a. Describe with the aid of sketches, an automatic self‑desludging centrifuge suitable for dealing with system oil of an auxiliary engine operating on heavy fuel oil.     
b. If the purifier is continuously discharging sludge from the heavy liquid outlet, discuss the reasons and suitable corrective measures. 
c. Explain how the feed rate, separation temperature and gravity disc of centrifuge described in (a) is selected for optimum purification

a. Automatic self-desludging purifier operation:

1. After getting the operational speed, first apply the bowl closing water then sealing water.
2. Perform manual Discharge to ensure satisfactory purifier operation
3. Then oil supplied at optimum throughput and correct temp.
4. Oil passes from the distributor to the space between the discs through distributor hole
5. Separation and clarification take place between the discs
6. Water and sludge moves outwards and purified oil moves inwards.
7. Sludge accumulated at the periphery discharged at regular interval.

Finally, water will come out from the heavy liquid outlet and purified oil will come out from light liquid outlet

b. Sludge discharge mechanism:

• During sludge discharge, oil supply to purifier is stopped by opening recirculation,
• Open sealing water for specific time to repel remaining oil from the bowl then stop water supply. 
• Now close bowl closing water then bowl opening water is supplied
• bowl opens and sludge discharge takes place 

c. Discharging sludge from the heavy liquid outlet:

Reasons of overflow:

1. Emulsification of oil, contamination due to sea or fresh water –difficult purification
2. De-sludging interval too long or irregular cleaning –resulting in sludge buildup in bowl periphery and in plates resulting high back pressure.
3. Interface is maintained too much outward on the periphery due to oversize gravity disc
4. Throughput is very high resulting in higher back pressure
5. Operating temp low
6. Bowl sealing ring not holding tight due to scoring and worn.

Action:

• Maintain the throughput at optimum level
• Use the correct size of gravity disc
• Maintain the correct operating temperature
• Ensure the good condition of main sealing ring
• Regular maintenance and cleaning of purifier.

d. Steps to be taken to ensure optimum purification:

Feed Rate:

• For lube oil purifier, the throughput should be about :20~25% of max capacity of purifier for detergent oils (4-stroke engine system oil)
• 50~60% of max capacity for straight mineral oils (2-stroke engine system oil)
• For fuel oil purifier, the throughput should be about 10% more than engine consumption
• this will ensure sufficient dwell time FOR effectively removal ofwater and the unwanted insoluble.

Separation Temperature:

• For lube oil purifiers, temperature should not exceed 90oC as this may damage the additives
• For fuel oil purifiers, the temperature should not exceed 98^oC as this may cause the sealing water to boil and loose bowl seal
• Higher temperatures assist in the separation process.

Selection of gravity disc:

• Select gravity disc using nomogram with the information of oil density, operating temperature, and throughput
• The diameter of gravity disc determine the location of interface, smaller diameter brings the interface inward and vice-versa
• Interface should be made between top disc and edge of disc stack.