TECHNICAL • LUBES
Matching lubricant to fuel type
The 2020 introduction of the global 0.5% fuel sulphur cap is expected to have an impact on lubrication issues – somewhat less than for the 0.1% ECA sulphur limit, but there are still considerations that shipowners and operators will have to bear in mind.
Heavy Fuel Oil (HFO) varies considerably in sulphur content, depending on the source of the basic crude oil, and the refining process. The current maximum permitted level is 3.5%. The two general underlying principles of operating with varying sulphur content are that the sulphur in the fuel adds an inherent lubricity, while on the other hand burning high sulphur oils can result in corrosion in the cylinders.
This means that auxiliary equipment such as fuel pumps, which traditionally have relied on the sulphur in the fuel for lubrication, risk increased wear when the sulphur content is reduced.
Cylinder corrosion, which has proved to be a potential problem with cool- running engines, such as at partial load when slow steaming, can successfully be countered by using a high-alkalinity cylinder lubricant. However, the degree of alkalinity – denoted by the total base number (TBN, or often BN) – must be carefully matched to the fuel content and running conditions.
While too low a TBN can result in the formation of sulphuric acid, and hence corrosion, using a lubricant with too high
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a TBN can result in calcium ash deposits, which become hardened, causing unwanted bore polishing. MAN B&W guidelines suggest that slight corrosion is beneficial, so the lubricant TBN should not prevent corrosion, but instead control corrosion. Much depends, too, on fuel quality, engine design and lube oil feed rates.
The major difficulty comes when switching between heavy fuel oil and distillate fuels, for example when entering or leaving a sulphur emission control area, where 0.1 maximum sulphur fuel must be used. This means a change of lubricant, to an ultra-low TBN to match the ultra-low sulphur fuel. This is still likely to be necessary with the 0.5% HFO post- 2020. There is a chance that refining capacity may not be sufficient to produce sufficient 0.5% fuel to meet demand, in which case distillate fuels will be more widely used by shipping, meaning a very low TBN cylinder oil and maybe a lubricity-enhancing additive.
Where operating conditions and fuel specification are reasonably consistent,
it should be possible to run on a standard TBN cylinder oil at a constant feed rate. However, when these parameters vary, it will be well worth undertaking regular analysis of used cylinder oil, either at a shore laboratory or using on-board test equipment from suppliers such as Parker Kittiwake, which will indicate whether adjustments need to be made to feed rate or TBN.
Another system worth considering is blending on board. The Sea-Mate equipment from Maersk Fluid Technology takes system oil from the engine, and blends it with a high-TBN additive to produce an optimised cylinder oil for the individual engine. This has the additional advantage that the system oil is slowly and constantly replenished, reducing wear risk and waste oil disposal costs.
Sea-Mate system by Maersk Fluid Technology
Finally, another environmental issue affecting lubricants is the need to use environmentally acceptable lubricants (EALs) in certain areas: primarily the US, but certain to be adopted elsewhere. This is mainly concerned with any possible interface between oil and sea (or river) water, such as stern tubes, rudder bearings, thrusters and so forth, but also applies to winches, wire ropes and any equipment where oil or grease may come into contact with the water. Various types of EALs, including biological and synthetic oils, are available from all of the major lubricant suppliers.
Seatrade Maritime Review • Quarterly Issue 2 • June 2017 85
PHOTO: SHELL
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