Most marine oil separators can run as a purifier or as a clarifier. It’s the same machine, set up two different ways. Yet the two terms are constantly used interchangeably in the engine room, and choosing the wrong mode for the job either wastes oil or delivers contaminated product. This guide explains the real difference: what each mode does, how the bowl is physically different, what each manufacturer calls the parts involved, and what it takes to switch from one to the other.
The short answer
A purifier removes both water and solids from oil. It is a three-phase separation producing clean oil, separated water, and sludge. A clarifier removes only solids. It is a two-phase separation producing clean oil and sludge and is used for oil that contains little or no water. Both rely on exactly the same centrifugal principle; the difference is what they’re set up to remove.
On most marine separators this is a matter of configuration rather than a different machine altogether. The same separator can be arranged for either duty by changing a small number of bowl parts, and both Alfa Laval and GEA Westfalia define the two modes the same way.
Purifier vs clarifier at a glance
The table below summarises the practical differences. Note the “interface part” row in particular: this is the component that sets the oil/water boundary in a purifier, and it’s the main thing that changes between the two modes.
| Feature | Purifier | Clarifier |
|---|---|---|
| Removes | Water and solids | Solids only |
| Phases separated | Three (oil, water, sludge) | Two (oil, sludge) |
| Liquid outlets | Two (clean oil + water) | One (clean oil) |
| Interface part | Gravity disc (Alfa Laval) / regulating ring (GEA Westfalia) | Clarifier disc / sealing (blind) ring |
| Sealing / operating water | Required (maintains the water seal) | Not required |
| Typical duty | Heavy fuel oil; lube oil with water present | Lube oil polishing; diesel / MGO with little water |
How a purifier works
In purifier mode the separator performs a three-phase separation. Contaminated oil is fed into the centre of the spinning bowl, where centrifugal force — up to roughly 7,500 times gravity — drives the heavier phases outward. Water forms a ring at the outer edge, solids pack against the bowl wall as sludge, and the cleaned oil stays toward the centre, leaving through the clean oil outlet via a paring disc or centripetal pump.
The key to purifier operation is the interface part — the gravity disc on an Alfa Laval separator, or the regulating ring on a GEA Westfalia. This precisely sized ring sets the position of the oil/water boundary inside the bowl and maintains the water seal that stops oil escaping through the water outlet. Because a purifier handles a water phase, it needs a separate water outlet and requires sealing water (also called operating or displacement water) to establish that seal before oil is admitted. Select the wrong interface part and the water seal breaks — but that’s a troubleshooting subject in its own right, covered separately.
How a clarifier works
In clarifier mode the separator performs a two-phase separation: it removes solids only, and does not separate water. Because there’s no water phase to manage, a clarifier needs no gravity disc or regulating ring, and no sealing water.
Instead of the interface part, a clarifier bowl uses a clarifier disc or sealing (blind) ring that closes off the passage to the water chamber, so the whole bowl is arranged for a single clean-oil discharge. In a typical conversion a blind disc is fitted at the base of the disc stack, forcing the incoming liquid to enter the stack from the outside. Accumulated solids are discharged periodically as sludge, exactly as in purifier mode. Clarifier duty suits oils that carry little or no water — most commonly lube oil polishing, and diesel or marine gas oil where water contamination isn’t a concern.
What’s physically different inside the bowl?
The difference between the two modes comes down to a handful of bowl parts. To convert a purifier to a clarifier you mainly change two things:
- The interface part, the gravity disc (Alfa Laval) or regulating ring (GEA Westfalia), is replaced by a clarifier disc / sealing (blind) ring that closes the water passage.
- The top disc or chamber cover arrangement is changed: the purifier version has an opening for water discharge; the clarifier version does not. On some GEA Westfalia bowls the purifier mode additionally requires a dedicated water seal component that the clarifier arrangement omits.
The rest of the bowl and the disc stack stay the same. Exactly which parts you need depends on the manufacturer, the generation and the specific model — which is why it pays to confirm the model from the nameplate before ordering. KET Marine supplies the separator bowls, service kits and conversion parts for both Alfa Laval and GEA Westfalia machines.
How do you convert a separator from purifier to clarifier?
At a principle level, the conversion follows four steps. This is an overview, not a substitute for the service manual for your specific model:
- Confirm the make, generation and model on the nameplate
- Replace the interface part (gravity disc / regulating ring) with the correct clarifier disc/sealing ring
- Change the top-disc or chamber-cover arrangement as specified in the manual, removing the water-seal components where applicable.
- Set the operation to clarifier mode, no sealing water is admitted.
Always work from the manufacturer’s service manual for your model and use the correct conversion kit. Get the parts wrong and the machine won’t seal or separate as intended. KET Marine stocks the conversion parts per make and generation, and can advise on the correct kit for your unit.
Where automatic systems change the picture
On modern generations the classic purifier/clarifier distinction has largely blurred. Alfa Laval’s ALCAP and GEA Westfalia’s UNITROL both do away with the manually selected interface part. A sensor continuously monitors the water content of the cleaned oil; when water begins to break through, the control system discharges it automatically.
The result is a machine that behaves like a clarifier but still deals with water whenever it appears, combining both functions without a gravity disc or regulating ring. For engineers running older gravity-disc or regulating-ring machines (Alfa Laval MAB, MAPX and MOPX; GEA Westfalia OSA and OSB), the classic purifier-versus-clarifier choice is still very much a live decision. For the full picture of which generation does what, see our separator generations guide.
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