A common concern of maritime chief engineers, regarding vessel engine damages, is the presence of highly abrasive 'cat fines' within marine fuel. To fully understand the seriousness of this issue, we first need to understand the source of the problem.

Oil refineries use catalysts in their Fluid Catalytic Cracking Units (FCCU), or 'cat plants', to process crude oils in the production of numerous products from the source crude. These catalysts are high in aluminium silicate (Al+Si) content, and over time and repeated use, can breakdown and fragment into smaller particles, or catalytic fines. These catalytic fines, known throughout the industry as cat fines, can eventually find their way into the heavier refining streams and consequently the HSFO and VLSFO fuels being burnt on board vessels. Higher levels of cat fines within the fuel can be extremely abrasive and damaging to ships engines.

It is critical to note the ISO 8217 specification limit for Al+Si for residual fuels delivered to the ship is 60 mg/Kg for all revisions post-2005. This level is significantly greater than the OEM engine inlet limit of 15mg/Kg, and therefore the fuel requires efficient onboard purification, monitored via a frequent fuel-system check process in order to avoid potentially catastrophic engine damages.

From a recent review of more than 3,000 fuel-system check samples taken pre‑purifier and post‑purifier, VPS found that 16% of post‑purifier fuel samples had cat fine concentrations above the 15 mg/kg OEM limit. This indicates that some 480 vessels had purifiers failing to operate to their optimum efficiency in order to protect the engine.

At this time of the Middle East military conflict, one might suspect that refinery output around the wider region to have been impacted by crude availability, thus reducing the ultimate throughput of any refinery. However, under certain refinery operating conditions, reduced throughput may lead to higher relative concentrations of cat fines in heavier streams. Thus, these cat fines can potentially end up in marine fuel supply chains.

In two-stroke crosshead engines, ineffective removal of cat fines can result in their entry into the combustion chamber, where they cause abrasive wear between piston rings and cylinder liners. Conversely, medium-speed engines tend to be generally more tolerant, due to continuous oil circulation and filtration, but still vulnerable if catfine levels are excessive.

Another consideration within this equation is that cylinder oil (used exclusively in two-stroke engines) is 'single use' and exhausted after first use. Thus, its overuse in any attempt to combat cat fines and minimise wear can be commercially punitive, unnecessary, and, in some cases, contribute to damages — exacerbating the 'grinding paste' effect occurring within the cylinder liners.

Cylinder lubricants are primarily a base oil, plus an additive 'pack' (a combination of carefully selected compounds focused on providing various qualities — e.g. Base Number (BN) — to combat fuel sulphur and detergency to clean etc). However, inappropriate cylinder oil selection, or excessive feed rates may contribute to deposit formation on piston crowns and ring zones.

Over-lubrication is not an effective mitigation strategy — as it may increase deposit formation and will certainly increase operating costs.

A first step to prevention of cat fine damage would be in the selection of a reputable supplier known for delivering low cat fine fuel. Data in relation to this is highlighted in the VPS PortStats application. However, it is recognized on numerous occasions that the owner/operator of the ship dealing with sub-standard fuel is not the entity who purchased the fuel. Therefore, further practical considerations should also be made, including appropriate tank cleaning, circulation and movement to avoid cat fine settling and concentration at much higher levels than the original test results obtained from the vessel manifold sample.

In the case of managing cat fine levels above OEM engine entry limits, effective purification remains key. Statistically, a vessel will suffer one to two instances of main engine damage over the course of her operational lifetime. These costs can reach millions of dollars — far more than the average main engine damage costs of $650K highlighted in the Swedish Club paper 'Engine Damage'.

Monitoring and Quality Control – Fuel System Checks (FSC)

It is essential that fuel samples are taken both pre- and post-purifier. This should be carried out at regular intervals (on a quarterly basis), with increased frequency when high cat fine levels are identified. In addition, whenever VPS sees a vessel manifold sample with a cat fine level >40kg/Kg, an alert is automatically sent to the vessel, with the instruction to take additional before and after purifier samples as a cautionary procedure.

There are numerous reasons why regular fuel system checks are critical:

• Help identify potential risks and operational issues before major damage occurs
• Confirm that the system’s flow rate, temperatures and discharge cycles are properly adjusted to handle the specific fuel that is being treated
• Verify that the fuel treatment system is properly maintained
• Reduce operating cost and increase lifecycles of critical components
• Identify the presence of unusual components that can enter fuel post- delivery.

Periodic sampling from the fuel treatment system will also identify problems such as water ingress from ballast systems, leaking heating coils and cargo contamination.

Core Operating Conditions

In addition to the FSC programme, it is key to undertake the following practices:

Maintain the correct inlet temperatures — as purifiers rely on centrifugal separation, enhanced by density differences. The fuel must be heated to the manufacturer-specified temperature for optimal separation and reduced carryover.

Ensure the separator is correctly configured and operated (e.g. gravity disc selection in conventional units or proper control settings in automated systems) to avoid reduced separation efficiency, which could allow water or cat fines to pass through to the engine.

Keep the feed rate of fuel slow and steady. Running purifiers in parallel at minimum feed rate improves separation and reduces carryover.

Match settings to oil density. Density changes with fuel grade, so adjusting settings ensures proper bowl separation.

Maintenance Practices that Directly Improve Efficiency

Clean filters and heaters regularly. Blocked filters or fouled heaters reduce flow and temperature stability, lowering purification quality.

Clean fuel tanks at scheduled intervals. This prevents high sediment and cat fine concentrations from overwhelming the purifier.

Avoid bypassing any fuel-treatment equipment. Bypassing risks engine damage and eliminates the purifier’s protective function.

Ensure adequate spare parts are on board. Worn seals, gaskets, or disc stacks reduce bowl pressure and separation efficiency.

Follow the vessel’s preventative maintenance schedule and manufacturer’s intervals. Planned maintenance ensures consistent cleaning, inspection and overhaul cycles.

Summary

So, with a recent VPS survey finding that 16% of post‑purifier fuel samples had cat fine concentrations above 15 ppm, this highlights the fact that these purifiers are failing to operate to their optimum efficiency and their primary function to protect the engine.

All the above monitoring, quality control and operational steps are key to ensuring a vessel’s engine is protected from the abrasive wear cat fines can cause. Plus, by following this advice, the vessel will see an improvement in combustion efficiency due to burning cleaner fuel. This will all result in reduced downtime and repair costs — avoiding purifier failures which can lead to major engine issues.

Article by VPS, with editing by Bunker Index Staff