Engine-damaging cat fines: a chief engineer’s nightmare

Composed primarily of aluminium and silicon, cat fines are hard, microscopic particles leftover from the refining process that are found in high sulphur and very-low sulphur fuels. When cat fines, usually between 5 to 75 microns thick, bypass the fuel treatment system, they act like a fine grinding paste between moving parts in marine engines. In two-stroke crosshead engines, this occurs principally between the piston rings and cylinder liners, wearing away the honed surface the liner relies upon for lubrication and sealing.

Fuel injectors and pumps suffer similarly, scoring precision surfaces with micron tolerances. Consequences range from increased lubricating oil consumption and reduced combustion efficiency through to holed pistons, scuffed liners and, in severe cases, complete loss of main engine power.

Their presence in HSFO and VLSFO is a direct consequence of how these fuels are produced. Refineries use catalysts within fluid catalytic cracking units, or cat plants, to break heavier crude fractions into lighter, more valuable products such as petrol and diesel. These catalysts are largely aluminium silicate, and through repeated use they fragment into microscopic particles, ranging from roughly one to 75 microns – smaller than a human hair.

“Purifier underperformance remains a live and widespread operational risk”

While much of this catalyst is retained within the process, some inevitably escapes into the heavier residual streams forming the basis of HSFO and VLSFO. Because these fuels are blended from the bottom of the barrel, rather than cleaner distillate fractions, they are inherently more likely to carry catalyst contamination through to the finished bunker product.

Industry standards reflect the scale of the challenge. ISO 8217 permits residual fuel to be delivered with an aluminium plus silicon content of up to 60 mg per kg, yet engine manufacturers generally require concentrations at the engine inlet to be reduced to between 10 and 15 mg per kg before combustion.

Effective purification is therefore fundamental to engine protection. The centrifugal purifier remains the principal defence, separating dense cat fine particles from lighter fuel oil through high-speed rotation. Fuel must be heated to the manufacturer’s specified temperature to achieve correct viscosity and density differential for separation. Purifiers should run at a slow, steady feed rate, with parallel operation preferred over series operation when cat-fine levels are elevated, since this increases residence time and improves efficiency.

Gravity disc selection and bowl configuration must match fuel density, and worn seals, gaskets or disc stacks must be replaced promptly, since reduced bowl pressure compromises separation. Tanks should be cleaned regularly to prevent settled sediment disturbing and overwhelming the purifier, and bypassing fuel treatment equipment must always be avoided.

Verification of this process matters as much as the process itself. Regular fuel system checks, with samples drawn both before and after the purifier, can confirm the equipment is performing as intended. Where a manifold sample returns an elevated reading, increased sampling frequency becomes essential. Recent industry data indicates a meaningful proportion of vessels are still delivering fuel to the engine above the recommended OEM limit, underlining that purifier underperformance remains a live and widespread operational risk.

Ultimately, managing cat fines successfully depends on treating fuel purification as a continuous discipline, rather than a single safeguard. Careful supplier selection, rigorous tank and purifier maintenance, correct operational settings, and consistent before-and-after sampling together form the only reliable barrier between contaminated bunker fuel and an engine failure that can cost a shipowner millions of dollars and weeks of vessel downtime.