Why is my fuel or oil purifier not separating properly?

Poor separation almost always comes down to one of three things: feed rate, temperature, or the water seal and interface. Work through them in that order before blaming the machine. First confirm the throughput is within the maker's rated capacity for the oil being treated; separation efficiency depends on residence time in the bowl, so overfeeding pushes dirty oil straight through to the clean side. Many plants deliberately run purifiers well below rated capacity for exactly this reason, and a purifier that was fine until someone raised the feed to catch up on a low service tank has not failed, it has been overfed.

Second, check the heater and the actual feed temperature at the bowl, not just the set point. Separation is extremely sensitive to viscosity: heavy fuel is normally separated at about 98 C, and lubricating oil typically in the region of 85 to 95 C depending on the maker's instructions. Cold, thick oil simply will not release its water and solids, and a heater element or control valve fault often shows up first as dirty clean-side oil.

Third, verify the interface position. On a conventional purifier that means the gravity disc: it must match the oil's actual density, taken from the bunker delivery note or lab analysis, not from the last batch of fuel. Too small a disc pushes the interface outward until the bowl floods and discharges oil to the water side; too large pulls the interface inward and lets water carry over with the clean oil. On modern units the interface is managed by the control system instead, so check its density setting and sensors. Confirm a proper water seal was established before the feed was opened, and that it was not lost mid-run through a displaced seal, a leaking bowl valve or a sudden slug of water.

Then look at the mechanical housekeeping: an overdue desludge, a scaled or sludge-packed bowl, damaged or wrongly stacked discs, a leaking bowl-hood O-ring, or abnormal vibration from an unbalanced bowl all destroy efficiency and justify a strip-down.

Change one variable at a time and recheck, and always sample the clean oil for water and appearance before returning it to service. If separation stays poor after all of this, suspect the fuel itself; densities close to that of water make separation marginal, and that is a fuel-quality conversation, not a purifier fault.