CONTAMINANTS OF LUB. OIL

a)         Fuel Combustion products - In internal combustion engines, due to improper combustion, there are materials such as unburnt and partially burnt hydrocarbons, carbon in the form of soots and sulphur acids. These gases, luquids and solids enter the crankcase due to blow - past of piston rings and slack stuffing box. These fuel - derived products oxidise to form varnish. Acids from fuel decomposition can increase wear and corrode non - ferrous parts such as bearings.

b)         Liquid fuel - Liquid fuel can contaminate engine Lub oil due to dripping injector, over fueling for power and leaks in fuel fittings. Fuel dilution reduces Lub. oil viscosity, can cause scuffing, bearing failure and crankcase explosion.

c)    Water and Coolant additives - Water reaches the crankcase as vapour in blow -past gases, condensation of water vapour and moisture from humid outside air and leaks in the cooler, mix with the oil. These conditions are enhanced while allowing jacket water temperature to be lower than normal and engine operating for short and infrequent periods. Larger quantities of oil are usually caused by leaks in the
cooling system.

Effects - (i) Sludge formation which may obstruct oil lines, (ii) Adverse effect on additives, even causing precipitation from oil solution in extreme cases, (iii) Rusting and corrosion resulting in high wear and bearing failure, (iv) Impaired lubricant film in heavily loaded bearings, (v) Corrosion preventive water additives are usually water      soluble salts containing sodium nitrate and sodium borate and generally are not
compatiable with the oil additives and can be extremely harmful. Products of mechanical wear - Engine wear metals and rust particles are normally separated by filtration but fine particles if present can cause bearing wear. Also non dispersed fuel soot can form deposits when it combines with soluble fuel and lubricant deterioration products.

e)    Biological elements-.Micro - organisms growth takes place due to the presence of water, oxygen and favourable temperature.

OXIDATION

Lubricating oils whether obtained from paraffinic oil or napthemic oil, are susceptible to oxidation forming organic acids and sludge deposits.

In virtually all machinery applications, lubricants are in contact with air and hence with oxygen. Lub oil react chemically with oxygen and is accelarated under high load, high speed and metallic contact. The metal surfaces throughout the engine act as Catalysts which speed up oxidation and the finely dispersed metal - particles from engine wear, particularly copper from bearings and bushings are even more potent as Catalysts. In general, oxidation is harmful as apart from acid attack on sensitive metals, sludge, particularly if excessive, tends to adhere to metal surfaces. If subjected to higher temperature, it will form insoluble resins, gums or varnish deposits. Oxidation also causes increase in viscosity which results in higher internal friction and less cooling effect.

In diesel engines, sludge can contribute markedly to piston deposits. Cold sludge, which is a complex mixture of water, solid contaminants and oil decomposition products can cause choking of lines and filters and oil viscosity increases beyond acceptable limits.

The oxidation rate of oil is roughly doubled for every 10°C rise in temperature. Below about 82°C, this effect is not considered to be of great importance but above this temperature, oxidation increases progressively rapidly with undesirable results.

Oxidation of mineral oil is a free chain radical reaction and this can be slowed down by the use of certain chemical additives termed anti - oxidants.

CLOUD POINT, POUR POINT FLOC POINT

The pour point of an oil is defined as 2.7°C above that temperature at which the oil just fails to flow under prescribed condition.

With paraffimic oils, as the temperature is lowered, wax crystals are formed which finally prevent flow. If however, the oil is agitated, these crystals will break up and the oil will continue to flow at an appreciably lower temperature.

Cloud point, therefore, relates to essentially paraffimic fuels or lubricating oils and it is the temperature at which a cloud or haze first appears when cooled under prescribed conditions in simple laboratory test equipment.

Cloud point is more important in fuels than in lubricants, as the formation of wax crystals may restrict flow from the bunkers to the fuel equipments and may block cold filters and cut - off fuel supply to the engine. The main objection to high cloud points in lub. oil is where the oil is supplied by simple wick feed oilers, as the formation of wax crystals will restrict oil flow by capillary action under cold operating conditions.

With conventional refrigerating oil, an additional test the "floc point" is carried out. This indicates how the oil flows at low temperatures in the presence of a refrigerant. Although refrigerating oils are predominantly napthenic in origin because of their naturally low pour points, nevertheless they contain a proportion of wax - containing paraffins. If the floe point is high, then trouble could be experienced if oil is carried over with the gaseous refrigerant, as wax deposition could occur in the cooling coils or other low temperature areas of the system.     

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