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Microbial infestation of lub oil

Microbial infestation of lube oil is the undesirable growth of bacteria, yeasts and/or moulds in  the oil. 
Infestation of the oil can be due to contaminated seawater, hydrocarbon source already 
onboard or due to poor housekeeping practices. 
The problems are like tank washing with contaminated seawater, water ingress from leaks of  seals (as in a stern tube, CPP etc.,), leaking cooling water, microbe infested fuel oil 
contamination, onboard contamination due badly maintained or stagnant tanks (long lay off),  bilge water etc., 

The effects of such microbial infestation can be
a. Slimy appearance of the oil; the slime tends to cling to the crankcase doors 
b. Rust films 
c. Honey-colored films on the journals, later associated with corrosion pitting 
d. Black stains on white metal bearings, pins and journals 
e. Brown or grey/black deposits on metallic parts 
f. Corrosion of the purifier bowl and newly machined surface 
g. Sludge accumulation in crankcase and excessive sludge at the purifier discharge 
h. Paint stripping in the crankcase 
i. Additive depletion 
j. Rancid or sulfitic smells 
k. Increase in oil acidity or sudden loss of alkalinity. (BN) 
l. Stable water content in the oil, which is not resolved by the purifier 
m. Filter plugging in heavy weather 
n. Persistent demulsification problems 
o. Reduction of heat transfer in coolers
To counter the problems of the microbial infestation, it is necessary to take steps to prevent 
such infestation. 
The microbial infestation is not possible without the presence of water. Some water in the oil  in inevitable due to leaks, condensation, etc., so it is necessary to constantly purify the oil. 
Tanks should be provided with drain cocks at the bottom most part to regularly drain the 
tank of water. The tanks should be designed to prevent any pockets where the flow is 
minimal or stagnant. The oil should be stored outside the 15~35˚C range. This temperature 
zone is very conducive for bacterial growth. Preferably, the tank should be kept at a higher 
temperature to facilitate sterilization. Lube oils have a maximum risk of infestation in the 
water-cooled pistons engine. Therefore, care must be taken not to allow any scope for water  leakage from the cooling system and the coolers should be kept leak free. 
Once the oil is infested, it is necessary to kill the bacteria by physical, chemical or other 

a. Settling: microbes settle the as they have a higher specific gravity ( 1.05) 
b. Centrifuges: they can be separated by centrifuging 
c. Filtration: microbes can be filtered by suitably staging filters 
d. Heat: This is a function of both temperature and time at that temperature. A 
temperature of over 70'C for 20 minutes is effective in killing the microbes. However 
this is difficult to achieve at the plate surfaces and it may be necessary to sterilize the 
tank first say by the use of steam lances before filling with oil for heat treatment 

Killing microbes using microbes is easy and effective, however the selection of chemicals 
appropriate for the system application and should be done with care. Such things as 
compatibility and hazards should be taken into account. However, if the infestation is acute,
 higher concentration of dosage of the chemical is required. This may render the oil unusable and has to be discharged to shore facilities. 

Other means of combating the microbes are available like 
a. Irradiation (UV Rays, Gamma Rays or X Rays)
b. By ultrasonic treatment of the infested oil
c. Using microwaves
d. Continuous pasteurization of the infested oil and heat control


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