Stability — will the fuel hold together?
A stable fuel keeps its heavy asphaltenes in suspension. An unstable fuel drops them out as sludge — blocking filters and separators and starving the engine. VLSFO blends, which vary widely in composition since the 2020 sulphur cap, are more prone to this than the old uniform HSFO. Total Sediment Potential (TSP) on a lab test is the key stability indicator; ISO 8217 caps it at 0.10% by mass.
Compatibility — the danger of mixing
Two fuels can each be perfectly stable on their own and yet be incompatible when mixed — the blend drops asphaltenes even though neither fuel would alone. This is why commingling different stems in the same tank is risky: a compatible-looking mix can sludge heavily. Best practice is to segregate bunkers by stem, burn down a tank before refilling with a different fuel, and run a compatibility spot-test before deliberate commingling.
Cat fines — the engine-killer
Catalytic fines are hard particles of aluminium and silicon (Al+Si) left over from the refinery's catalytic cracking. They are abrasive; if they reach the engine they grind away liner and injection surfaces, causing expensive wear. ISO 8217 limits Al+Si to 60 mg/kg at delivery, but that is far too high to burn — the engine maker typically wants under 10-15 mg/kg at the engine inlet. The gap is closed onboard by settling and centrifuging.
Total Sediment Potential: max 0.10% m/m (ISO 8217). Cat fines (Al+Si): max 60 mg/kg at delivery, but target <10-15 mg/kg at engine inlet. Both are only as good as the sample they are measured on.
How to protect the engine
Draw a proper representative sample at the manifold and test before burning — pre-burn testing catches problems before they reach the engine. Let bunkers settle, keep purifiers correctly set up and running at the right temperature and flow, and monitor cat fines at the engine inlet, not just at delivery. Segregate incompatible stems. Seven Ocean supplies to ISO 8217 spec and supports representative sampling so you have a documented basis if a claim is ever needed.
Frequently Asked Questions
What are cat fines?
Catalytic fines — abrasive aluminium and silicon particles from refinery cat-cracking. They wear engine liners and injection equipment. ISO 8217 caps them at 60 mg/kg at delivery, but engines need them reduced to under 10-15 mg/kg at the inlet by settling and purification.
Why is mixing two fuels risky?
Two fuels can each be stable alone yet incompatible when mixed, dropping asphaltene sludge that blocks filters and separators. Segregate stems and run a compatibility test before any deliberate commingling.
What is fuel stability?
The fuel's ability to keep heavy asphaltenes in suspension rather than dropping them as sludge. Total Sediment Potential is the lab measure; ISO 8217 limits it to 0.10% by mass.
How do I avoid these problems?
Representative sampling and pre-burn testing, proper settling and purification, monitoring cat fines at the engine inlet, and segregating incompatible stems.