BLENDING OF OIL ON BOARD SHIP :

Shipboard blending of lower viscosity of fuel for use in auxiliary diesel engines is a cost saving practice. For this the heavy fuel used for the main engine is diluted with a gas oil or marine diesel fuel.

However, there is nevertheless a risk of incompatiability and consequent sludge formation that is more probable than might be experienced when mixing two fuels of similar viscosity.

The possibility of incompatiability occuring depends upon the stability reserve of the heavier fuel, the solvency of the dilutent and the degree of dilution. In a stable fuel, the ashphaltenes remain in suspension. However, should the equilibrium of the suspension be disturbed (possibly by mixing with a second fuel which is itself stable but is nevertheless incompatiable with the first fuel), precipitation of ashphaltenes will occur as sludge.

For a compatiable blending, there is little change in sludge formation irrespective of the quantity of dilutent incorporated. If however, the two fuels are incompatiable, then a point will be reached when sludge forming tendency will increase as dilutent incorporation increases. Further addition will cause more sludge to be formed, the maximum normally occuring over the heavy : light blending range of 60:40 to 30:70. Sludging tendency then falls rapidly to zero as the blend approaches 100% light fuel. Critical blending ratios can vary and will depend on many parameters including source of residue and nature of refining i process employed. To minimise fuel handling and treatment problems, an indication of compatiability can be determined by using a modified form of ASTMD2781. The light and heavy fuels should be mixed in the required ratio and then evaluated using the spot test rating procedure.

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