Figure 2. Sodium (ppm) with and without homogeniser
Figure 3. Al+Si (ppm) with and without homogeniser
A drop of 5℃ will reduce separation efficiency to such a degree that particles with a 10% larger size will slip through the system.
In many older installations, the best investment for improved separation efficiency is to spend the money on a new heating and temperature control system.
3.6 Avoid Fuels Containing Used Lubricating Oil.
This is a well-discussed topic. Joint studies between BP Marine and Alfa Laval  has proven that adding used lubricating oil to the bunker will reduce separation efficiency. In the study, different types and amounts of lube oils were used, and different types of separators were tested. See Figure 4. In one case the presence of 5% of used lubricating oil reduced removal efficiency from 65% down to 50%.
3.7 Use a Correctly Dimensioned Separator.
There is a basic relationship between the throughput of a separator and separation efficiency. The higher the throughput the lower the separation efficiency. The urge for competitiveness has driven the separator market towards quoting higher recommended capacities for the equipment. With reference to the data in figure 4, a comparison can be found between three different separator models with the same stated recommended throughput capacity for a 380 cSt oil. As can be seen, there are large differences in the actual performance. These differences in performance are increased when used lubricating oil is present in the fuel.
3.8 Ensure Good Maintenance and Cleaning
As always, good maintenance of the equipment is essential for good performance and high availability. Unplanned production stops for repair, or breakdowns due to non-genuine spare parts, will reduce the overall treatment enffciency of the system. Proper cleaning is also important, as a dirty, or partly clogged, disc stack in a separator will reduce the cleaning efficiency with a two-digit number.
If you have picked up a bunker in, let's say, East USA with an Al+Si content of 43 ppm and you have a treatment system onboard can you then rest peacefully? The cat fines content is anyhow well below the maximum limit of 80 ppm!
If you have succeeded in avoiding the pitfalls mentioned above, then you are on the safe side. But if your heater has started to clog, there is a return pipe going to the suction side of the pump, and you have a slug of used lubricating oil in your bunker, then you will start to get into problems, especially if you have an undersized separator. In this case, it could be that a separator with a higher separation efficiency would save your day.
If your ship is normally bunkering in the Far East or Middle East, can you then afford to have a less efficient treatment system? The average cat fines levels are lower or pretty close to what is regarded as acceptable by most engine builders. If you are lucky you might get away with it for some time, you could perhaps even consider using a homogeniser upstream of the separator. But remember, cat fines are not the only contaminant, some sand might have entered the oil during transport, salt water might have been added to replace some of the oil that "disappeared" before the oil was delivered to one of the dealers. Or you could be the unlucky operator that got the 290 ppm A1+Si onboard - and have to use it.