Category Archives: Purifiers

Centrifugal Purifiers Basic Principle and Working on Ships

Gravity Separation

Consider gravity separation, as occurs in a settling tank, over a gradual period solids such as sludge, dirt, etc., will settle out at the bottom of the tank. The heavy liquids, such as water, will settle out above the solids and the lighter liquids, such as oil, will be at the top of the tank. The application of heat will speed up the separation process.

Separation occurs due to the specific gravities or densities of the liquids and solids.

centrifugal purifiers


Centrifugal Purifiers

In the above case it is gravity that is responsible for the settling out and this takes quite a while. If the gravitational force is increased, by using centrifugal force, the separation effect is much greater. This is shown in figure below by imagining settling tank on its side and rotated, which is the basic principle of centrifugal purifiers.

centrifugal purifiers

The centrifugal purifiers or separator consists of a bowl containing a set of discs stacked one on top of the other. The dirty oil flows down the distributor and up through the holes in the discs.centrifugal_purifier_basic

What is gravity disc ?

  • The gravity disc is important part of purifier, which set the location of the oil, and water interface line, which is variable according to the maker’s design.

How to choose the correct size of gravity disc ?

Correct size is selected using:

  • Separation temperature
  • Density of oil at this temperature
  • Desired throughput of oil and by using of nomogram from the purifier manual.

What is paring disc ?

  • It is a stationary impeller mounted in a chamber at the neck of the bow.
  • Its function is to convert the rotating energy of the liquid into a pressure head.

What about water washing ?

  • Water washing is widely used techniques to remove acids, salts, and other impurities from the oil.
  • The oil must be straight mineral oils or without dispersant additive oils. 
  • Water should be injected into the oil before purifying at a rate of between 3 to 5% of the oil flow that is continuously renews the water seal in the purifier bowl.
  • The oil temperature should be around 75 °C and water temperature about 5 °C higher than oil.
  • It can be done continuously or intermittently. 

What is steam jetting ?

  • By blowing steam into the engine lube oil just pair to its purification.
  • Coagulation of colloidal carbon will occur.
  • Purifier will remove this carbon more effectively.

Compare purifier and clarifier ?


  1. Remove water and suspended solids particles from oils
  2. Two outlets water and clean oil     
  3. Gravity disc on top        
  4. Blind disc on the top of disc stack     
  5. Sealing water required


  1. Remove finer and lighter particles from oil
  2. One outlet for clean oil 
  3. No gravity disc only sealing ring 
  4. Blind disc at bottom.
  5. Sealing water not required 

How do you change purifier to clarifier ?

  1. Open up the purifier and set the blind disc at the bottom of the disc stack.
  2. The water outlet is blocked by a seal on the gravity disc.
  3. Blank off the sealing water inlet line.

What is purifier, clarifier ?

  • Purifier is a centrifuge, which is arranged to separate water and solid impurities from oil.
  • Clarifier is a centrifuge, which is arranged to separate finer solid impurities from the oil.

How to change purifier from HFO to DO ?

  1. Replace the gravity disc, which is smaller than the heavy oil
  2. Open heater by pass vale. 
  3. Close the FO heater steam in/out valves.
  4. Open heater drains v/v.
  5. Pure DO purifier cannot change to HFO, it has no heater.
  6. Pure clarifier cannot change to purifier, it has no water outlet.

Why multidisc provided inside purifier ?

  1. To separate the liquid into thin layer & create shallow settling distance between discs.
  2. Improving separation of oil from heaver liquids & solids particle

Cause of excessive vibration on purifier ?

  1. Sludge too much inside the bowl
  2. Foundation damper & spring failure
  3. Bearing failure
  4. Worn gear
  5. Uneven wear of frictional clutch
  6. Motor speed too high or too low

What are reaseons for purifier over flow ?

  1. Incorrect purifier disc size (inside diameter too large)
  2. Too low fuel oil temperature
  3. Too much rate of throughput
  4. Too much sludge inside the bowl
  5. Low speed (rpm) of bowl rotation
  6. Sealing water failure
  7. Operating water failure
  8. Worn out main sealing ring

Why purifier is not building up speed while running ?

  1. Improper touching with friction clutch (worn out frictional clutch)
  2. Touching with break
  3. Excessive sludge in the bowl
  4. Bearing failure
  5. Motor running at overload
  6. One phase power failure (Single phasing)
  7. Sump oil level too high
  8. Vertical shaft and horizontal shaft are out of alignment

How to adjust purifier bowl height ?

  • By shim by bowl body

Effect of reduced purifier bowl height ?

  • Oil goes through the water outlet.

Why need sealing water ?

  • To seal the water outlet & to prevent the overflow of oil from the water outlet.


“Operation and Maintenance of Machinery in Motor Ships” by N.E. Chell

Oral Guide by – MIN ZAR TAR