

Water in a lubricating oil or fuel oil, oxygen and appropriate temperature conditions will result in the growth of bacteria and infestation of a system. The removal of water, or ensuring its presence is at a minimum, is the best method of infestation prevention. The higher the temperature in settling, service and drain tanks holding fuel or lubricating oils, the better.

Test kits are available to detect the presence of bacteria, and biocides can be used to kill all bacteria present in a system. The system must then be thoroughly flushed out.

Exhaust emissions

Exhaust gases from engines and boilers contain atmospheric pollutants which are principally nitrogen oxides (NOx), sulphur oxides (SOJ, carbon oxides and unburnt hydrocarbon particulates. These various pollutants contribute to smog and acid rain, and carbon oxides contribute to the greenhouse effect, which is increasing global temperatures.

The IMO Marine Environment Protection Committee is considering ways to reduce the pollutants in exhaust emissions. IMO is to add a new Annex to MARPOL 73/78 to deal with atmospheric pollution.

The SOx content of emission may be reduced by either a reduction of the sulphur content in fuels or an exhaust gas treatment system. New engine technology may reduce NOx formation and thus emissions, while carbon oxides can be reduced by good plant maintenance.

Selective Catalytic Reduction Systems are in use on some vessels, which are said to reduce NOx emissions by 90 per cent and carbon oxides by 80 per cent. The equipment has been successfully operated on new-buildings and more recently as a retrofit on existing ships.

Major research initiatives are underway by engine builders, and classification societies, in cooperation with shipowners, in order to obtain data regarding achievable targets and suitable methods of measurement. This data will enable IMO and National Authorities to develop realistic legislation with which owners can readily comply on new and existing vessels.

The IMO Sub-Committee on Bulk Chemicals has prepared a draft of the new Annex to MARPOL 73/78, which will be considered at a conference to be held in 1996. Procedures to bring the amendments into force will be considered and the designation of special areas with specific emission criteria.

Solar Stirling Engine Basics Explained

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