A bunker analysis report arrives onboard showing sulphur, viscosity, density, flash point, water, sediment and catalytic fines within the specified limits. The fuel is recorded as meeting ISO 8217. It should therefore be safe to burn.
That assumption can be expensive.
A routine specification report tells the operator whether a tested sample meets a defined set of limits. It does not guarantee that the fuel will remain stable during storage, mix safely with fuel already onboard, separate efficiently in the purifiers or burn satisfactorily in a particular engine.
ISO 8217:2024 defines marine fuel requirements before onboard storage, settling, centrifuging, filtration and heating. The vessel must still convert the fuel as delivered into fuel that is suitable at the engine inlet.
“Within Specification” Is Not the Same as “Fit for Every Engine”
The expression ISO-compliant fuel is sometimes used too loosely.
A standard laboratory screening normally covers the listed specification parameters. However, harmful chemical components may not be identified by routine testing. If advanced investigation later discovers material that makes the fuel unsuitable for marine use, the fuel may have failed the wider general requirements of ISO 8217 even though its routine table results were within limits.
ISO compliance should therefore be treated as an essential starting point—not a complete machinery warranty.
This distinction has become more important as fuels are increasingly produced from different refinery streams, blend stocks, recycled components and renewable feedstocks. ISO 8217:2024 has expanded its coverage to include permitted synthetic, renewable, recycled and FAME-containing fuels, but not every possible material that might enter a marine fuel blend is acceptable.
1. The Fuel May Be Unstable
Fuel stability describes whether the fuel can keep its asphaltenes suspended during normal storage and handling.
A residual fuel may initially appear homogeneous but later begin depositing asphaltenic sludge. Ageing, excessive heating, prolonged storage or changes in the fuel’s chemical balance can accelerate this process.
The operational consequences can include:
- Rapid purifier sludge accumulation
- Frequent automatic filter backflushing
- High filter differential pressure
- Blocked strainers and heater passages
- Reduced fuel pressure at the engine inlet
- Loss of propulsion or generator load
Total sediment testing provides useful evidence of stability, but it cannot reproduce every temperature, storage period or operating condition the fuel will experience onboard.
The fuel supplier is responsible for delivering a stable fuel. The ship’s engineers are responsible for storing and handling it in a way that does not create further instability.
2. Two Acceptable Fuels May Be Incompatible
Stability and compatibility are not the same.
A fuel can be stable when stored alone but become unstable when mixed with another stable fuel. The two products may have different aromatic and paraffinic characteristics. Once mixed, the combined liquid may no longer keep the asphaltenes suspended.
Sludge can then form almost immediately.
This commonly occurs when:
- New bunkers are loaded over an existing tank balance
- Fuel is transferred through common lines without adequate flushing
- Settling or service tanks contain residues of a different batch
- Fuel grades are mixed during changeover
- Purifier recirculation returns fuel to a tank containing another product
Where compatibility has not been confirmed, different bunker batches should be segregated. An onboard compatibility spot test can provide an initial warning, but laboratory compatibility testing is more reliable when significant quantities may need to be commingled.
A small quantity left in a tank can still be enough to destabilise a much larger parcel.
3. Routine Testing May Miss Unusual Blend Components
Routine ISO testing measures specified physical and chemical properties. It is not a complete molecular investigation of every substance in the fuel.
Organic chlorides, acidic materials, unusual waste-derived components or other chemical species may remain undetected until the vessel experiences sticking fuel pumps, damaged injection equipment, corrosion, abnormal deposits or loss of combustion performance.
Advanced tests such as gas chromatography–mass spectrometry, commonly referred to as GC-MS, may be required when:
- Several vessels report problems from the same supplier or port
- The fuel has an unusual smell or appearance
- Filters block despite acceptable sediment results
- Fuel pumps or injectors begin sticking
- Abnormal corrosion or deposits appear
- The machinery symptoms cannot be explained by routine results
Recent industry concern over cashew nut shell liquid and related components illustrates the limitation. A fuel may resemble a conventional or biofuel blend while containing material that is not recognised as a normal ISO 8217 fuel component.
4. Catalytic Fines Can Be “On-Spec” but Still Dangerous
Catalytic fines are hard aluminium and silicon particles originating mainly from refinery catalytic cracking processes.
Certain residual fuel grades may contain up to 60 mg/kg of aluminium plus silicon before onboard treatment. Yet major engine manufacturers commonly require the concentration at the engine inlet to be below approximately 15 mg/kg, with some recommendations being even lower.
This difference is critical.
The fuel can satisfy its delivery specification only because the ship is expected to remove most of the contamination through settling, centrifuging and filtration. If the purifier is overloaded, incorrectly adjusted or operated at insufficient temperature, unacceptable quantities can reach the engine.
Catalytic fines cause abrasive wear to:
- Cylinder liners and piston rings
- Fuel pumps and plungers
- Injector components
- Piston ring grooves
- Other closely fitted fuel-system parts
They can also settle inside tanks and accumulate over time. Heavy weather may suddenly stir the deposits back into the fuel, creating high engine-inlet concentrations even though the original bunker report was acceptable. CIMAC records cases of machinery wear caused by this mechanism without the delivered fuel being off-specification.
5. Water and Sediment Are Operational Problems, Not Just Numbers
Water can enter fuel through the supply chain, condensation, leaking heating coils, tank boundaries or poor bunker handling.
Even when the reported quantity is within the specification limit, free water accumulating at the tank bottom can disturb purifier operation and carry contaminants into the treatment system.
Water can contribute to:
- Corrosion
- Fuel-pump and injector damage
- Reduced effective heating value
- Unstable purifier interfaces
- Microbial growth in susceptible distillate or biofuel blends
- Increased sludge and waste generation
Similarly, sediment that remains below the laboratory limit can still become operationally serious if it accumulates in tank bottoms or is concentrated by repeated transfers.
Settling and service tanks must therefore be drained regularly. Remote indications should not replace physical verification of drains, sludge quantities and purifier discharge performance.
6. Acceptable CCAI Does Not Guarantee Good Combustion
The Calculated Carbon Aromaticity Index, or CCAI, is derived from fuel density and viscosity. It can identify unusual density–viscosity relationships and provide an indication of possible ignition behaviour.
It does not fully measure actual combustion quality.
Fuels with similar density, viscosity and CCAI values can behave differently inside an engine. A fuel with an acceptable CCAI may still produce prolonged ignition delay, poor pressure development, after-burning or heavy deposits—particularly in sensitive medium-speed engines or during prolonged low-load operation.
Warning signs include:
- Difficult starting
- Unstable running at low load
- High exhaust temperatures
- Excessive exhaust-temperature deviation
- Smoke or after-burning
- Turbocharger and exhaust-valve deposits
- Increased cylinder or system-oil contamination
Where combustion quality is doubtful, the engine maker and testing laboratory should be consulted before unrestricted use.
The Ship’s Practical Defence
Before and during bunkering
The bunker order should clearly state the required ISO 8217 edition and grade. The vessel should prepare a tank plan that keeps the new delivery separate from existing fuel wherever possible.
Representative continuous-drip samples should be obtained at the receiving vessel’s manifold. Tank contents, sampling arrangements, seals and bunker delivery note details must be verified before the documents are signed.
IMO guidance recommends that purchasers assess suppliers, define technical requirements clearly and establish procedures for dealing with non-compliant or unsuitable fuel.
Before using the fuel
Where operationally possible, the new bunker should remain segregated until laboratory results are received.
The analysis should be reviewed against:
- The contracted ISO grade
- The relevant engine-maker limits
- The actual capability of the vessel’s treatment plant
- Any unusual characteristics or warnings from the laboratory
Compatibility should be checked before mixing batches. Advanced screening should be considered when routine results do not explain suspicious fuel behaviour.
During purification
Purifiers should be operated continuously at the correct temperature and at the lowest practical throughput consistent with engine demand. Controlled testing cited by CIMAC shows that reducing separator throughput can improve particle-removal efficiency.
The crew should also:
- Confirm the correct purifier configuration and density settings
- Drain settling tanks frequently
- Monitor sludge discharge and filter backflushing
- Avoid unnecessary disturbance of tank bottoms
- Sample before and after the separator when performance is doubtful
- Check fuel quality at the engine inlet
- Clean settling and service tanks periodically
- Follow the latest purifier and engine-maker instructions
Using two separators or increasing flow does not automatically improve cleaning. The arrangement must follow the vessel’s approved system design and equipment-maker guidance.
The Real Meaning of Fuel Quality
An ISO 8217 report confirms that a sample has been assessed against a recognised specification. It does not remove the need for engineering judgement.
Machinery safety depends on the full chain:
Procurement → sampling → testing → segregation → storage → purification → filtration → combustion monitoring
A weakness at any stage can convert apparently acceptable bunker fuel into a machinery casualty.
The correct question is therefore not simply, “Is the fuel on-spec?”
It is:
“Can this ship safely store, clean and burn this particular fuel?”
Final Thoughts
ISO 8217 compliance is essential, but it should never be treated as a guarantee of trouble-free engine operation. A fuel may pass routine laboratory limits and still create serious problems through instability, incompatibility, abrasive catalytic fines, water accumulation, unusual blend components or poor combustion behaviour.
The real protection lies in disciplined fuel management onboard. Proper sampling, segregation, compatibility checks, pre-use analysis, correct purifier operation and close machinery monitoring are just as important as the bunker specification itself.
For engineers and operators, the safest approach is to judge every fuel against the vessel’s actual treatment capability and engine-maker limits—not only against the bunker delivery report.
In practical terms, “on-spec” does not always mean “safe to burn without precautions.”