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The method

Dilution

Scavenge drain oil is never pure used cylinder oil. Flame Marine was the first to understand and use dilution and contamination correction in drain oil sampling. Every figure in a Flame report is corrected with Flame ADT before it is analysed.

What is in a drain oil sample

Cylinder oil is sprayed onto the liner, where it lubricates the piston and protects the liner from the products of combustion: it neutralises the acids and carries away the ash and debris. On its way down the liner it collects the products of combustion and the iron, chromium and other metals of wear between piston and liner. The oil that drips from the bottom of the liner is the evidence.

But it does not stay pure. In the under-piston space it mixes with system oil that has passed the piston rod stuffing box, unburned fuel and condensed water. What drains out, the scavenge drain oil, is that mixture. Flame Marine's measurements show that drain oil samples are diluted by a factor of between 1.5 and 4, and the proportions differ from unit to unit and from sample to sample.

Why it matters

The laboratory measures iron, BN and everything else per kilogram of the mixture. System oil carries almost no wear iron and has a BN below 10, against 15 to 140 for the cylinder oils in service, so every per cent of system oil in the sample lowers the measured iron and lowers the measured BN. A sample that reads 100 mg/kg of iron, within limits, can be 400 mg/kg once corrected, well above normal wear. A BN reading dragged down by system oil looks like acid depletion, and the usual response is to raise the feed rate, which the engine did not need.

The diluents also bring their own elements with them, so the distortion is not a simple scaling. Uncorrected, soot, dispersancy and fuel contamination all read lower than they are, and combustion problems are missed. Two units in the same engine, with the same liner condition and the same oil, return different numbers if one stuffing box passes more oil than the other, so the comparison between units that every drain oil programme relies on is not a fair one.

Worked example

One sample, two readings

Illustration with example figures: cylinder oil BN 70, system oil BN 6, a sample that is 60% used cylinder oil by mass.

One sample, two readings A scavenge drain oil sample made up of 60 percent used cylinder oil, 38 percent system oil and 2 percent fuel and water. As measured, iron is 95 milligrams per kilogram and residual BN is 50 percent of fresh. Corrected to a cylinder-oil basis with Flame ADT, iron is 158 and residual BN is 78 percent. What is in the sample Used cylinder oil 60% System oil 38%, past the stuffing box Fuel and water 2% Iron, total (mg/kg) As measured 95 Corrected with Flame ADT 158 Residual BN (% of fresh) As measured 50% Corrected with Flame ADT 78% As measured: wear normal, BN low. Corrected: wear rising, BN adequate.

Flame ADT: Advanced Dilution Theory

Correcting a sample means solving for what is in it. A drain oil sample has seven unknowns: the proportions of used cylinder oil, system oil, fuel and water; the composition of the used cylinder oil itself; and what fuel ash and burnt cylinder oil have added. The measurements alone do not pin them down.

Flame ADT is the part of Flame Marine's mathematical model of the engine and the combustion process that solves for them, built on the data collected by Flame Diagnostic. The equations have no closed form, so Flame ADT evaluates many hundreds of thousands of candidate solutions for each sample and takes the statistically most probable one. The model was validated against more than 25,000 Flame Diagnostic measurements, and has been in use in Flame reports for many years.

With the composition of the sample known, every result is restated for the used cylinder oil as it left the liner:

  • Wear. Iron and the other wear metals on a cylinder-oil basis, so that liner and ring wear is neither hidden by dilution nor made to look worse than another unit's only because its sample is less diluted. The copper carried in the system oil component can also show stuffing box wear.
  • BN. The residual BN of the used cylinder oil itself, which is what the margin against acid corrosion depends on, rather than a figure dragged down by low-BN system oil. This is what stops feed rates being set higher than the engine needs.
  • Combustion. Soot, dispersancy and fuel contamination at their true levels, which give early warning of combustion problems that do not yet show in the pressures and temperatures the ship records.
  • The dilution itself. A rising share of system oil in one unit's drain oil is a stuffing box finding; rising fuel is an injection or combustion finding.

Flame ADT is applied in every Flame SDA and Flame Diagnostic report. It is a method inside both, not a third product.

First, and now in the designers' guidance

Dilution has been part of the reading in Flame reports for as long as Flame Diagnostic has existed. The report extract on the Flame Diagnostic page, from 2006, attributes the variation in BN between units to dilution by fuel and system oil, and reads the alkaline reserve accordingly. The engine designers' guidance has since moved the same way: Everllence's SL2023-738 asks for samples of the fresh cylinder oil and the system oil to be sent with the scavenge drain oil, so that the laboratory can correct for system oil dilution. Flame SDA and Flame Diagnostic kits take both.

Flame Marine was the first to understand and use dilution and contamination correction in drain oil sampling. Without it, drain oil analysis gives only a coarse assessment of engine condition, and can mislead.

Talk to us about your fleet

Tell us the engine types and the number of vessels, and we will come back to you with how sampling would work for you and what the reports would cover.

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