Regular testing of the crankcase lube oil is important to assure that deterioration has not taken place. The results of in service deterioration could be a reduction in the engine protection or actual attack on working points by corrosive deposits. Oil samples are normally tested every 3 to 4 months depending on the system & experience. Shipboard testing is taking a rising prominence to permit monitoring of the oil condition between testing.
To assure good representation, care should be taken where the sample is drawn
Correct
- Main supply line
- inlet or outlet from the lube oil cooler
- Outlet from the main lube oil pump
Incorrect
- standpipes
- purifier outlet
- purifier direct sump suction
Samples should be drawn over the 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 & leakage.
The viscosity of a lube oil may fall due to fuel dilution if running on the gas oil, & rise if running on the heavy fuel oil Viscosity may also increase due to the heavy soot loading if purifiers & filters not operating efficiently. Oil ageing caused by the oxidation & thermal degradation increases viscosity.
A simple shipboard test is the Mobil flow stick where the drops of the new & used oil are placed in the separate channels on an inclined stick. The rate the oil flows down the stick is proportional to its viscosity.
Water content
Initially determined by the crackle test. The presence of Na & Mg in a 4:1 ratio indicates salt water contamination.
Limits are laid down by manufacturers, but as a rule of thumb a limit of 0.2% should cause investigation into source & remedial action at 0.5% Gross contamination can be remedied by placing the charge in a separate tank & heating to 70°C & circulating through purifier.
Spectrometry
Indicates the presence of the metal element composition & identifies additive & contaminant levels.
Zinc(Zn), Phosphorus(P) – are components of many oils such as diesel engine oils, hydraulic oils & gear oils, to improves antiwear & over properties of the oil
Calcium(Ca) – primarily a component of the engine oils, provides detergency,alkalinity & 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 the ratio of 1:4 of Na
Chromium(Cr) – Piston rings, hydraulic actuator cylinders
Manganese(Mn) – Cylinder wear
Aluminium(Al) – Normally comes from wearing piston skirts, levels rise where new piston is fitted to the old engine. Generally 10ppm, but rises during bedding in. May also indicate the presence of the catylytic fines in the residual fuels.
Iron(Fe), Molybdenum(Mo), Chromium(Cr)- metals alloyed for the 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 the shell bearings, & 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 the clay based (bentonite) greases
Sodium(Na) – With Mg indicates the presence of sea water contamination, possible coolant system & fuel derivative
Vanadium(V) – Normally indicates the presence of fuel oil
Alkalinity and acidity
TBN – TOTAL BASE NUMBER- measure of the alkaline additives available for the neutralisation of the acids from combustion products & oxidation. Level governed by type of the fuel.
For crosshead engines the TBN will tend to rise due to the contamination by liner lubrication, it should not be allowed to raise more than twice that of new charge.
As a guide, the TBN of the 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 & acid reagent to a 30ml sample. The quantify of the acid reagent added is determined by the required level of TBN, for TBN2.5 0.5ml are added, for TBN20 4ml is added. After 3 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 the organic acid & strong acid content of the oil. Where SAN is nil, the TAN represents the acidity in the oil due to both the acids in the additives & the oxidation of the hydrocarbons in the oil. The TAN of fresh oils varies with the oil type, and tends to climb with age. A high TAN may indicate that an oil should be changed or freshened by the top up. A high TAN may be accompanied with the increased viscosity.
SAN-STRONG ACID NUMBER – indicates the prescience of the 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 the oil
MD – MERIT OF DISPERSANCY – Ability of an oil to disperse contaminants, such as the soot, wear debris & water & thereby carry them away from the critical areas. Measured by oil blot test & should not be allowed to fall below 50
DP – DEMERIT POINTS – combination of IC & MD: the lower the value, the healthier is the condition of the oil.
Shipboard water content test
- The flask is filled to mark ‘A’ with kerosene
- A capsule of the reagent (calcium hydride) is added. Any water in the kerosene will react with the calcium hydride & any gas vented off. T
- The container is topped to mark ‘B’ with sample oil
- The screw valve & cap are closed.
- The flask is inverted & shaken
- After 2 minutes the screw valve is opened. The hydrogen produced by the reaction between the reagent & 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.
- 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.
- If water is detected its type, sea or fresh , should then be determined by the use of a special reagent the water.
