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3.2 Oil Samples − Piston Underside Drain or Scrape-down
WinGD recommends to get piston underside drain oil (scrape-down oil) samples at
regular intervals from each cylinder and to make an analysis to monitor the engine
condition.
These analyses are used to make an estimate of the cylinder liner and piston ring
wear and to set the applicable alkalinity and feed rate of the cylinder lubricating oil.
The data given in paragraph 3.1 (this document), 7218−1 Cylinder Lubrication and
7218−3 Feed Rate − Adjustment are calculated values. The applicable values for
each engine can be different, related to the engine and operating conditions.
You can adjust the cylinder lubricating feed rate related to the analysis of the piston
underside drain oil. The permitted maximum feed rate is 1.2 g/kWh (see 7218−1
Cylinder Lubrication and 7218−3 Feed Rate − Adjustment). If the analysis of the
piston underside drain oil shows that an adjustment to a higher feed rate than
1.2 g/kWh is necessary, you must change to a higher BN cylinder lubricating oil.
The recommended intervals for an analysis of the piston underside drain oil are:
D At each bunker change of the HFO (very important if the sulphur content of the
HFO is more than 2.5% m/m).
D At each change of more than 10% CMCR of the average engine (24 hours).
D A minimum of one time each week.
Wear metals, the residual BN, viscosity, fuel components and water are measured.
The quantity of system oil additive metals in the sample gives an indication about the
piston rod gland box condition. It is important to monitor trends and not full values,
and to think about the actual quantity of drained oil relative to the analysis results.
For data about the procedure to get an oil sample from the piston underside, see
8016−1 Lubricating Oil System, paragraph 2.1 Dirty Oil Samples.
The total iron in the scrape down oil is measured to determine the corrosion of the
liners and steel parts. A large quantity of system oil can be mixed with the used
cylinder lubricating oil in the piston underside space. To get an accurate view of the
used cylinder lubricating oil, a correction is necessary to remove the effect of the
system oil on the results. The iron and residual BN values are corrected in relation to
the phosphorus and/or zinc content of the system oil in the used cylinder lubricating
oil. This correction analysis must be done carefully because some cylinder lubricating
oils also include phosphorus and/or zinc.
The analyses of many piston underside samples from a wide range of engines that
operate with a high sulphur content in the range 0.5% m/m to 3.5% m/m and cylinder
lubricating oil from BN 40 to BN 100 has shown:
D The safe corrected piston underside residual BN to prevent piston ring and liner
corrosion is more than 25 mg KOH/g but less than 50 mg KOH/g (see Fig. 2).
D The alert corrected limit for piston underside residual BN to prevent excessive
corrosion is between10 mg KOH/g and 25 mg KOH/g.
D The danger corrected limit is less than 10 mg KOH/g piston underside remaining
BN. It is possible that there will be excessive corrosion and fast piston ring and
liner wear if not corrected. Scuffing and the fast failure of piston rings and very
fast corrosive liner wear is possible.
2017−08
Lubricating Oils
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