Category Archives: Maintenance Management

Not all scavenge drain oil analyses are the same

I can report that not all scavenge drain oil analyses are the same. There are many approaches and the end user needs to be specify the right approach.

Scavenge Drain or Used Cylinder Oil analysis cannot be viewed as a conventional lube oil sample. There are a great many variables that have to be accounted for and normalised before any diagnosis can be made.

Firstly there is no sump so there is a need to understand the relevance of the results to the engine condition at the time of sampling. Therefore feed rate and engine conditions need to be known RPM, %MCR, coolant and scavenge air temperatures, relative humidity and air intake temperatures. These factors at a minimum will indicate the likely conditions that the CLO will experience in terms of dwell time on the liner wall and exposure to water laden air. Both are factors in determination of the potential for corrosive conditions being present.

To get this information you require accurate power information as the feed rate will be normally calculated using grams per kWh, also power loss due to weather, hull condition rudder angle will impact upon the power to rpm relationship. Knowledge of the fuel quality during sampling allows you to understand what abrasive materials may be present and to what degree the presence of unburned or partially pyrolised fuel has on the lubricating qualilty and the determination of BN and viscosity values.

Clearly this information is not required by all analysis providers – FOBAS Engine provided by Lloyd’s Register, a service which I lead, is one service which considers the holistic view in this way – noting that FOBAS Engine has a wider remit for engine performance optimisation and training and is not simply provided to check feed rates etc, though this is performed and opportunities for CLO cost reduction are usually the first to be targeted.

What is clear however, is that there are significant differences between onboard analysis and lab analysis as well as differences between oil analysis suites.

As corrosive ear generates smaller wear particles which are not within the visibility of onboard magnetic PQ devices, these devices should only be used to warn of unexpected scuffing – therefore permanently installed in the drain line. Using the same technique onboard but offline for periodic sampling is often cited as being used to demonstrate that feed rate has not be “over optimised” to the extent that the oil film is being routinely disrupted and producing adhesive/scuffing wear debris. My view is that in reality this level of feed rate optimisation leaves no room for error and is in all probability a false economic approach in this application.

For lab analysis – there are the issues of immediacy for obvious reason but if your laboratory uses a sensible approach to dilution correction to normalise the effects of contamination by system oil, fuel, water and feed rate variability. Then it is perfectly reasonable to use such a test suite to help optimise feed rate in accordance with variations in Sulphur content and differences in CLO BN performance.

I would recommend to check that your lab based CDO analyses are subject to at least a system oil correction and I would also strongly recommend that a scavenge space cleaning regime is built into the schedule to ensure that each sample represents oil arriving via the liner and not simply that which has accumulated over the previous few days which will have no useful relevance to the operation at the time of sampling and thus weaken any diagnosis.

I would suggest that samples be drawn during steady state operation at a point approaching port which will allow the samples to be passed to the courier in the shortest time possible in order that your laboratory can get the results back to the ship in the shortest time thus increasing operational relevance to the task.

Quote 

“It cannot be stated how much care must be taken when assessing scavenge drain oil …..
Unless some extensive form of dilution correction is undertaken based upon knowledge of the fuel oil, new and used system oil, feed rates and operational data collected from the engine at the time of sampling, use as a routine condition monitoring tool is limited to the measurement of significant changes issues such as high iron, water or BN change.”

CIMAC Used Engine Oil – User Interpretation Guide
Guidance Number 30 issued 2011

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