Recent bunker alerts have pointed to a growing presence of high-abrasive fuels containing cat fines in several regions.
As explained by Jordan Hatch, the latest VPS Bunker Alerts confirm this upward trend, which is increasingly contributing to operational challenges and disputes within the shipping sector.
If not properly managed, cat fines can cause severe engine wear and, in extreme cases, lead to complete engine failure in a relatively short period of operation. Damage involving multiple cylinder liners and piston rings can be particularly costly and disruptive.
Because vessels typically carry only one or two spare liners on board, extensive damage can quickly exceed onboard repair capacity. This often results in prolonged off-hire periods, as additional spare parts must be sourced and shipped in, with supply chain delays further extending repair timelines.
ISO standards vs engine reality
One of the challenges lies in the disconnect between the recognised international fuel standards and the engine manufacturer’s requirements.
The international marine fuel standard, ISO 8217, historically permitted cat fine concentrations (Al+Si) of up to 80 mg/kg. This limit has been reduced to 60 mg/kg (1) in later and current revisions. References to older versions still appear in charterparties and supply agreements, so ordering fuel to the latest edition of ISO 8217 helps reduce cat fine risks.
In addition, while established testing methods and standards are in place, every recognised testing method includes an inherent margin of error, known as repeatability and reproducibility (r&R). Each specific test has its own defined R&R values.
In practice, this means that although suppliers (e.g. charterers) must meet the limits set by ISO 8217 standards, the recipient, (e.g. the shipowner) has an adjusted acceptance limit that accounts for this testing uncertainty. As a result, the shipowner may receive a fuel with a parameter which is higher than the published ISO limit when they test independently.
For example, in cat fine (Al+Si) testing, the recipient limit can be up to 72 mg/kg for a fuel with a specification limit of 60 mg/kg. ISO 4259 is the standard used for interpreting test results and a useful guide can be found here by the International Council on Combustion Engines (CIMAC) which correlates this to marine fuel use.
Consequently, shipowners may receive fuel that complies with all relevant standards yet still exceeds what engine manufacturers consider safe for operation. Most engine manufacturers specify a maximum Al+Si content of around 15 mg/kg at the engine inlet, with some recommending limits as low as 7 mg/kg.
Any bunker stem that has a significant cat fine content more than the engine manufacturer’s instruction will require treatment on board before the fuel is suitable for injection. Achieving this reduction can be challenging for engineers aboard vessels.
This creates a difficult legal and financial position for owners. If fuel supplied by charterers complies with ISO 8217 and 4259, but it has excessive cat fines, it becomes extremely challenging to pursue claims for any resulting damage.
In essence, responsibility shifts from fuel quality at delivery to fuel management on board. The focus is on shipowners and crew to ensure that cat fine levels are reduced to safe limits before reaching the engine.
Cat fine trends
The graph below presents cat fine test results for residual fuels with ≤0.50% sulphur in 2025. It shows that 0.8% of samples exceeded the ISO 8217 limits. However, a much larger proportion, 24% of samples, fell within the 41–60 mg/kg range.
While these values remain within ISO specifications, they still present a heightened risk and underline the importance of effective on-board fuel management to reduce cat fines to safe levels at the engine inlet.
This concern is reflected in industry practice. For example, when VPS, a leading fuel testing laboratory, identifies a vessel manifold sample with cat fine levels exceeding 40 ppm, it typically triggers a request for additional ‘before-purifier’ and ‘after-purifier’ samples. This enables verification of purifier efficiency and highlights whether the vessel’s fuel treatment system is adequately removing abrasive particles.
Managing cat fines on board
Fuel testing
After bunkering, send a representative sample for independent lab analysis against the relevant ISO 8217 parameters and carefully review the results. Don’t focus solely on ‘out of spec’ results, as elevated values can still cause handling issues even if they’re within specification. Where possible, wait for the analysis before using the fuel.
Settling and storage management
Settling tanks are designed to aid the separation of water and sediments from the fuel, with solids like cat fines sinking to the bottom. Draining these tanks regularly from dedicated points keeps any settled contaminants and water out of the fuel system and provides an indication to engineers on the quality of the fuel inside the tank. The same procedure is advised daily on service tanks, where the fuel is then further cleaned and prepared for the engine.
Tank cleaning practices
Regularly clean and inspect settling and service tanks, as far as practicable, to remove catalytic fines, which naturally settle and accumulate at the bottom of the tanks over time. Even when fuel initially complies with specifications, this buildup can result in quantities exceeding acceptable standards. While these sediments typically remain undisturbed during stable conditions, factors such as rough seas and tank stripping may resuspend them, leading to unexpectedly high concentrations entering the fuel system and potentially exceeding the capacity of separators.
Optimising centrifugal separation
Centrifugal separators serve as the main safeguard against cat fines. Operating at extremely high speeds, these devices spin fuel rapidly, pushing contaminants outward so they can be removed as sludge. Maintaining optimal and efficient operation, with the right settings, is crucial for minimising fuel related risks.
To maximise efficiency:
- Operate separators at the correct temperature relative to the density of the fuel which was bunkered.
- Reduce flow rates to ensure adequate residence time, relative to fuel consumption rates and manufacturer guidelines. Longer residence time enhances fuel purification for a given volume. Running purifiers in parallel can also achieve this benefit as each purifier processes only half the total fuel flow, which increases residence time without reducing the overall output to the service tank.
- Ensure all other parameters and conditions are within manufacturer limits, such as back pressure and operating water parameters.
- Some separators enable clarifier configuration to efficiently remove solids only; this is helpful when cat fines are elevated. Afterwards, conventional separator setup should be used in series to remove remaining impurities like water.
- Whenever possible, recirculate fuel from the service tank through the purifiers to ensure ongoing cleaning and prevent cat fines from building up. Some vessels achieve this automatically by allowing the service tank to overflow into the settling tank, maintaining constant circulation.
- Ensure planned maintenance regimes are followed in line with manufacturer requirements.
Filtration and final barriers
In-line filtration protects the engine by capturing larger particles before fuel arrives. Though filters can’t remove all cat fines, they are crucial for blocking sudden contamination spikes, with finer filters placed closer to the engine. Regular filter checks and maintenance help maintain performance. Keep spare clean filters ready to minimise downtime when cleaning is needed.
Testing and monitoring
On board testing kits can provide an estimation of cat fine levels using centrifuge-based methods. Although these tests are not highly accurate, they provide an indication which can aid the crew.
Regularly sending samples taken both before and after the separator to a laboratory for analysis is recommended, as this helps verify that the on-board separators are operating efficiently and effectively removing contaminants.
Standards and management
The damage caused by cat fines can be severe. Repairs can involve major engine overhauls, off-hire time, and financial loss. The key takeaway is that compliance with fuel specifications is only the first safeguard.
Effective on-board treatment and vigilant operational practices are the only reliable safeguards against cat fine damage.