Lubricant operating temperatures of crankcase oils in wet engines are normally sufficient to control and prevent infestation. Any problems are usually associated with ships which have shut down their lubricant system, allowing temperatures to drop and water to accumulate.
There are many thousands of types of microbes but only a few are able to grow in lubricating oils at the elevated operational temperatures in use. Provided the plant treatment equipment is functional, the purifier heater temperature is maintained and the water content kept at a minimum, the lubricating oil system will be both environmentally and nutrient deficient. Such conditions will prevent microbes from establishing themselves. Should heavy contamination of the system occur, however, this self-regulating mechanism will be unable to prevent microbial proliferation (Figure 9). Fortunately, the symptoms of microbial contamination will not occur immediately, allowing engineers to implement remedial physical and/or chemical decontamination programmes.
|LUBE OIL CONTAMINATION|
Figure 9 Interactive problems associated with microbial corrosion (Lloyd’s Register)
The visual symptoms of microbial contamination of lubricants are:
- Slimy appearance of the oil, the slime tending to cling to the crankcase doors.
- Rust films.
- Honey-coloured films on the journals, later associated with corrosion pitting.
- Black stains on whitemetal bearings, pins and journals.
- Brown or grey/black deposits on metallic parts.
- Corrosion of the purifier bowl and newly machined surfaces.
- Sludge accumulation in the crankcase and excessive sludge at the purifier discharge.
- Paint stripping in the crankcase.
Operational symptoms of microbial contamination in lubricants are:
- Additive depletion.
- Rancid or sulphitic smells.
- Increase in oil acidity or sudden loss of alkalinity.
- Stable water content in the oil which is not resolved by the purifier.
- Filter plugging in heavy weather.
- Persistent demulsification problems.
- Reduction of heat transfer in coolers.
Microbial growth occurs in the water associated with the lubricant, and the phenomenon is therefore characteristic of crankcase oils in wet engines, particularly those with water-cooled pistons. Lubricant infection can be identified by slimy film formation on the crankcase doors. There may also be a rancid odour and whitemetal parts may be stained black. As the problem progresses, filters choke up, organic acids are formed and the oils tend to emulsify.
If sulphate reducing bacteria (SRB) are present—a problem particularly associated with laid-up tonnage—copious pitting of ferrous and non-ferrous metals may occur (Figure 10).
Since micro-organisms feed upon the additives within the oil the lubricity of the oil may be impaired and its viscosity altered; and there is a greater resultant acidity and increased potential for emulsification and corrosion. Hydrogen sulphide may also be produced as a byproduct. Serious corrosion problems will result within weeks of the initial contamination if these factors occur at the same time.
Sources of contamination within the lubricant are the fuel, cooling water and seawater. Cooling water in particular has featured as a common contaminant of crankcase oil at engine operating temperatures, since the use of chromates as corrosion inhibitors is banned. The use of chromates also acted as an effective anti-microbial biocide.
Prevention is better than cure and it is known that microbial growth is retarded by extreme alkaline conditions. To date, there have been no microbial problems reported with medium speed engines during operation when using highly alkaline lubricants of BN 12 to 40. At elevated temperatures lubricating oils tend to be self-sterilising. Unfortunately, however, there still remains the possibility of infestation
from contaminated bilges and tanktops which may inadvertently leak into the oil system.
Figure 10 Anaerobic corrosion by sulphate reducing bacteria (SRB)
Since the majority of operational microbial problems in lubricating oils arise due to infection from cooling water, seawater from cooler leaks and overflows of contaminated bilges, the effectiveness and efficiency of plant treatment are important remedial measures.
The following procedures are recommended to avoid microbiological problems:
- Ensure that the water content of crankcase oil does not rise above 0.5 per cent by weight.
- Check that purifier suction is as near to the bottom of the sump as possible.
- Maintain a minimum oil temperature after the purifier heater of 70°C or higher for at least 20 seconds and/or 80°C for 10 seconds.
- Circulate the volume of oil in the sump via the purifier at least once every eight to ten hours.
- Regularly check that coolant corrosion inhibitor concentrations are at the manufacturer’s recommended values.
- Monitor the microbial population of the cooling water and prevent water leaks into the oil system.
- Test for microbial contamination of the oil system and monitor whether the oil after the purifier is sterile.
- Prevent ingress of contaminated bilge water.
- Inspect storage tanks and regularly check for water.
(The above information is based on advice from Lloyd’s Register’s Fluid Analytical Consultancy Services (FACS) department which has considerable theoretical and practical expertise in assessing and resolving shipboard microbial contamination problems with fuel and lube oils.)