Low sulphur fuel concerns top of the list

When talking about low sulphur fuel for marine trunk piston engines, is a 30BN needed or could a 12BN, as for a distillate, be used asks Keith Saddler, manager of Global Marine OEM, ExxonMobil Lubricants & Specialties.

The overwhelming opinion, he says, is that a higher BN, and the proper selection of BN chemistry, is probably going to be required regardless of the fuel sulphur level because with engines in some cases looking at a lube oil consumption of 0.2g/kWh, a higher BN is needed just to maintain alkalinity and detergency.

Non-reacted

The question then becomes is there any issue about having non-reacted BN in certain cases where there might be a higher lube oil consumption? “I don?t think it?s going to be an issue ? it is an issue in two-strokes but we haven?t seen that in four strokes. There is everything out there from a standard 30BN all the way up to a 65BN, so we?ve got a wide range to choose from and I think they will be necessary primarily for the detergency purpose,” he explains.

The two-stroke market appears to be struggling more than the four-stroke market with the whole issue of low sulphur fuel. There were a number of papers on the issue presented at CIMAC in Kyoto and there have been a great deal of technical discussions within the industry about the best way to handle varying sulphur levels in two-stroke engines.

“It?s almost a consultative process and one of the key factors is that there is not a terrific amount of experience in the marine, or any, environment where there is extended experience of (a) running long term on low sulphur fuel or (b) intermittent use of low sulphur fuel,” says Jerry Hammett, technical development manager for Shell.

In a paper presented at the last Motor Ship Marine Propulsion Conference in Amsterdam Lubmarine highlighted that the problem is not running on low or high sulphur fuels but when switching fuels. “It is a challenge because today we have one lubricant for low sulphur fuel and one normal lubricant for quite high sulphur fuel. But what is the upper limit of the fuel we can accept?” asks Claude Ouvrier-Buffet, head of Total Lubmarine?s technical department. Lubmarine is currently in discussion with its customers. “We know we can run with low sulphur fuel but is it possible to run with the same lubricant up to 1.5% sulphur? ? I think nobody knows today ? I guess it is a question for the future,” he says.

According to Ouvrier-Buffet it will also be interesting to find out what the limit is for the BN70, especially on modern engines. He believes that there is no problem with running high BN lubricants on old engines but that there might be problems on modern big bore engines.

There are three different options open to owners/operators, although only two are currently feasible. One is to have two oils onboard ? a 40/50BN and a 70BN ? and switch between them, but this requires additional tankage and the additional problem for oil companies of having to stock two products in upwards of 900 ports.

The other option is to carry on using 70BN oil but adjust the feed rate. “But this could be a bit risky because it is difficult to know exactly how low to decrease the feed rate when in a low sulphur operating environment,” says Keith Saddler. Under certain operating conditions, if the feed rate is too low there will be insufficient alkalinity even with low sulphur and there is also the worry about whether there is enough oil to fully cover the surface of the liner. “We feel it is extremely important if feed rate is going to be used to control the lubricant in low sulphur applications that there needs to be a diagnostic tool onboard the vessel to check the residual BN of the scrape down oil and also the amount of iron in each cylinder,” he told The Motor Ship. It is also a good safety check to make sure that it hasn?t gone too low. “We have found this is an excellent way to monitor the engine not only in a low sulphur environment but also in routine operation.” This ensures the optimum amount of lubricant is going to each cylinder.

Suspected difference

It is also suspected that, in some cases, low sulphur fuels could also have different combustion characteristics. ExxonMobil has seen some cases in four-stroke engines where using low sulphur fuels has resulted in very high amounts of insolubles getting into the crankcase from less than optimum combustion. It is for this reason that Saddler thinks there is a need to better understand what the combustion characteristics of these low sulphur fuels will be.

Operators using low sulphur fuels don?t need to make gross changes to their BN or feed rates ? it is really dependant upon the make and model of the engine and how long it is going to be operating on it. “I think that caution has to be communicated to the industry,” says Saddler and that people shouldn?t overreact. “If you?re doing it for a few hours, it?s not a big deal; when you?re doing it for an entire ocean crossing, it could be a big deal,” he explains.

Acceptable practice

ExxonMobil has been in contact with the OEMs and for short-term operation they see operating with a 70BN as an acceptable practice. He told The Motor Ship. “What we haven?t done is put a stake in the ground and say when short term operation becomes long term and you need to make an adjustment either BN wise or feed rate wise. This is only likely to come from experience.”

However, Shell?s Hammett believes that people are concerned whether having a high BN cylinder oil and the base material not being used, would actively contribute to deposits, which may lead to a disruption of the oil film.

There are some key factors that are defined by engine design and which will have an affect. Engines that employ piston cleaning rings or anti-polishing rings ? depending on which OEM, or how a particular device is described ? goes a long way to controlling deposits. “If you have an anti-polishing ring it gives a primary means of deposit control but it is still considered good practice to prevent the deposits building up in the first place, explains Hammett.

He told The Motor Ship: “Depending on the customer?s outlook and the trading pattern of the vessel, we would sit down with customers and go through the question of managing the risk and also discuss the previous track record of the engine ? is it well behaved in terms of building up deposits?” The other key factor is feed rate. Certainly MAN B&W has focussed heavily on this and is quite clear on the need to keep to the recommended feed rate at all times. It is also well known that over lubrication of engines is not good.

Greater flexibility

“One of the key areas for lower BN lubricants such as our Shell Alexia LS would be in flexibility in feed rates ? that is it may give the flexibility to promote better running conditions in line with what OEMs require,” Hammett explains. It may also help in preventing deposits in the short term.

MAN B&W believes that some corrosion must be allowed to take place says Hammett. This permissible corrosion helps maintain the good tribological properties of the liner material and allows the lubricant to work at its best. He told The Motor Ship: “It may be that with a 70BN oil over a certain period of time one might start to loose what is called the open grain structure of the liner surface so it could be that, for a short period of time, it is okay to stay on the 70BN oil.”

However, for longer periods of time on certain ships, depending on the trade and a number of other factors, it may be that to maintain good running conditions it could be necessary to change over to a low BN oil. It is something that Shell intends to consult its customers about. “It?s going to be a learning curve for sure and in some cases quite a steep one,” concludes Hammett.

Sadler adds: “We are going through a difficult period in the industry and there is a lot of uncertainty about the future impact on engine operation. Shipowners should be encouraged, however, by the fact that OEMs and the oil companies have these issues clearly in their sights and, in time, will have the necessary answers.”

Ready to go

One company that is ready to go with a lubricant specially developed for low sulphur fuels is Lubmarine. Talusia LS 40 has completed in-service tests and is approved by MAN B&W and Wärtsilä Sulzer designs.

“There are not a lot of vessels only running on low sulphur fuel today so we intend putting the LS 40 into ports at the end of this year and the beginning of next year, depending on demand,” says Ouvrier-Buffet.

He told The Motor Ship: “Everything is ready and we are waiting for our first customer to put stocks of oil on the route of the vessel. We can currently manufacture it but have not put stocks in place ? we are waiting for the market.”

He believes it could be one or two years before the company has significant volume demand for its lubricants, depending on the availability of the low sulphur fuel. n