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 70oC and circulating through purifier. Metals Content 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 anti wear 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 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 catalytic 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 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 neutralization 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)
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
