MAN Diesel & Turbo has warned against using existing mid-range cylinder lube oils in its two-stroke engines when burning fuel with a high sulphur content of 1.5 % or more. It is a warning that should be heeded by many tanker operators, whose ships represent a large proportion of those affected by the advice.
The engine maker issued a market update note to alert shipowners that it will withdraw its no-objection letters for cylinder lube oils of 50-60BN (base number) for all MAN B&W two-stroke engines operating on high-sulphur fuels from 1 January 2016. The note said the decision was based on “numerous engine inspections and feedback,” from which it had concluded that in many cases mid-range cylinder lube oils are not being used at sufficiently high feed rates when operating on high-sulphur fuels.
“The result has been a weaker protection against cold corrosion and, eventually, significantly increased wear rates,” making it “not a sustainable situation,” the note said.
MAN Diesel & Turbo’s two-stroke engine operations department claims that experience had shown that higher BN oils allowed significant reductions in oil consumption. Viswa lab claims that by regular testing of cylinder drain (scrape down oil from under piston space) oil samples, it is very well possible to bring the feed rate down enough to save at least US $60,000 a year.
- Cylinder Lube oil is very expensive and it cannot be re-used.
- A saving of 0.4 gm/bhp on a 6 cylinder engine can generate a saving of nearly US $60,000 in a year.
- Over Lubrication could mean money down the ‘cylinder’ drain.
- Cylinder drain oil analysis will not only help you cut costs by optimising cylinder lubrication but also provide important information on –
- Combustion conditions inside engine,
- Wear rate of rubbing parts of the cylinder liner and piston rings,
- Potential for major problems as a result of serious blow past the rings and by contamination of lube oils.
- Additionally Viswa Lab tests for contamination of cylinder drain with system oil – where this can also reveal information on loss of main engine system oil.
- The best way to start optimizing on cylinder lubrication is to perform a ‘Sweep Test’ and then to optimize the feed rate.
The latest generation of 2-stroke crosshead engines have long strokes to deliver high power at low speeds. They are especially sensitive to corrosion because the temperatures and pressures found in their cylinders tend to promote sulphuric acid condensation. Although one solution is to increase cylinder oil feed rate, it has a limited effect because there is a physical limit to the thickness of the oil film and adding any more is not efficient. The corrosion is caused by sulphuric acid condensing on the cylinder liner, so another approach is to increase the cylinder temperature to prevent this condensation. The Engine mark numbers effectively reflect their generation of technical development, and engines with a mark number above 8 are sensitive to corrosion.
Source: Viswa Lab, Man Diesel & Turbo.