The booklet aims to provide crew members on board ships going to bunker and use compliant fuels, with information related to the 'compatibility' and 'cold flow properties' of such fuels.


It also regards the risks rising from such fuels and the measures to be taken to address them.

Measures to prevent sludge formation

In order to prevent sludge formation, due to the mixing of fuel oils or to prevent issues regarding sludge formation in the machinery, ClassNK recommends the following:

Changing over in fuel oil storage tanks

  • Empty tanks as much as possible before bunkering a new batch of fuels;
  • In case mixing within storage tanks cannot be avoided, check compatibility by spot test, whenever possible at bunker time. If incompatibility is confirmed, then add sludge dispersant to the remaining fuel oil in the storage tank before bunkering;
  • Since compatibility may change over time, and sludge may be generated after long periods of storage even when the results of initial spot testing were good, it may be wise to use sludge dispersant whenever different fuel oils are mixed.

Changing over in fuel oil pipelines

When changing over in fuel oil pipelines, in order to change to compliant fuel oil that is stored in a storage tank, the different types of fuel oils are generally mixed together in settling or service tanks. In this case:

  • To prevent the mixing of different types of fuel oils in settling/service tanks, operators should maintain the amount of the remaining fuel oil in these as minimal as possible. Then they should transfer the compliant fuel from a storage tank to the settling/service tank;
  • Conduct a spot test, and add a sludge dispersant to the settling tank if incompatibility is confirmed;
  • Use up the fuel oil which had been mixed together in tanks or pipelines as soon as possible;
  • In the cases were sludge is frequently confirmed in fuel oil pipelines or where the differential pressure in the pipeline between before and after the filter rises, operators should: limit the interval backwashing; maintain the differential pressure of backwashing lower; and clean the filter more frequently than usual.
  • In cases where sludge is commonly found in filters placed before purifiers, precipitation of sludge in the fuel oil purifier could also take place. In case a large amount of sludge generation could occur, operators should: Reduce the flow rate. They should operate two sets of purifiers and treat half the amount of fuel be each purifier; Limit the interval of sludge discharge; Keep a high temperature of oil flow, according to the purifier manufacturer's manual; and Limit the maintenance interval of the separating disc.

For more information, click the PDF below