Purifier Types: Purifier vs Clarifier, Disc vs Tubular

1. Purifier versus clarifier

This is the classification everything else hangs on, and it is the one most often answered badly. The two machines use identical physics but are plumbed differently, and the difference decides what each can remove.

The purifier

A purifier is used when the oil contains both water and solids. It performs oil-water separation and oil-solid separation at the same time, and discharges water and sludge by separate routes.

It requires a water seal so that the separated oil and water follow their correct outlet paths, and it uses a gravity disc to set where the interface between the two liquids sits.

The clarifier

A clarifier is used to remove solids from oil. It has no water seal in the same arrangement, no heavy-liquid outlet used for water separation, and no interface to control. The bowl simply provides more space for oil and solids.

The practical consequence of getting this wrong is expensive. Run a purifier without an established water seal and oil escapes through the heavy-liquid outlet. Run a clarifier on wet oil and the water has nowhere to go, so it leaves with the clean oil.

2. Terminology and machine names

The same machine is called different things by different makers, and the syllabus uses the terms loosely. Settle the vocabulary first.

The last two entries are worth separating clearly, because they are the source of a common operational mistake. Rated capacity is a catalogue figure measured under stated standard conditions. Actual capacity depends on the density, viscosity, water content and solids content of the oil in front of you, and on how clean the oil has to be. For difficult contamination, especially catalyst fines, the actual capacity you should use is well below the rated figure.

3. Disc-type and tubular-bowl centrifuges

Marine oil-treatment centrifuges fall into two bowl families.

The disc type won the marine application for a simple reason. A tubular bowl has to run extremely fast to compensate for the long distance a particle must travel, and high speed limits the bowl size and the amount of sludge it can hold. The disc stack buys the same separation at a moderate speed in a bowl large enough to hold useful quantities of sludge — which is exactly what a ship needs when treating residual fuel.

4. Manual and self-cleaning purifiers

The second classification is how the accumulated solids are removed.

Manual, or non-self-cleaning

Solids collect at the bowl periphery until the sludge space is full. The machine is then stopped, the bowl is opened up and the sludge is cleaned out by hand. Treatment stops for the duration.

This arrangement is confined to small machines and to oils with a light solids load, where discharge intervals are long enough to make manual cleaning acceptable.

Self-cleaning

A self-cleaning purifier has a sliding bowl bottom held closed by operating water under pressure. At the chosen interval the feed is interrupted, the bowl opens hydraulically and the sludge is thrown out through discharge ports, then the bowl closes and the liquid seal is re-established. Treatment resumes without the machine being dismantled.

Residual fuel carries enough sludge that a manual machine would need constant attention, so self-cleaning operation became standard for fuel treatment at sea. It is also what makes unattended machinery space operation possible.

5. Partial and total discharge

Self-cleaning machines are further classified by how much of the bowl contents is ejected. This matters because the two modes interrupt treatment differently and are used for different reasons.

Both modes are described step by step, with the operating water that drives them, in the Working chapter. What matters here is the classification: the two modes are not alternatives chosen at random. The partial discharge keeps the machine running, and the total discharge cleans it out.

6. Batch versus continuous bypass purification

This is how the purifier is connected into the engine, and it is a syllabus question in its own right.

Continuous bypass purification

Oil is taken continuously from the engine sump or drain tank, passed through the purifier and returned to the lubricating oil system. The engine runs normally throughout. The purifier treats part of the circulating oil rather than requiring the whole charge to pass through it, so the oil is cleaned gradually and continuously.

This is the normal at-sea arrangement for system lubricating oil. It keeps the oil in acceptable condition while the engine stays in service, and it costs nothing in downtime.

Batch purification

The entire sump charge is emptied into a renovating tank, heated to about 75-80°C, and purified slowly back into the emptied and cleaned sump tank. The engine must be stopped, or the sump isolated, for the duration.

Batch treatment achieves the higher degree of cleanliness. The throughput is low and the residence time long, so the limit-size particle is smaller and both water and fine solids come out far more effectively than they can at continuous bypass rates.

The distinction to carry into the exam is this: continuous bypass maintains oil quality while the engine runs, and batch restores it when it has already deteriorated. They are not competing methods — they are used at different times for different reasons.

Fuel oil is handled differently again. Distillate fuel normally needs a single-stage centrifugal purification. Residual fuel is commonly treated in two stages: the purifier removes water and heavy solids first, and a clarifier downstream removes the remaining solids before the fuel reaches the service tank.

7. Modern self-cleaning families

Two families dominate modern tonnage. The names differ but the principle is the same, and both represent the same shift: from a machine that follows a timer to a machine that reads its own conditions.

Mitsubishi Selfjector

A self-cleaning disc-type purifier built around the sliding bowl bottom and a set of operating-water solenoid valves. Depending on the model and build, the control package can include:

• A control panel with manual and automatic modes, and timer-controlled discharge.
• Separate partial and total discharge sequences, selected by the controller.
• A water detector, which senses water arriving at the clean-oil side and can trigger a discharge.
• A discharge detector, which confirms that a discharge actually took place.
• A leakage monitor, which detects leakage from the bowl or the seals.
• A Multi-Monitor display presenting operating data and trends.

Alfa Laval ALCAP and similar systems

ALCAP stands for Alfa Laval Clarifier And Purifier, and describes a separation system in which the interface is controlled automatically rather than set once by a fixed gravity disc. A water transducer senses where the interface actually sits and the controller adjusts accordingly, so the machine adapts to the oil rather than relying on a fixed mechanical setting.

What changed, and why it matters

The practical benefit is fewer manual adjustments and earlier warning of a developing fault. The practical caution is that the control system can mask a mechanical problem: an alarm that keeps being reset by the controller is still telling you something is wrong.

Model names, panel layouts, timer ranges and detector behaviour vary widely between makers and between generations of the same machine. Always work from the manual for the specific unit on board — never from a general description.

8. Matching the machine to the duty

Bringing the classifications together, the choice of machine follows the duty almost mechanically.

Why the rated figure is not the answer

A machine is often selected, and then operated, from its rated capacity. That figure is measured under the maker's standard conditions and is not a promise about your oil. The rate you can actually use depends on:

• The density of the oil at the separating temperature, and therefore how much density difference exists
• The viscosity, and therefore the drag resisting particle movement
• The water content and the solids content
• The particle size of the contamination — fine catalyst fines need a much lower rate than coarse sludge
• The cleanliness actually required at the engine inlet

This is why a purifier that performed perfectly on one bunker can appear to fail on the next without anything having broken. The machine is the same; the oil is not.

Never select or set a purifier from a catalogue figure alone. Start from the maker's selection method, then adjust the throughput downwards until the oil leaving the machine meets the cleanliness you actually need.

Where this leads

This chapter has classified the machines. The next chapters take the standard marine arrangement — a disc-type, self-cleaning purifier — and go inside it:

• Construction — the bowl, disc stack, distributor, spindle, worm gear and sliding bowl bottom.
• Working — the three-phase flow, the interface line and the desludging cycle.
• Operations, maintenance, safety and troubleshooting — running it, keeping it fit, stopping it safely and diagnosing it.