Question: What is a purifier? state its purpose?
Answer: A centrifugal purifier is essentially a container which is rotated at high speed while contaminated oil is forced through, and rotates with the container. However, only material that are insoluble in the oil can be separated by centrifugal force, distillate e.g., gas oil cannot be separated from lubricating oil, nor salt can be removed from sea water by centrifugal force. Water can be separated from oil because water and oil does not form a true solution when they are mixed. Furthermore, there must be a difference in the specific gravity of the materials before they can be separated by centrifugal force.
When separation process is required, two conditions could be met:
1. To separate solid particles from a liquid, e.g. sludge / dirt from oil.
2. To separate liquids of different densities which are mutually insoluble e.g. water from oil 
Purpose of purifier is to purify oil and remove dissolved impurities. Oil that is fed into an engine requires controlled characteristics to maintain the engine operational reliability and extend the life usage.An Oil Purifier separates various contaminants (Aluminum, silicon, sludge, water, etc.) from oil in such specific condition to maintain oil characteristics to certain limits.
Q. state the principle of centrifugal purifier?
Ans: When centrifugal force is applied, the main separating force is equal to the difference between the centrifugal force acting on the solid and water, and the centrifugal force acting on the oil due to the difference in the density.
As per Stoke’s law, the separating force in centrifugal separation can be expressed:
F= (pid3v2/6r) x (d2-d1)
Where:
d = diameter of particle
v = linear velocity of particle
r = radius of rotation of particle
In the formula above, the separating force acting between the two liquids or liquid and solid particle is directly proportional to the difference in their densities and the square of linear velocity. This method of separation is very useful when the difference in densities of water and oil in a mixture is very less. Since the separation force is directly proportional to square of linear velocity, it can be increased many fold by rotating the container at permissible high speed.
When a centrifugal force is acting in the rotating container, all solid particles and water which is heavier than oil is displaced outward from the center of rotation. The lighter liquids tend to form in the center and moves upward. Centrifugal force application increases the separation process in a rotating container. 
In a centrifugal purifier, the separated water is discharged from water outlet, the oil from the clean oil outlet and the solid remain in the rotating unit. The separation by centrifugal force is further affected by the size of the particles, the viscosity of the fluids, and the time during which the materials are subjected to the centrifugal force. In general, the greater the difference in specific gravity between the substances to be separated and the lower the viscosity, in case of fuel oil the greater will be the rate of separation.
Q. What is interface and why it is important?
Ans: The interface is the boundaries of layers between the separating liquid and it is important to maintain inside the bowl of the centrifugal separator to attain the effectiveness of purification. If the separator is running as purifier, it is necessary to hold the oil and water interface in the bowl within the definite range. The position of interface is controlled by varying the outlet diameter of the heavy liquid (water) side and achieved by using gravity discs of different inside diameters.
Q. What are the factors affect the position of interface?
Ans: 
1. Size of the Gravity disc
2. Density or specific gravity of oil
3. Viscosity of oil
4. Oil inlet temperature and
5. Oil feed rate
Q. what are the factors required maintaining the correct position of Interface?
Ans:
1. Correct size of gravity disc.
2. Clean disc stack.
3. Maintain the following feed condition:
4. Constant oil properties i.e. Viscosity & Density
5. Constant Flow rate
6. Constant Temperature
Q. What are the factors effects the selection of a gravity disc?
Ans:
The selection of the gravity disc depends upon the following factors:
1.Specific gravity of feed oil
2.Feed Rate
The adjustment of feed rate is mainly governed by daily consumption (as in fuel oil) and the effectiveness of
separation/purification desired (as normally applied for lube oil). The recommended feed rate for the following: 
i) For fuel oil, it is recommended that the separator should operates at about 50~70% of the rated capacity. In case that the fuel oil quality is over specification and its properties exceed company specifications, it is recommended to operate available purifiers in parallel operation and reduce the feed rate to about 30% for each purifier.
ii) For lube oils, the recommended feed rate is the minimum possible feed rate at which separation temperature can be maintained. This minimum feed rate will help in removing the finer impurities. For a continuous separation system, it is better to circulate the full system oil through the separator 5~6 times/24 hrs rather than circulating it 10~15 times/24 hrs with increased feed rate.
The minimum feed rate is limited by the controllability of constant temperature. When the feed rate is changed drastically, it is necessary to recheck whether the gravity disc is appropriate and replacement is required, as feed rate is one of the governing factors in the selection of gravity disc.
3.Oil inlet temperature
The oil should be heated to obtain an appropriate reduction in viscosity and specific gravity as to effect an efficient and quick separation of water and other impurities. However, the oil temperature should be maintained less than 100 degree C to avoid evaporation of the sealing water and damage to o-rings.
The treating capacity of the oil purifier depends mainly on the viscosity of feed liquid and can be plot on the Temperature-Viscosity diagram below. To ensure that the oil purifier operates efficiently.
refrigeration system raises the pressure of the vaporized refrigerant, causes its saturation temperature to raise, so that it is higher than that of the sea water or air cooling the condenser. Generally condensing temperature should be 5 or 6 degree C above the sea water inlet temperature. It also acts as a pump in the refrigeration system and promotes circulation.