Exam Question

(a) Cause and effect of microbial attack on lubricating oil:
Microbial attacks on crankcase lube oil are due to contamination of oil by water from leaks and condensation, the fuel, atmospheric air, cooling water, and even seawater. Cooling water, in particular, is a biological source of contaminant of crankcase oil. Bacteria are of two types: one grows in the presence of oxygen, and the other does not require oxygen. These microbes thrive in small amounts of water at the oily water interface, and they dislike movement of oil (favourable to grow when lube oil is not circulating, i.e. engine in stopped condition). The ideal temperature condition to grow is 25 - 40C. The additives in lube oil are consumed as nutrients by the bacteria. Under ideal conditions, bacteria grow very quickly.
The microbial attack causes lubricating oil to become slimy and will increase the viscosity, which results in frequent choking of filters, there will be a rotten egg smell and severe pitting corrosion of white metal in the crankcase. Fuel injection will be affected (Electronic engines), leading to misfiring and lack of power. The increased viscosity and an increase in the acidity of lubricating oils will cause overheating and corrosion within the bearings.
(b) Letter to Ship Owner/Operator