Higher BNs could save millions claims Castrol Marine

Castrol Marine has presented evidence that higher Base Numbers (BN) in marine engine lubricants can prevent cold corrosion.

Castrol has presented case studies to CIMAC (International Council of Combustion Engines) in support of more cost-effective control of cylinder liner corrosion.

Marine cylinder oils have a higher alkalinity (BN) than automotive lubricants. Their role is to neutralise any acidic residues which are formed by combustion. The BN also calibrates the alkali concentration present in the lube and the lubricant replenishment rate.

With an industry drive to reduce bunker (fuel) consumption in vessels, slow steaming and lower loads have been seen as part of the solution.  However, slow steaming presents other problems which include:

• a high sulphur content (vessels with lower loads often use a fuel with sulphur content over 2.5%.)
• difficulty in achieving optimal cylinder liner temperatures and, thus, finding the right cylinder oil for the job
• lower engine load leading to the lubricant staying the cylinder for longer (lubricant residence) and therefore a reduced refresh of the lubricant.

Another solution to high bunker consumption has been the introduction of longer-stroke engines. But, according to the Castrol report, this in turn increases cylinder liner corrosion and increased combustion pressures with cooler cylinder liners showing evidence of increased sulphuric acid condensation.

The bottom line, states Castrol, is that shipping companies are faced with huge bills for the effects of corrosion on cylinder liners. Shortened liner life could cost as much as $3.5m in replacements, excluding the cost of piston rings and liners. The implication being that by improving liner wear through better lubricants can extend the life of both liners and vessels.

The case studies compared vessels using mid-BN cylinder oils to 80 BN oils and found conclusive evidence of the high BN oils arresting corrosive wear as well as improving the cleanliness of the piston rings and crowns.