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Thursday, April 30, 2020 

After 100 days of operation on very low sulphur fuel oil (VLSFO), Lukoil Marine Lubricants is sharing recent experience on VLSFO operation with Navigo MCL Extra (40BN), drawing on learnings over the past few months as well as experience from the transition from HFO to distillate fuels with Navigo MCL Ultra (20BN).

Lukoil says that using a cylinder oil with extra detergency is only the first step to avoid excessive liner wear on VLSFO. Feed rate optimisation is equally important for good engine condition, in combination with engine inspections and monitoring the wear metal content of drain oil samples.

The fuel that operators are switching from, the new fuel and how the transition is managed all play a major role in ensuring a smooth changeover. HFO is stable because it contains many polar hydrocarbons. Distillate fuels are non-polar and can cause instability in mixtures. Many ultra low sulphur fuel oil (ULSFO) and VLSFO blends are mixtures. When changing over from HFO to these fuels, the sludge, asphaltenes and cat fines which may have accumulated over time while on HFO can remain on the tank bottom and walls or in the fuel system unless these are fully cleaned or flushed as it was recommended before the transition to VLSFO.

This issue can be made worse because the ability of distillates, ULSFO and VLSFO to absorb cat fines and asphaltenes can cause them to be carried through to the filter, purifier and engine. Some ULSFO and VLSFO are treated with additives to improve their stability, but these further increase the fuel’s absorbance and must be handled with care. Mixing fuels can cause problems due to incompatibilities which cannot be completely avoided.

Some 100 BN cylinder oils offer improved detergency. Therefore, in some cases high BN cylinder oil is used for a short time to counteract deposits in the engine. In the long run, however, using excessive BN with VLSFO will lead to deposits in the piston top land and ring pack, which can ultimately lead to scuffing and, consequently, loss of liners and off-hire of the vessel. Navigo MCL Extra has detergency comparable to a 100 BN cylinder oil but a lower alkalinity or neutralization capacity (BN). Another risk related to using high BN cylinder oil continuously for VLSFO operation is that it will change the surface structure of the liner and counteract the controlled corrosion that MAN favours in order to keep the surface rough. In this situation new rings - such as those with Cermet coating - are required to reduce the risk of scuffing.

An optimal feed rate is always better than an excessive feed rate. Each engine must be evaluated individually and the condition of liners and rings dictates the required feed rate. It is also necessary to consider potential loss of efficiency in old, worn lubricators. The resulting deviation is more critical on VLSFO then it may have been on HFO. An insufficient feed rate and BN while on VLSFO can also lead to cold corrosion and resulting high wear.

If an engine is not equipped with a piston cleaning ring or mini-sac nozzle, increased fouling of the exhaust gas and piston running components, such as top land, may lead to oil film disturbance and ultimately scuffing.

An elevated feed rate is required to flush out wear particles to prevent further abrasive wear by the piston ring. The correct feed rate and BN level of a cylinder oil with low alkalinity but increased detergency is needed to keep liner, piston and ring pack clean and with controlled wear levels.

Lukoil says that generalizing high feed rates as harmful is incorrect and may even be dangerous for certain engines, so the company recommends following OEM recommendations and lubricant manufacturers’ advice at all times.

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