Lubricating Oil Degradation

Lubricating Oil Degradation

A lubricating oil may become unsuitable for its intended purpose as a result of one or several processes.

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Oxidation. Oxidation occurs by chemical reaction of the oil with oxygen. The first step in the oxidation reaction is the formation of hydroperoxides. Subsequently, a chain reaction is started and other compounds such as acid, resins, varnishes, sludge, and carbonaceous deposits are formed.

Water and air contamination.

Water may be dissolved or emulsified in oil. Water affects viscosity, promotes oil degradation and equipment corrosion, and interferes with lubrication.

Sources of Water Contamination

! Heat exchanger leaks

! Seal leaks

! Condensation of humid air

! Inadequate reservoir covers

! Temperature drops changing dissolved water to free water.

Forms of water in oil

! Free water (emulsified or droplets)

! Dissolved water (below saturation level).

Typical oil saturation levels

! Hydraulic--200 to 400 ppm (0.02 to 0.04%)

! Lubricating--200 to 750 ppm (0.02 to 0.075%)

! Transformer--30 to 50 ppm (0.003 to 0.005%).

Results of water contamination in fluid systems

! Fluid breakdown, such as additive precipitation and oil oxidation

! Reduced lubricating film thickness

! Accelerated metal surface fatigue

! Corrosion

! Jamming of components due to ice crystals formed at low temperatures

! Loss of dielectric strength in insulating oils.

Galvanic corrosion. Water may act as an electrolyte between dissimilar metals to promote galvanic corrosion. This condition first occurs and is most visible as rust formations on the inside top surface of the fluid reservoir

 Effects of water on bearing life. Studies have shown that the fatigue life of a bearing can be extended dramatically by reducing the amount of water contained in a petroleum based lubricant

 Effect of Water on Bearing Fatigue Life

Lubricant Water Concentration Relative Life Factor

SAE 20 25 ppm 4.98

SAE 20 100 ppm 1.92

SAE 20 400 ppm 1.00

(Reference: Effect of Water in Lubricating Oil on Bearing Life, 31st annual ASLE meeting, 1975.)

 Effect of water and metal particles. Oil oxidation is increased in a hydraulic or lubricating oil in the presence of water and particulate contamination. Small metal particles act as catalysts to rapidly increase the neutralization number of acid level.

Results of Dissolved Air and Other Gases in Oils

! Foaming

! Slow system response with erratic operation

! A reduction in system stiffness

! Higher fluid temperatures

! Pump damage due to cavitation

! Inability to develop full system pressure

! Acceleration of oil oxidation

c. Loss of additives

Two of the most important additives in turbine lubricating oil are the rust- and oxidation-inhibiting agents. Without these additives, oxidation of oil and the rate of rusting will increase.

Water may react with oxidation additives to produce acids and precipitates that increase wear and cause system fouling.

 Antiwear additives such as zinc dithiophosphate (ZDTP) are commonly used for boundary lubrication applications in high-pressure pumps, gears, and bearings.However, chemical reaction with water can destroy this additive when the system operating temperature rises above 60 °C .The end result is premature component failure due to metal fatigue

d. Accumulation of contaminants.

 Lubricating oil can become unsuitable for further service by accumulation of foreign materials in the oil. The source of contaminants can be internal or external.

Internal sources of contamination are rust, wear, and sealing products.

External contaminants are dirt, weld spatter, metal fragments, etc., which can enter the system through ineffective seals, dirty oil fill pipes, or dirty make-up oil

 Water can act as an adhesive to bind small contaminant particles into clumps that plug the system and cause slow or erratic operation. If the condition is serious, the system may fail completely.

e.Stresses on Lube oil

The main stresses experienced by Lube oils in diesel engines operating on heavy fuel oils are expressed as follows

Acid Stress- Caused by sulphuric and oxidation acids. This leads to increased corrosive wear, deposits, reduced Base Number and shorter oil life.Rapid depletion of the BN is the clearest sign of oil stress

Thermal/Oxidative stress-This caused by elevated temperatures leading to increased rates of thermal/oxidative breakdown of lubricant and fuel. This leads to increased levels of deposits, sludges, corrosive wear of bearing material, oil thickening and reduced oil life. In addition deposits on the under crown side of the piston can lead to increased hot corosion on the piston.

Asphaltene Stress-This caused by fuel contamination of the lube oil and can lead to increased levels of deposits, sludges, lacquers, oil thickening and reduced oil life. In addition deposits on the under crown side of the piston can lead to increased hot corosion on the piston

f.Cross Contamination with other oils due to mixing on account of seal failure (such as stuffing box leaks resulting in admixture of cylinder oil into crank case of a main engine) , results in stratification of suspended additives at working temperatures , leading to scoring of bearing metals due to stratification of calcium from its dissolved form (added as a TBN enhancer)

g.Electrostatic damage to bearings – leading to deterioration of oil by way of Lacquering due to electrical discharge , leading to damage of oil properties, hence stratification of additives when mixed with bearing metal debris, coupled with high temperature generated at area of operation.

h. Biological deterioration.

 Lubricating oils are susceptible to biological deterioration if the proper growing conditions are present. Bacterial attack

Certain bacteria will attack oil but water must be present. The bacteria may exist in a dormant state in the oil but water is required if they are to reproduce.

Microbes vary in size from 0.2 to 2.0 μm for single cells and up to 200 μmM for multicell organisms. Under favorable conditions, bacteria reproduce exponentially. Bacteria may grow by digesting oil, into an interwoven mass that will clog the filters, breakdown emulsions, acidity increases and forming corrosive films on working surfaces.

The severity of microbial contamination is increased by the presence of air.

In summary their must be three essential conditions for microbiological growth;

•        There must be a source of carbon- present in the oil

•        There must be some bacteria or fungal spores present-these are almost universally present in the atmosphere

•        There must be free water present

 

Types of growth depend upon the infestation as shown under:

Aerobic bacteria: Completely oxidized products (CO and H O) and some acids. Occasionally generate ammonia ,  forming slime during aggravated growth

Anaerobic bacteria : Incompletely oxidized and reduced products including CH₄ , H₂ , and H₂S - forming slime during aggravated growth adhering to steel surfaces.

Yeasts : Oxidized and incompletely oxidized products resulting in fall of  pH . often follow bacterial infections or occur when bacteria have been inhibited. Sometimes filamentous

Fungi (molds) : Incompletely oxidized product organic acids accumulate. Filaments of cells forming visible mats of growth. Spores may resemble yeasts. Both yeasts and molds grow more slowly than bacteria.

Two other factors which encourage the growth are a slight acidity in the water (pH 5 or 6) and a slightly raised temperature (20 to 40oC) which can lead to rapid growth.

Biocide additives are available but they are not always compatible with other desired additives and can lead to large organic blockages if treated in the machinery. The best solution is to avoid the presence of water. If mild attack takes place the oil may be heated in the renovating tank to above 90oC for 24hrs before being returned to the sump via the centrifugal separator.

For a severe attack the only solution is complete replacement of the charge followed by sterilisation of the system. It may be noted that on replenishment the bacteria may be present in a dormant state in the new charge.