ST10 On-site Fuel Stability and Compatibility Tester ASTM D4740
Bulk fuel stored for long periods can become unstable the asphaltene content can precipitate out of solution causing the formation of sludge. This has the potential to block filters and pipes, leaving tanks with an unpumpable residue.
The 'break up' is dependent on the nature of the liquid hydrocarbons in which the asphaltenes are suspended. If the medium is aromatic (hydrocarbons in ring formation) then they will remain in suspension but if it is paraffinic (linear hydrocarbon formation) the asphaltenes may have a propensity to coalesce into sludge. Once a fuel has chemically broken down there is no way to satisfactorily reverse the process. Precipitated asphaltene cannot be re-dissolved.
Although incompatibility is not a common phenomenon its likelihood increases in tandem with fuel switching, such as when entering and leaving an ECA. Compatibility problems must be treated as a critical concern as it can result in power failure, fuel systems can become paralyzed and the work for cleaning up the results is both difficult and time consuming. It is important to remember that a stable fuel oil can become unstable when mixed with another stable grade although generally fuels of the same viscosity grade with similar densities will be compatible.
The marine fuel market is undergoing complex and far-reaching change, largely as a result of a growing body of environmental legislation. Issues surrounding fuel stability and compatibility have never been more relevant, especially with the new 0.10 per cent sulphur level for fuels used in Emission Control Areas (ECAs)