Key parameters analysed during laboratory testing of lubricating oil for ship machinery, and how results indicate oil and machinery condition
Samples of lube oil (main engine and auxiliary engine crankcase oil) are sent to a laboratory and analysed. Key parameters:
- Viscosity (at 40 C or 100 C): indicates if the oil has thinned (dilution by fuel, and/or shear) or thickened (oxidation, contamination, soot/gel). Increased viscosity suggests oxidation/contamination; a fall indicates fuel dilution - both affect film strength.
- Total base number (TBN): for the alkaline additive reserve neutralizing acid from combustion (sulphur). Falling TBN indicates the neutralising reserve is being consumed - time to change oil. It relates to deposit/acidity control.
- Insolubles (particles - soot, carbon, wear metals, debris) measured by pentane/hexane insolubles and total insolubles: rising insolubles indicates combustion blow-by, contamination and approaching oil-change limit.
- Wear metals (ppm of iron, copper, aluminium, chromium, lead, tin, silicon, sodium, etc.): rising iron/copper indicates abnormal wear of bearings/liners; aluminium - aluminium piston/head; chromium - ring wear; sodium - sea-water/water contamination; silicon - dust; these identify the source of metal release (worn bearings, rings, liners).
- Water content (K.F./distillation): any water indicates contamination (sea water, cooling leak) which promotes corrosion and premature breakdown.
- Fuel dilution (%): indicates fuel/piston-ring blow-by diluting and thinning the oil, risking bearing failure.
- Acid number / total acid (TAN): whether the oil is becoming acidic from oxidation and contamination, indicating acid attack and degradation.
- Flash point: low flash point indicates fuel dilution, a safety/temperature concern.
- Additive/detergent (base) content and presence of glycol/other contaminants - tests for cross-contamination.
How results help determine oil and machinery condition
- A healthy oil shows viscosity, TBN, acid and insolubles within expected bands; rising wear metals correlate with specific component wear, so a sudden increase of iron/copper warns of impending bearing/liner failure.
- Fuel dilution, water and acid problems indicate a fault (injector/piston-ring blow-by, cooling leak) requiring attention before catastrophic damage.
- Trending the results between analyses lets the engineer predict when to renew the oil, identify the need for preventive maintenance (e.g. ring/injector overhaul), detect contamination sources, and protect the machinery by taking corrective action (change oil, investigate, adjust). The lab report's limits and comparison with previous results thus direct maintenance and prevent expensive damage.