The good and the bad of EALs

We know from experience that there are very good bio oils, with better life time and performance than mineral oils and creating no technical or environmental problems. However, we also see that some bio oils are creating serious issues. We therefore fully endorse DNV GL’s recent decision to conduct a deeper investigation into EALs.

There are four very different types of EALs currently in use: vegetable oil, modified mineral oil, polyglycol (antifreeze) and esters. According to our experience so far – and we are still learning – EALs which are fully saturated synthetic esters are giving no problems. Some manufacturers give an oil life warranty with the possibility of an extension after five years while re-using the old oil. That is the good news.

From numerous cases we concluded that certain oil makers have not fully tested their products in this application. Certain oils recommended as EALs by reputed manufacturers for use in the stern tube are clearly inappropriate, and ship owners who use the wrong oils are confronted with expensive repairs and downtime.

For owners using biodegradable oils, some makers of propulsion equipment are now suggesting expensive ‘solutions’, turning the issues into their advantage. Examples include oil circulation or filtering systems, or even systems which should get rid of the water in the oils. Air seal conversions are also nsuggested. Although they may be very good systems, we feel that with the use of a good biodegradable oil this costly upgrade as should not really be necessary.

EMULSIFYING OILS

From what we see in dry dock and hear from ship owners, it appears that emulsifying oils suffer from more issues than non-emulsifying oils. Emulsifying oils can absorb water and still lubricate. This is excellent in case of an emergency water ingress and the makers of these oils use it as a sales argument to protect the stern tube bearings.

However, it appears that certain emulsifying EALs decompose even faster if mixed with water. Bear in mind also that the emulsified oil is more difficult and sometimes impossible to drain, especially when it has turned into ‘mayonnaise’, causing expensive shaft withdrawal and cleaning. For how long can oil makers guarantee their product once it has been mixed with water, if at all?

The sealing system usually has an aft seal tank (if not it can and should be retrofitted), allowing water leakage to be contained before it enters the stern tube. Crew often closes this tank after a high-level alarm, overflowing the tank and thus allowing water to enter the stern tube – or oil to flow into the sea in case of a leak.

This often happens in ballast condition or heavy seas when pressure fluctuations cause the rubber seals to temporarily leak as an inherent property of the seal. The crew should regularly drain the tank for as long as manageable to avoid water entering the stern tube. An experienced seal service company can offer advice to manage the situation if the leakage persists in calm conditions.

We recommend that operators use a non- emulsifying oil as standard, so any water coming into the stern tube can be drained. Moreover, we recommend that crew manage any water ingress (or oil loss) by using the aft seal tank effectively instead of closing the valves to the aft seal, unless there is too much leakage to manage.

ISSUES EXPERIENCED

Other issues that we have noted include decomposing oil in the chambers between the sealing rings, degrading to a black sludge that smells like sewage that hardens over time – even after the oil has been used for less than 1.5 years. The oil in question was an emulsifying oil.

Oil that stops lubricating will eventually affect the seals and bearings. We have seen oils with a lifetime often not lasting until next special survey because oil analyses show excessive acidity level and a loss or increase of viscosity.

Increased bearing temperature and other overheating issues have also been recorded with EALs. The forward seal housing of one customer’s vessel reached a temperature close to 80°C at 1.5 years after overhaul, but when 10 litres of oil from the forward seal tank was replaced by fresh oil, temperatures turned back to normal immediately. The oil that was drained was extremely liquid, indicating strong viscosity loss.

We occasionally see oil sample reports showing increased lead content, indicating lead leaching. We have yet to establish whether this is coming from white metal bearings, lead paint or (in older vessels) from deposits cleaned out by EALs. We also see increased bearing failure and high metal content in oil, consistent with bearing wear. But as with the lead content, this could also come from old metal deposits in the system.

Last but not least, we see leaking seals likely caused by overheating. All seal makers have developed sealing ring material that is compatible with EALs and these products were not harmed by the EALs they were tested with, or by chemicals resulting from a combination of water mixing with the EAL.

OVERHEATING CONCERNS

Overheating of an aft sealing ring should not be possible since it is cooled by an endless amount of seawater. As a rule of thumb, the running surface of a seal will not get hotter than 25-30°C higher than the surrounding cooling liquid.

Considering a stern tube oil temperature of 45°C, the running surface of the sealing ring should not exceed 75°C, which is totally unproblematic for fluoroelastomer (FKM) seal material. Investigating this phenomenon, it was pointed out by a lube oil manufacturer that they discovered large differences in the lubrication film strength of the different types of EALs.

Considering the fairly low rpm of a typical propeller shaft on a panamax size vessel, resulting in 3.5-4.5m/s surface speed, film strength is a critical factor because the speed is so slow that hydrodynamic lift is minimal. Slow-speed operation clearly doesn’t help matters.

In other words, bad oils that are also deteriorating in the chambers between the sealing rings may suffer from oil film collapse, which could explain the overheating seals and bearings and, in some cases, their total failure. This phenomenon, I expect, at least partly explains the tail shaft bearing failures which DNV GL is currently investigating.

Overall, we estimate that over 50% of the ship owners we speak to that are using EALs in their stern tubes have problems that appear to be related to the use of these oils.

There are several potential solutions. An air seal is accepted for use with mineral oils by the US Vessel General Permit. However, it is a serious investment, with return on investment often longer than 10 years. And ship owners are still liable if the air seal loses oil. It also adds complexity to the stern tube, although an air seal could be viable for newbuilds. Similarly, water lubricated bearings are environmentally acceptable, but also require a high investment due to corrosion protection, anti-wear and water filtering requirements.

For the 90% of vessels that do not have air seals or water lubricated stern tubes, there is one clear solution: Find a good EAL.