These are the five properties of residual fuel oils that should be further paid attention when using compliant fuel oils:

  • Compatibility
  • Cold flow properties
  • Low viscosity
  • Cat-fine (contents of Aluminum and Silicon)
  • Ignition and combustion quality

In particular, the issues of “Compatibility” and “Cold flow properties” would directly cause the adverse influence for engine operation due to excessive sludge formation and wax crystal formation as follows:

  • “Difficulty of transferring fuel oil and engine stop due to the blockage and chock of fuel oils in the pipe lines” and
  • “Operation inability of fuel oil purifiers due to the capture and/or accumulation of sludge and/or wax in purifier bowls.”


Preventive measures

The following are effective in view of the fundamental of fuel management:

-Compatibility: Measure for preventing sludge formation

Fundamental actions: Never mix the fuel oils in storage tanks and pipe lines, or Minimise the mixing ratio as low as possible among different types of oils.


  • If the incompatibility is found, please add sludge dispersants or sludge solubilisers to the storage tanks.
  • Please keep cleaning and take action on the oil purifier management in order to avoid sludge accumulation; reducing the flow rate,shortening the interval of discharge of sludge, maintaining a high temperature of oil flow, and shortening the maintenance interval of separator disc.
  • It is recommended to store the reserve of sludge dispersant on board.

-Cold flow properties: Measure for preventing wax crystal formation.

Fundamental actions: Keep or heat the fuel oil temperature at least 10℃ above the cold plugging point (CFPP+10℃).


  • If the temperature of sea water, surrounded temperature of pipe lines, engine room, ambient temperature can’t be 10℃ above the cold plugging point (CFPP+10℃), please add additives to mitigate any wax formation on board before wax crystal formation
  • The treatment for both strainers and purifiers are the same as above measure for preventing sludge formation.
  • It is recommended to store the reserve of additives to mitigate any wax formation onboard.

ClassNK has recently published a booklet, which includes technical information from manufacturers that provide the sludge dispersants and sludge solubilizers, to let crew members onboard ships know "compatibility" and "cold flow properties" of such fuels as well as the risks associated with such fuels and the measures to be taken to mitigate such risks.