Cold corrosion is an unintended consequence of progress. Longer piston strokes in new generation, fuel-efficient engines bring a change in operating conditions within the cylinder liner resulting in a cooler temperature that is below the dew point, and a higher pressure. Previously, sulphur in the engine was in a gas form as SOx. Now it combines with the condensation to form liquid sulphuric acid, resulting in corrosion on the liner surface. The resulting iron compounds formed are flushed into the cylinder oil, leading to excessive wear of the cylinder liner.
Over-lubricating to tackle cold corrosion
Increasing the feed rate of an alkaline lubricant is one obvious solution to counteract increased acidity within the cylinder liner, however, as lubricant is over-applied, operating costs quickly climb. With often four times the normal level of required lubricant being used, this amounts to a substantial cost increase.
Minimising both fuel and lube oil consumption is high on the agenda in this efficiency-conscious era. Even a 0.1 g/kWh reduction in cylinder oil dosage represents a significant yearly saving for the owner. Successful feed rate optimisation relies upon identifying exactly how much cylinder lube oil injection can be acceptably reduced to achieve the optimum operating conditions.
Monitoring to optimise feed rate
Several different monitoring methods are available that allow a user to optimise cylinder lube oil feed rate. The appropriate solution will depend on the requirements of the specific application.
Electronically-controlled lubricating systems
The MAN Alpha Lubricator, Wärtsilä’s Pulse Lubricating System (PLS) and Hans Jensen SIP system all aim to inject cylinder oil into the cylinder at the exact position and time where effect is optimal. These systems have achieved significant savings, for example the guide feed rate for Wärtsilä RTA and RT-flex engines equipped with the Pulse Lubrication System as original equipment is 0.7 – 0.8 g/kWh of cylinder lubricating oil when previously the average would have been 1 – 1.2 g/kWh.
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.
Laboratory testing to monitor scrapedown oils is a primary method used to inform adjustment of lubrication levels. Each of the oil majors provides a drain oil analysis programme; Exxon Mobil’s Signum Oil Analysis and Shell’s Rapid Lubricants Analysis heighten savings by monitoring the engine to determine its sensitivity to particular parameters, optimise lube oil feed rate, and improve maintenance management and extension of overhaul periods.
Flame Marine’s service is perhaps the best-known independent provider of detailed diagnostics. The company estimates that over-lubrication of cylinder lubricating oil in slow speed two stroke marine diesel engines can cost over US$100,000 per year per ship.
There are two main challenges in adopting this approach in isolation. Firstly, results depend upon engineers understanding how to draw and test a representative sample. Secondly, inherent delays accompany the collection of information, meaning a costly repair opportunity could be inadvertently missed.
The Shell Onboard Alert and Exxon Mobil Scrapedown Analyser are portable, touch screen devices, helping monitor cylinder liner wear by providing daily onboard measurement and recording of metallic iron content in scrape down oil.
Although wear rate is monitored by measuring magnetic iron particles, these devices do not monitor corrosive iron oxide particles. To manage this, Total Base Number should be measured post fuel switching, ensuring sufficient additive content.
Constant real-time monitoring is arguably the ultimate tool for safely optimising cylinder lube oil feed rate by improving efficiency.
Parker Kittiwake’s LinerSCAN, used by companies including AP Möller-Maersk Group, Hapag Lloyd, Ernst Russ and Reederei F. Laeisz, informs feed rate reduction and helps achieve maximum penetration of the lubrication safety buffer.
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.
LinerSCAN minimises liner wear, improves maintenance scheduling, decreases sampling and testing costs, and detects ingress of catalyst fines. With fuel testing agencies continuing to correlate declining sulphur levels, increasing levels of catalyst fines and subsequent engine damage, online testing to safeguard vessels from costly engine damage is crucial.
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. When used in conjunction with Parker Kittiwake’s Cold Corrosion Test Kit, the shipowner will receive the most comprehensive analysis of corrosive wear in cylinder lubricants, with the Test Kit providing a measurement of non-ferrous iron compounds present in a sample almost instantly, negating the need to send samples to a laboratory for analysis and avoiding the time and cost this incurs. By having quick and simple access to this information onboard, operators can easily identify where adjustments need to be made to alter the operating conditions within the cylinder in order to minimise corrosive wear and reduce cost. The Test Kit, when used alongside ferro-magnetic analysers, will indicate the levels of both metallic and non-metallic iron compounds so that operators can isolate and address the different processes which result in these corrosive elements being formed.
Many OEMs are now advocating the use of higher base number (BN) lubricants in engines with a longer stroke and so it becomes more important to monitor cylinder oil to understand the effects of this change on conditions within the cylinder chamber. Scrape down oil is continually exposed to acidic combustion products that need to be neutralised before they corrode engine parts. By frequently testing residual BN of used cylinder lubricant, operators can ensure that alkaline reserve levels within the oil are sufficient to neutralise these acidic products. Parker Kittiwake recently updated the Digi TBN Test Kit that measures residual BN levels in used cylinder oil, providing an onboard figure in minutes. This allows operators to monitor efficiency of cylinder lubricants over a long period of time, maximising potential life of the product as well as saving both the time and cost incurred with repairs resulting from corrosive damage.
Choosing to optimise not compromise
Effective feed rate optimisation is not based on a single test at a fixed point in time – it is a dynamic, ongoing exercise. For this reason, achieving the optimal feed rate without damaging the engine relies upon a combination of traditional methods with a reliable, real time feedback system. You may certainly adjust lubrication according to the OEM’s instructions, though the risk may become apparent at a later date, when you notice the loss of more liners. With an average insurance claim for unexpected liner loss at over $250,000, this realisation comes at a price.
Article contributed by Parker Hannifin Hydraulic Filtration Division, Parker Kittiwake, Littlehampton West Sussex, UK
Other posts related to filtration include: