Asphaltene stability and fuel compatibility

Asphaltenes are the heaviest, most aromatic fraction of a residual fuel, held in colloidal suspension by the aromatic components around them. Disturb that balance and they flocculate, dropping out as sludge that blinds filters, overwhelms separators and can stop an engine.

The practical danger is not instability in a single fuel but incompatibility between two. ISO 8217:2024 Annex H puts it plainly: mixing fuels which are individually deemed to be stable can result in a product which is not stable, and those fuels are described as being incompatible. The usual mechanism is dilution of an asphaltenic fuel with a paraffinic one, which reduces the solvency of the continuous phase and precipitates the asphaltenes. It is why commingling different stems in one tank is avoided, and why a change of bunker grade is planned as a tank-by-tank sequence rather than a top-up.

Annex H is informative rather than normative. It names supplementary methods including ASTM D4740, the spot test, alongside ASTM D7060, ASTM D7112 and ASTM D7157 and ISO/PAS 23263, but it sets no compatibility limit, endorses no spot-test grade threshold and requires no compatibility test at all. The full article will cover the colloidal model, the flocculation mechanism, each test method with its edition, the operational sequencing that avoids the problem, and how an incompatibility claim is proved.