L.O ANALYSIS AND TESTS

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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