What is the difference between a purifier and a clarifier?

The distinction between a purifier and a clarifier comes down entirely to whether the bowl is designed to separate two liquids of different density or just to strip solids from a single liquid, and getting this wrong in operation, running a bowl set up for one when the oil condition calls for the other, is a classic cause of poor separation or a lost bowl.

In a purifier, the bowl holds an interface between the denser water phase and the lighter oil phase, and separated water is continuously discharged through its own outlet while clean oil is drawn from the centre. Positioning that interface correctly inside the bowl is critical: traditionally this was done by fitting a gravity disc of the right bore for the oil's density, so that the interface sits at the correct radius, too small a disc pushes the interface out toward the oil outlet and oil is lost with the water, too large a disc lets water carry over into the clean oil. Modern self-cleaning purifiers increasingly control this electronically with a water transducer that senses the interface and adjusts automatically, removing much of the old trial and error with gravity discs.

A clarifier has no water outlet and no interface to manage, because it is built to handle oil that is already largely free of water. Its bowl disc stack is arranged purely to fling out fine solids, catalytic fines, carbon, rust, and other particulates, toward the periphery, where they collect and are periodically discharged during the bowl's desludging cycle. Because it does not have to accommodate a water layer, a clarifier can achieve a finer degree of solids polishing than a purifier handling the same throughput.

On many ships the two stages run in series on the fuel oil system: a purifier first strips out the bulk water and larger contaminants, particularly important with fuel that arrives with significant water content or after tank cleaning, and the partially cleaned oil then passes through a clarifier for final polishing, which matters a great deal for protecting fuel injectors and exhaust valves from catalytic fines, a genuinely destructive contaminant that abrades injector components and cylinder liners if not removed. Correct throughput rate for the separator's bowl size and correct oil temperature, typically around 95 to 98 degrees Celsius for heavy fuel purification, matter as much as the choice between purifier and clarifier configuration, since a separator run too fast or too cold will not separate properly regardless of setup.