How an oil purifier work?

Bunker oil for combustion on ship's engines is of very low grade. They contain various impurities like particles and water droplets. Large particles in the fuel oil are allowed to settle down in the settling tank before they are pumped into the combustion process piping. After the particles have settled down, the fuel oil will be passed through some sort of filtration using coarse and fine filters.

However, these processes are not sufficient to remove very fine particles and water droplets in the oil.

If you have a bucket of a mixture of dirty oil and water and you leave it in a quiet place to settle, what will happen? You will find that after a long time, the oil will separate out from the water. And if the solid particles are heavy enough, they will also settle at the bottom of the bucket.

Notice that the separation is due to gravity (or specific gravity). Heavy bunker fuel oil has an SG of about 0.95, diesel oil about 0.85 and fresh water has an SG of 1. Because of the difference in Specific Gravity, or SG, the oil will float on top of the water. The solid particles that is heavier than water will sink down. But the above method will take a long time. Furthermore, if the SG's of the mixture are very close, the oil and the water may not be able to separate very well.

An oil purifier uses the same principle for separating dirt or water from oil. Instead of using gravity, it uses centrifugal force.

Through a system of gears, a centrifuge bowl is rotated at high speeds. Oil to be purified is allowed to enter the bowl while it is rotating. The heavier components in the oil are thus forced outwards. The solid particles that are too fine to be removed by filtration are forced towards the circumference of the bowl.

The oil is also heated so as to reduce the SG of the oil. The difference in SG's between the oil and the water will thus be widening. This will enable a better separation between the oil and the water.

Oil purifiers usually maintain a layer of water inside the bowl to act as a seal for the oil. Without the water layer to act as seal, the oil can flow out together with the particles and be lost.
If removal of water is not needed, the centrifuge can be modified so that no water layer is needed. The centrifuge then becomes a clarifier.

Instead of using gravity, the oil purifier makes use of centrifugal force to separate the water from the oil. The solids are removed together with the water. (Hint: Just turn the picture of the bowl 90 degrees and you will see the similarity between the gravity model and the centrifugal model. )

The actual construction of the purifier will depend on the manufacturer. The most common designs have conical plates to enable the particles to clump together. The larger particles formed will weigh more and are able to be separated from the oil easier. There will be a ring that acts like a barrier between the oil layer and the water layer. The selection of the size of the ring aperture will depend on the SG of the oil. Wrong selection of the ring will cause either water seal loss or oil loss. Some purifiers are designed for auto bowl cleaning operation. Some components in the bowl assembly are able to be open by water pressure coming from a separate control system. The bowl cleaning operation is operated at timed intervals. This reduces the need for frequent manual bowl cleaning. Like all machinery, bearings do wear down, o-rings and seals do become brittle, and bowl contact surfaces do get worn out. The high rotational speeds of the purifier can cause vibrations if the sludge accumulates unevenly. Shear pins or couplings do get broken if there is too high a torque between the bowl shaft and the motor drive.