Importance of Maintaining Lube Oil Quality
Maintaining good lube oil quality is essential for the proper health and reliable operation of marine diesel engines.
Lube oil provides:
- lubrication of moving components,
- reduction of friction and wear,
- cooling of components,
- removal of contaminants, and
- protection against corrosion.
Degraded or contaminated lube oil can result in bearing failure, piston-ring sticking and, ultimately, serious or catastrophic engine damage.
Therefore, the lube oil system uses several stages of filtration, purification, inspection and condition monitoring to ensure that the oil remains fit for service.
Automatic back-flushing filters act as the primary full-flow filtration unit. They are normally installed directly before the engine lube-oil inlet and remove solid particles larger than approximately 10–15 microns, depending on the engine type.
Role in Maintaining Oil Quality
They continuously remove solid contaminants such as:
- combustion soot,
- wear metals, and
- external dirt.
The major advantage is that the filter can be cleaned automatically without manual cleaning or stopping the lube-oil system.
Working Principle
The filter operates using differential-pressure (ΔP) monitoring.
When the differential pressure across the filter reaches a predetermined set point, for example approximately 0.6–0.8 bar, an automatic back-flushing cycle starts.
A burst of compressed air or clean oil is used to back-flush a small section of the filter mesh. The accumulated dirt and sludge are removed and discharged into a dedicated sludge tank.
Effect on Engine Health
Automatic back-flushing filters:
- prevent abrasive particles from reaching critical engine components,
- reduce abrasive wear of main bearings and crankpin bearings,
- protect piston cooling spaces, and
- ensure a continuous supply of clean lube oil to critical components.
Lube oil separators, or purifiers, normally operate as a bypass system, treating a portion of the sump oil continuously.
They use centrifugal force to separate:
- water, and
- fine heavy solid contaminants
from the lube oil.
Role in Maintaining Oil Quality
The separator is particularly important for removing:
- water resulting from condensation or cooler leakage,
- very fine particles that may pass through the main filters,
- catalytic fines, and
- fine wear metals.
Operating Parameters
For effective separation, the purifier must be operated at the correct optimum temperature, typically around 90–95°C.
Heating the oil reduces its viscosity and helps maximise the effective density difference between the:
- oil,
- water, and
- solid contaminants.
Correct gravity disc selection or an automatic density-control system, such as Alfa Laval Alcap, is also required where applicable.
Effect on Engine Health
Removing water is essential because water contamination can cause:
- emulsification,
- loss of lubricating properties, and
- corrosion of bearings, particularly white-metal bearings.
Removal of catalytic fines and fine abrasive particles is also important because they can cause severe abrasive wear of:
- cylinder liners,
- fuel pumps, and other engine components.
Magnetic filters are installed in suitable return lines or before main pumps to capture ferrous or magnetic wear particles.
Role in Maintaining Oil Quality
They specifically collect abrasive:
- iron particles, and
- steel particles
that may be too small to be effectively removed by other filtration arrangements.
They can also act as a pre-filter, thereby reducing the contaminant load on other purification equipment.
Effect on Engine Health
Magnetic filters have an additional important function: they provide an early warning of abnormal mechanical wear.
For example, excessive ferrous particles may indicate abnormal wear in:
- gear trains,
- cams, or
- liners.
Regular, particularly daily, inspection of the magnetic core provides immediate visual evidence of abnormal metallic wear and possible developing mechanical failure.
The duty engineer should carry out daily visual checks of lube-oil samples taken from the engine sump or purifier outlet.
What to Check
Visual inspection provides a quick qualitative assessment of the condition of the oil.
The following should be checked:
- Colour: Excessive blackness may indicate high soot loading.
- Clarity: Changes may indicate contamination.
- Smell: A burnt smell may indicate oxidation or blow-by-related contamination.
- Water: Cloudiness or visible free water indicates possible water contamination.
A simple "crack test", such as dropping a small amount of oil onto a hot plate, can also be used to quickly identify water contamination.
Effect on Engine Health
Visual inspection allows the engineer to identify abnormal oil conditions at an early stage.
This enables:
- immediate operational adjustments,
- further investigation, and
- corrective action
before serious engine damage occurs.
Periodic laboratory testing provides a comprehensive condition assessment of the lube oil.
Oil samples are sent to a shore-based laboratory at regular intervals, for example every 3–6 months or as specified by the PMS (Planned Maintenance System).
Parameters Checked
Laboratory analysis can determine:
Wear Metals
- Fe – iron
- Cu – copper
- Pb – lead
- Sn – tin
These indicate wear of different engine components.
Oil Condition and Additives
- TBN/BN depletion
- additive condition
- oxidation-related deterioration
Physical Properties
- viscosity at 40°C
- viscosity at 100°C
Contamination
- water content (%)
- insoluble content (%)
Effect on Engine Health
Laboratory analysis provides long-term trend analysis, which is extremely useful for predictive maintenance.
It can indicate developing abnormal wear or contamination before the condition becomes serious.
The results help determine whether the lube oil should be:
- sweetened, i.e. partially replaced,
- further purified/treated, or
- completely condemned and replaced.
It prevents continued operation with oil that has lost its important chemical protective properties, such as:
- anti-corrosion protection, and
- dispersancy.
Quick Revision
Method | Main Function | Main Benefit |
Automatic back-flushing filter | Full-flow filtration of approximately 10–15 µm particles | Protects bearings and other components; automatically cleans itself based on ΔP |
Lube oil separator/purifier | Bypass centrifugal purification | Removes water and fine solids; normally operates around 90–95°C |
Magnetic filter | Collects ferrous/steel particles | Detects abnormal gear, cam or liner wear at an early stage |
Visual inspection | Daily qualitative condition check | Identifies abnormal colour, smell, clarity and water contamination |
Laboratory test | Periodic detailed oil analysis | Provides trend analysis of viscosity, TBN, wear metals, water, insolubles and oxidation |
Important Difference: Filtration vs Purification
The examiner may ask why both filters and separators are required.
Full-flow filtration
Automatic back-flushing filter:
- Oil passes through the filter as part of the full-flow system.
- Removes relatively larger solid particles.
- Protects the engine immediately before the lube-oil reaches critical components.
Bypass purification
Lube oil separator/purifier:
- Only a portion of the oil is treated at a time.
- Uses centrifugal force.
- Removes water and very fine heavy contaminants that may not be removed effectively by the main filter.
Therefore, filtration and centrifugal purification complement each other rather than performing exactly the same function.