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Lubricating oils

Lubricating oils.

miranda-buckner
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Lubricating oils

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  1. Lubricating oils Lubricating oil fractions extracted from crude oil are a widely varying mixture of straight and branched chain paraffinic, napthenic aromatic hydrocarbons having boiling points ranging from about 302o to 593oC. Some specialty lubricants may have boiling point extremes of 777 and 815oC. The choice of grade of lubricating oil base is determined by the expected use.

  2. General capabilities expected from an engine lubricant • Dispersivity or capacity to the cold parts of an engine clean • Detergency or capacity to keep hot parts of an engine clean • Thermal strength or capacity to withstand temperature changes • Anti-oxidant or capacity to resist the action of oxygen • Anti-wear or capacity to contain wear • Anti-scuffing or capacity to preserve oil film even in the presence of high pressures • Alkalinity reserve or capacity to neutralise acids formed during combustion or other sources thereby preventing corrosive wear • Demulsibility or capacity to separate contaminants • Resistance to hydrolysis or capacity to withstand the action of water which can affect additives • Centrifugibility and filterability or capacity to separate insoluble elements • Anti-rust, anti-corrosive and anti-foam are just some of the other properties required

  3. Properties ideal for bearings • Soluble for high speed fluid film hydrodynamic lubrication, hence, low viscosity with reduced oil film friction. • moderate bearing loads • improved heat transfer behavior • corrosion protection • cooling • low friction • good low temperature viscosity • good high temperature viscosity

  4. Cylinder lub oil properties • Normal properties required are: • adequate viscosity at working temperature so that the oil spreads over the liner surface to provide a tough film which resists the scraper action of the piston rings • the oil must provide an effective seal between the rings and liner • only a soft deposit must be formed when the oil burns • alkalinity level (total base number or TBN) must match the acidity of the oil being burnt • detergent and dispersant properties are required in order to hold deposits in suspension and thus keep surfaces clean • Behaviour depends upon the temperature of the liner, piston crown and piston rings. TBN and detergency are closely linked. This can have an adverse effect when running on lighter fuels with lower sulphur content for any period of time. Coke deposits are can increase.

  5. Properties ideal for gear case • high film strength to prevent metal to metal contact. Hence, high viscosity adhesive to resist sliding and centrifugal forces • corrosion protection • cooling • reduces friction • good low tempo viscosity • good high tempo viscosityThe thicker the oil film the greater the cushioning against shocks. Also less tendency for pit formation by hydraulic action in cracks, • sound damping properties with cushioning effects • antifoam properties

  6. Turbine oil • Compromise between above two requirements • Generally a good quality refined mineral oil derived from paraffanic base stock used with various additives including EP additives for highly loaded gearing. • Anti-foaming properties important

  7. Additives • Improvements in lubricating oil over the last twenty years have come about almost entirely from the use of additives. • These are added for three main reasons; • to protect the lubricant in service by limiting the chemical change and deterioration • To protect the mechanism from harmful combustion products and malfunctioning lubricating oil • To improve existing physical properties and to create new beneficial characteristics in the oil

  8. Typical additives are; Barium, calcium, phosphorus, Sulphur, chlorine, zinc • Oxidation inhibitor-increases oil and machinery life, decreases sludge and varnish on metal parts • Corrosion inhibitor-protects against chemical attack of alloy bearings and metal surfaces. • Antiwear improvers-protects rubbing surfaces operating with this film boundary lubrication. • Detergent-tend to neutralise the deposits before formation under high temperature and pressure conditions, or as a result of using a fuel with high sulphur content. The principle detergents are soaps and alkaline metals, usually calcium (often referred to as 'metallo-organic compounds'). They are usually ash forming and spent additive will contribute to the insolubles loading of a used oil. It should be noted that additives which do not burn cleanly without ash tend to be avoided for use with Cylinder Lubricating Oils.

  9. Dispersant-used to disperse or suspend the deposits forming contaminants. Typical dispersants, such as polyesters and benzlamides, are usually clean burning. The molecules have a polar charge at one end which attracts and holds the deposits • Alkaline agents-neutralises acids, these form the TBN of the oil and includes additives such as the above dispersants and detergents. An excess of acid neutralising alkalis are present in the oil and these help to keep parts clean. Failure to keep an oil alkaline can lead to damage to bearings due to acidic attack as well as increased liner wear. • Rust inhibitors- • Pour point depressants-improves low temperature viscosity • Oiliness agent-reduces friction seizure point and wear rates • EP additives-increases film strength and load carrying capability • Antifoam agents-prevents stable bubble formation • Metal deactivators-prevent catalytic effects of metal • Antiseptic-bactericide. • Oxidation - Oxidation degrades the lube oil producing sludges, varnishes and resins. Presence of moisture, and some metals particularly copper tend to act as a catalyst. Once oxidation starts, deterioration of the properties of the oil is rapid.

  10. Recharging - When recharging no more than 10 % of the working charge should be topped up due to heavy sludgeing that can occur due to the heavy precipitation of the sludge. • EP additive oils - Can assist in healing of damaged gear surfaces but should be used as a temporary measure only due to risk of side effects • EMULSIFICATION - This occurs due to water contamination; also, contamination with grease, fatty oils, varnish, paint and rust preventers containing fatty products can also promote emulsification. The presence of an emulsion can be detected by a general cloudiness of the sample. Salt water emulsifies very easily and should be avoided. Water entrained in the oil supplied to a journal bearing can lead to loss of oil wedge, rub and failure. Fresh water contamination whilst not in itself dangerous can lead to rusting. The iron oxides catalyses the oil to form sludge's. The additives in the oil can leach out to change the water into an electrolyte. Salt water contamination is very serious as it causes tin oxide corrosion, and also leads to electrochemical attack on the tin matrix in the white metal. The sea water act as then electrolyte. A major problem of water within a lub oil is where the mix enters a bearing, here it is possible for the water to be flashed off collapsing the oil wedge.

  11. OIL ANALYSIS • Regular testing of crankcase lub oil is important to ensure that deterioration has not taken place. The results of in service deterioration could be a reduction in engine protection or actual attack on working points by corrosive deposits. Oil samples are generally tested every 3 to 4 months depending on the system and experience. Shipboard testing is taking a rising prominence to allow monitoring of oil condition between testing. • To ensure good representation, care should be taken where the sample is drawn • Correct • Main supply line • inlet or outlet from lub oil cooler • Outlet from main lub oil pump • Incorrect • standpipes • purifier outlet • purifier direct sump suction • Samples should be drawn over a period of several minutes

  12. Viscosity • The viscosity is the most important property of the oil. Oil of correct viscosity will provide optimum film strength with minimum friction losses and leakage. • The viscosity of a L.O. may fall due to fuel dilution if running on gas oil, and rise if running on heavy f.o. Viscosity may also increase due to heavy soot loading if purifiers and filters not operating efficiently. Oil ageing caused by oxidation and thermal degradation increases viscosity. • A simple shipboard test is the Mobil flow stick where drops of new and used oil are placed in separate channels on an inclined 'stick'. The rate the oil flows down the stick is proportional to its viscosity.

  13. Water content • Initially determined by 'crackle' test. The presence of Na and Mg in a 4:1 ratio indicates salt water contamination. • Limits are laid down by the manufacturer, but as a rule of thumb a limit of 0.2% should cause investigation into source and remedial action at 0.5% • Gross contamination can be remedied by placing the charge in a separate tank and heating to 70oC and circulating through purifier.

  14. Spectrometry • Indicates the presence of metal element composition and identifies additive and contaminant levels. • Zinc(Zn),Phosphorus(P)- are components of many oils such as diesel engine oils, hydraulic oils and gear oils, to enhance antiwear and over properties of the oil • Calcium(Ca)- primarily a component of engine oils, provides detergency,alkalinity and resistance to oxidation. Residual fuel engine oils have higher Ca levels • Nickel(Ni)- Bearings, Valves, gear plating, fuel derivative • Barium(Ba)- Multi purpose additive, declining importance • Magnessium(Mg)- as for Ca, may also be due to sea water contamination if found in Ratio of 1:4 of Na • Chromium(Cr)- Piston rings, hydraulic actuator cylinders • Manganese(Mn)- Cylinder wear

  15. Aluminium(Al)- generally comes from wearing piston skirts, levels rise where new piston fitted to old engine. Typically 10ppm, but rises during bedding in. May also indicate the presence of catylytic fines in residual fuels. • Iron(Fe), Molybdenum(Mo), Chromium(Cr)- metals alloyed for piston ring etc, a rise in level may indicate ring pack/liner wear. • Copper(Cu), Lead(Pb) , Tin(Sn), Silver(Ag) - soft metals used in the overlay of shell bearings, and phosphor bronze gears.Note that high copper content can also occur when samples are drawn from copper pipes which have not been flushed as well as gear wear. • Silicon(Si)- Indicates poor air filtration, possible fuel derivative • Sulphur(S)- May indicate the presence of clay based (bentonite) greases • Sodium(Na)- With Mg indicates the presence of sea water contamination, possible coolant system and fuel derivative • Vanadium(V)- Usually indicates the presence of fuel oil

  16. Alkalinity and acidity • TBN-TOTAL BASE NUMBER- measure of alkaline additives available for the neutralisation of acids from combustion products and oxidation. Level governed by type of fuel. • For crosshead engines the TBN will tend to rise due to contamination by liner lubrication, it should not be allowed to raise more than twice that of the new charge. • As a guide, the TBN of fresh oil should be at least: • 10 x fuel sulphur content (%) for trunk piston engines (10mgKOH/g) • 20 x fuel sulphur content (%) for cyl oil in x-head engines (20mgKOH/g) • A simple shipboard go,no-go test is available for measuring the TBN, it involves the addition of an indicator and acid reagent to a 30ml sample. The quantify of acid reagent added is determined by the required level of TBN, for TBN2.5 0.5ml are added, for TBN20 4ml is added. After three minutes the colour is checked against a chart • Purple:Good level of TBN • Green:Borderline • Yellow:Low level of TBN

  17. TAN-TOTAL ACID NUMBER-measure of organic acid and strong acid content of oil. Where SAN is nil, the TAN represents the acidity in the oil due to both the acids in the additives and the oxidation of the hydrocarbons in the oil. The TAN of fresh oils varies with oil type, and tends to climb with age. A high TAN may indicate that an oil should be changed or freshened by top up. A high TAN may be accompanied with increased viscosity. • SAN-STRONG ACID NUMBER-indicates the presence of strong, highly corrosive (inorganic) acids, usually formed from combustion products. If SAN is not zero the oil should be changed immediately • Oil cleanliness • IC-INDEX OF COMBUSTION-measures soot loading of oil • MD-MERIT OF DISPERSANCY-Ability of an oil to disperse contaminants, such as soot, wear debris and water and thereby carry them away from the critical areas. Measured by oil blot test and should not be allowed to fall below 50 • DP-DEMERIT POINTS- combination of IC and MD: the lower the value, the healthier is the condition of the oil

  18. Shipboard water content test • The flask is filled to mark 'A' with kerosene. A capsule of reagent (calcium hydride) is added. Any water in the kerosene will react with the calcium hydride and any gas vented off. The container is topped to mark 'B' with sample oil. The screw valve and cap are closed. The flask is inverted and shaken. After 2 minutes the screw valve is opened. The hydrogen produced by the reaction between the reagent and water exerts a pressure which forces the kerosene through the open valve into the graduated cylinder. The amount discharged is proportional to the water content in the oil sample. If the water content is greater than 1.5% then the test should be repeated this time using a smaller sample by filling only to mark 'C'. The second scale on the graduated cylinder should then be used. If water is detected its type, sea or fresh , should then be determined by use of a special reagent the water

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