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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 l.o. cooler
• Outlet from main l.o. pump
Incorrect
• standpipes
• purifier outlet
• purifier direct sump suction

Samples should be drawn over a period of several minutes

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.

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 70^oC and circulating through purifier.

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

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 sw contamination, possible coolant system and fuel derivative

Vanadium(V)- Usually indicates the presence of fuel oil

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

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 prescience of strong, highly corrosive (inorganic) acids, usually formed from combustion products. If SAN is non 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

Shipboard water content test

1. The flask is filled to mark 'A' with kerosene
2. A capsule of reagent (calcium hydride) is added. Any water in the kerosene will react with the calcium hydride and any gas vented off.
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3. he container is topped to mark 'B' with sample oil
4. The screw valve and cap are closed.
5. The flask is inverted and shaken
6. 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.
7. 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.
8. If water is detected its type, sea or fresh , should then be determined by use of a special reagent the water