The global 0.5 per cent sulphur cap has been in operation for nearly three months. The effects of low sulphur fuel oil (LSFO) and very low sulphur fuel oil (VLSFO) on cylinder condition compared with conventional high sulphur fuel oil (HSFO) are coming to light with preliminary field experiences showing issues associated with the changeover.
According to Hans Jensen Lubricators, a Danish supplier of lubricants, there are cases of high wear rates, severe scuffing, heavy deposits and piston rings sticking leading to heavy blow-by/ring breakage. Some of these issues have started occurring just a few days after fuel changeover.
The Danish firm says that as sulphur no longer has a significance presence in fuel oil, the cylinder lubrication method has to be managed correctly in order to prevent the aforementioned issues.
With high wear rates and scuffing, Hans Jensen says that the common procedure is to increase the cylinder lubrication oil feed rate but warns that this is not always the right solution. Increasing the dosage of lubrication oil does not mean less frictional losses and wear and it can be as damaging as low oil dosage, due deposit formation.
If the cylinder lubrication oil feed rate and the base number (BN) are matched to the running conditions of the engine and the amount of sulphur in the fuel oil, then the wear of piston rings in the cylinder liner can be reduced to an acceptable level. Hans Jensen says it is important to schedule the lubrication oil changeover correctly in connection to a fuel changeover. Experience has shown that issues related to over-lubrication can be observed within the first 24 hours of engine operation. Each batch of new fuel will vary in quality and sulphur content, which means that the engine condition must be monitored closely when switching to the new fuel batch.
Deposit formation can occur due to:
- Non-combusted fuel components or contaminants.
- Neutralisation products.
- Metal debris from the abrasive wear.
According to the Danish company, the most critical areas for deposit formation in two-stroke marine engines are the piston crown and ring area.
To counteract deposit formation, BN additives can be used with the cylinder lubrication oil. This can cause unwanted particles to remain in suspension, preventing coagulation, which makes flushing them out much easier. According to Hans Jensen, this cleaning effect is reduced in operation with low sulphur fuels with a matching low BN cylinder lubrication oil, thus leading to a higher risk of deposit formation.
In order to prevent deposit formation, the following can be applied:
- Ensuring a correct cylinder oil feed rate.
- Use the correct type of cylinder oil (BN level).
- Ensure the proper distribution of the cylinder oil.
- Deliver frequent injection of the cylinder oil.
In one example cited by Hans Jensen Lubricators, over lubrication and a mismatched BN caused increased deposit build-up and scuffing of piston rings.
The vessel had switched from heavy sulphur fuel oil (HSFO) to LSFO to comply with IMO 2020. Until the changeover to LSFO, the cylinder condition has been normal with a lubrication feed rate of 1.18 g/kWh and a 70BN cylinder lubrication oil. After the changeover, the vessel suddenly experienced high wear and scuffing. The bunker analysis prior to the changeover showed that the content of cat fines in the fuel was within the normal limit of 35 ppm.
Scavenge-port inspections before and after the changeover showed an increased deposit build-up and scuffing of piston rings. As high wear was observed in the onboard scrape-down analysis, the engine operators dealt with the issue by increasing the cylinder lubrication oil feed rate to approximately 1.5 g/kWh. This was without any success, as the following port inspection 7 days later showed (Figure 4) an even worse cylinder condition with an increased build-up of calcium deposits.
According to Hans Jensen, scrape-down results after the changeover confirmed high iron content as well as high residual BN, which indicates over-lubrication and mismatch in BN level.
The vessel was recommended to change cylinder oil type from 70BN to 40BN, and reduce the cylinder oil feed rate in order to prevent the issues from calcium deposit build-up. The instructions where followed, and the following inspection and scrape-down results showed improved cylinder condition.
Read more on the case study by clicking here.