An overview of engine lubricants on ships
12 April 2017
All machines regardless of type need lubrication between moving parts to prevent heat build-up and wear. On most equipment such as pumps, compressors, winches and the like, lubricant levels need only regular checks and the occasional change. However, engine lubricants on ships are a much more challenging area and need constant attention, regular adjustment and special attention as to choice of lubricant dependant on fuel being used.
Engine lubricating oils can also be used as a means of ascertaining the health of an engine and proper analysis can give early warning of possible failures long before an incident occurs, allowing preventative measures to be taken. Several companies and classification societies offer analysis services and it is possible for crew to make basic tests themselves using proprietary test kits.
Used and waste lubricating oils also pose a problem with regards to their disposal and unless some means of reducing their volume is available on board such as the PureDry described in Chapter Three, there will inevitably be a large cost involved for disposal ashore.
A 2015 report from The Swedish Club shows that incorrect maintenance and repair continues to be the most frequent cause of main engine damage – a trend which has continued unabated since the Club began monitoring the issue nearly ten years ago. The report investigated more than 1,000 Hull and Machinery claims relating to over 5,400 vessel years of statistics and its findings make interesting reading.
Main engine damage makes up nearly 35% of machinery claims costs and is the most expensive category of claim with an average cost of over US$500,000 per claim. And with an average cost per claim of US$926,000 lubrication failure is still the most costly cause of damage to the main engine, due to consequential damage to expensive parts such as crankshafts etc.
According to the club the root causes include crew with insufficient experience and training; experts not in attendance at major overhauls; contaminated lubrication oil and contaminated bunkers. Despite technical advances since the Swedish Club published its last report in 2011, vessels with low speed engines still suffer proportionally fewer claims than those with medium and high speed engines, with 57% of club entries in this category responsible for only 40% of main engine claims cost.
Different engine types have differing lubricating requirements. The main determining factor is whether residual or distillate fuels are the main fuel type. There are also two areas of the engine that may or may not have different lubricating requirements – Cylinder and Crankcase or circulating oil.
In any engine operating on residual fuels containing sulphur, cylinder lubricants have three main purposes; to provide a barrier to metal to metal contact between piston rings and the cylinder liner, neutralising any sulphuric acid to control corrosion and to clean the cylinder liner and piston rings preventing damage from combustion and neutralisation residues.
In four-stroke trunk piston engines the same oil is used for cylinder lubrication and cooling. Some of the cylinder oil by-passes the piston rings and ends up in the combustion space, where it is “consumed”. However, the piston in a four-stroke trunk piston engine has an oil scraper ring that scrapes most of the oil supplied to the cylinder liner back to the engine’s oil pan, from where it is drained, cleaned and recycled.
In general four-stroke engines are less complicated as regards lubrication than two strokes. The two-stroke crosshead engine is different from the four-stroke in that it has no connection between the piston underside space and the oil pan. The cylinder lubricant is not recycled and will be consumed in the combustion process making precise dosing imperative and allowing for different oil types to be used in the two zones of the engine.
Two-stroke cylinder lubricating oil has a number of important parameters including viscosity, Base Number and detergent additives. The viscosity is typically SAE 50 across most suppliers although other grades might be specified under certain operating conditions. The Base Number (BN) can vary and is normally in the range of 40 to 70. The BN indicates the content of alkaline additives used to neutralise the sulphuric acid produced during combustion. Higher sulphur content fuels require a lube with a higher BN and low-sulphur fuels a lube with a low BN.
Before the advent of SECAs and controls on SOx emissions, most operators would choose a lube with a BN that could deal with the 4.5% global sulphur cap. This would normally require a lube of BN70. As low-sulphur fuels became more common there was a great deal of debate over what strategy to adopt. In order to avoid problems with deposit build up on piston crowns it was accepted wisdom to reduce the BN of lubes when operating on low-sulphur fuels. Short periods of operating on low-sulphur fuels normally present no problem and would not require a switch away from the high BN lube normally used.
In order to avoid problems connected with the use of lubes with the wrong composition, many lube suppliers experimented with producing universal lubricants which were typically of BN40 or 50 and these are now readily available and favoured by some operators. The use of these universal products is not without a degree of controversy and some engine makers have retracted previous approval of their use under some conditions and in particular engines.
Clearly it is unwise to ignore OEM recommendations but the benefits of using a single lubricant are attractive and relieve crew of having to cope with the problems of changing lubes as well as fuel oils during approaches to ECAs. Owners should therefore communicate with their engine and lube suppliers to determine the best and safest operational strategy.
Most operators chose to use lubricants from a single maker whenever possible but even the best laid plans can go awry and it is sometimes necessary to use products from a different supplier. Except for a few specific lubricants, most manufacturers have equivalent products that can safely be used alongside competing products. Ensuring the compatibility of products before mixing is relatively simple as most makers produce charts listing compatible rival offerings.
The reduction of the IMO’s global cap on fuel sulphur levels in 2020 will lead to demand for a new generation of lubes and if refiners choose to produce HFO with the 0.5% limit rather than continue with the current 3.5%, some of today’s products will become obsolete except for those ships with scrubbers that can continue to use the higher sulphur fuels.
Leaving aside the issue of BN and sulphur content, ensuring accurate cylinder lubricating oil dosage is vitally important in a two-stroke crosshead engine. The challenge is to make the oil properly and efficiently fulfil its tasks before it is “lost”, partly to the combustion space where it is burned, and partly to the piston underside space as sludge. Exactly how complex the issue of cylinder lubricant dosage can be is illustrated by the fact that MAN Diesel & Turbo’s PrimeServ Academy offers a four-day course devoted entirely to the subject.
In the early to mid-2000s, computer controlled lubricating systems that could dose according to engine load, speed and sulphur content were introduced by leading engine makers. Among these were the MAN Diesel & Turbo Alpha and Wärtsilä Pulse Lubricating systems. These were introduced before slow-steaming became a popular strategy among container operators after the economic crash of 2008 but their benefits were extolled by the engine makers as helping to overcome the problems caused by operating engines outside of design parameters and many systems have been retrofitted since. Automated dosage systems are now standard equipment on most new engines.
It is not only controlling SOx emissions that has created problems in the tribology aspect of modern engines. Addressing the twin effects of controlling NOx and meeting the EEDI requirements has led to a new phenomenon – cold corrosion.
Cold corrosion is when sulphuric acid forms on the liner walls in an engine cylinder and corrodes the liner surface. This abnormal corrosion then creates excessive wear of the liner material. In order to comply with NOx and EEDI regulations engine makers have needed to increase pressure and reduce operating temperatures this has been done by way of lower rotations per minute, longer strokes and increased scavenge and combustion pressures. This creates conditions below the dew point that allows water to condense on the cylinder liner walls. This then combines with sulphur from the combustion process to form sulphuric acid, which leads to cylinder liner wear or cold corrosion.
The NOx and EEDI rules only apply to newer engines but older engines can also suffer from the problem if they are modified, as many have been, to allow for slow steaming when engine loads below the original design parameters become routine rather than exceptions.
Among the modifications are turbocharger cut-out, retrofit of variable turbo charger nozzle rings, exhaust gas by-pass valve fitted, and engine tuning changes. Some modified engines become mildly corrosive whereas others may be more seriously affected. The problem can be exacerbated because in order to avoid cold corrosion occurring, a simple solution is to keep coolant temperatures elevated. However, this is precisely what engineers on board have tried to avoid in the past and it is counter intuitive to years of training and experience.
Another mistake is over lubricating in order to reduce acidity within the cylinder liner. To meet the ideal acidity level it is not unknown for lubricant use to quadruple which is expensive and creates other problems. It is therefore advisable to monitor feed rate and seek advice.
Electronic lubrication systems can reduce cylinder oil consumption, but as an open loop system it does not provide feedback on the impact of such reduction and, sensibly, a safety buffer is often applied. Without a reliable feedback system to accurately monitor the effect on the engine, changing feed rates based solely on OEM’s
recommendations could increase associated wear caused by under-lubrication and seriously harm the engine. To penetrate the lubrication safety buffer, safely achieve the true optimum feed rate and realise maximum savings, offline or online tools are available to closely monitor lubrication conditions.
In order to overcome the problem, most engine makers have issued advice on measures to identify the severity of the problem and what can be done to counteract it. MAN recommends the use of a ‘Sweep Test’ and Wärtsilä have a similar ‘Quick Test’. In addition, most lubricant manufacturers have developed test kits and can offer more advice and assistance on conducting tests.
An independent test kit is available from Parker Kittiwake which can provide accurate results on-board in less than five minutes, negating the need to send samples to a laboratory to be analysed. Unlike typical cylinder oil test kits, which give a total iron figure, the cold corrosion test kit gives a measurement of the parts per million value of Fe2+ and Fe3+ compounds in used scrape down oil, corresponding to the wear attributable to cold corrosion. Using a colour-matching test the kit alters the colour of an oil sample, indicating the concentration of non-ferrous iron compounds. The resulting colour is matched to a reference colour wheel that provides a measurement of the corrosive wear present in the sample.
Implementing a condition monitoring programme to monitor the performance of the engine and cylinder oil is crucial. This programme should monitor the parameters of iron wear and the residual Total Base Number (TBN) in scrape down oil.
Besides their primary role of reducing friction and keeping the engine running smoothly, lubricants other than cylinder lubes can be used as indicators of developing problems in engines. Different components of an engine are made from different metals and as they wear minute particles find their way into the oil. The scrape down oil from the cylinders shows up wear in rings and liners as well as giving clues to the combustion process, while circulating oil can carry traces of bearing wear from the crankshaft and connecting rods.
Analysis of scrape down oil can sometimes show evidence of over-lubrication and suggest adjustments can be made to the feed rate to reduce consumption of lubricant as well as preventing fouling of turbocharges, valves etc. As well as showing evidence of engine wear, analysing lube oils also gives information on the quality of the oil itself. Lubricants breakdown gradually and their effectiveness reduces as it does so. Topping up is necessary
at times but if continued for very long periods can actually result in increased consumption because all of the work of the oil is being done by just a small amount of fresh oil while the remainder may be practically useless.
Changing the circulating oil in any marine engine is an expensive task because of the quantities involved so is undertaken only when strictly necessary. As with machinery components, changing at set intervals is normal practice but this could result in product that has a significant amount of ‘life’ remaining being sent to the waste tank. Regular analysis will reveal what the true quality of the oil is and can mean changes can be postponed for significant time periods.
Almost every major lubricant producer runs a testing and analysis service and there are also several independent service providers such as Kittiwake and Intertek. Engine makers are also becoming more involved with MAN Diesel & Turbo’s Fluid Monitoring service being a prime example.
Some of the analysis carried out by laboratories is complex and requires specialist equipment but some basic tests can be carried out on board using test kits provided by the analysis service providers or purchased directly. These tests usually allow for identifying viscosity, water content, salt content, base Number (alkalinity) and the presence of insolubles.
As well as testing onboard or by shore laboratories, there are some other options available. One such is Parker Kittiwake’s LinerSCAN, which helps optimise feed rate. Using magnetometry, LinerSCAN‘s sensors fit to each cylinder of the engine and report changes caused by abrasive wear, highlighting periods of increased physical or thermal stress. By monitoring change trends and wear particles in real time, engineers are alerted to escalating cylinder liner damage, facilitating a fast reaction time, enabling preventative maintenance during the ship’s passage to the next port, insuring against costly downtime.
The LinerSCAN system is particularly useful for reducing risk of high wear in unfamiliar environments. In areas of high humidity water can enter the combustion chamber with the air from the turbo charger, disturb the oil film and lead to wear and scuffing, endangering the liner. The system is best used in conjunction with a cold corrosion test kit.
Another product from the same company is the DIGI TBN test kit. With this kit ship operators using high BN lubes now have the ability to test residual TBN levels in their oil. Allowing crew to ensure they are following OEM guidelines to prevent cold corrosion. This quick and simple test can be used in conjunction with laboratory testing, but gives an on-board reading within two minutes to meet OEM’s testing recommendations.
Lubricants are usually produced using a mineral oil base with additives but even with the best quality controls in place, natural variability in the base oils means that performance can sometimes be less than expected and furthermore the base oils themselves are sometimes not ideally suited to the environment they are used in.
In attempt to gain more uniform and improved performance, most leading manufacturers now offer synthetic products. Most will claim that synthetic lubricants offer superior protection and performance, when compared to conventional mineral oils and that they have a longer service life in the engine, decaying at a known rate and therefore giving predictable performance. Other claimed advantages include high-temperature capability, low-temperature flow properties and a resistance to oxidation.
Test do tend to show an improved performance that agrees with the makers’ performance claims but for the operator the downside is that synthetic lubes come with a premium price tag. It will be for the operator to decide if the benefits outweigh the price premium or if the extra cost can indeed be clawed back through longer life. It is however important to investigate miscibility with other products if synthetic lubes are chosen.
Custom made lubes
Almost all engine lubes are sourced from specialist manufacturers but the ability to produce a useful lube on board the vessel is now possible on some vessels. Maersk Fluid Technology (MFT) – an A P Møller subsidiary –has been experimenting and developing blending on board (BOB) as an alternative solution for some years.
The system has been implemented on some Maersk vessels and is now on the brink of commercialisation as the SEA-Mate Blending-On-Board system. The concept is based on proprietary technology designed to enable the operator to custom blend a fit-for-purpose cylinder lubricant from recycled two-stroke system oil and a cylinder oil concentrate with a base-number up above 300 BN. MFT says that independent tests document a fuel consumption reduction up to 1.5% and the possibility to reduce lube oil consumption by up to 50%.