Centrifugal seperators

Question 1

1. With reference to centrifugal purifiers:
   a) State the function of the gravity disc; (4)
   b) Explain the consequences of fitting the following:
   i) Too large a gravity disc; (6)
   ii) Too small a gravity disc. (6)

Answer 1

a) The gravity disc is required to create sufficient back pressure on the water seal to maintain the oil water interface as close to the edge of the bowl as possible without loosing the water seal. This results in the oil which has to be purified having a longer dwell time in the bowl in order for centrifugal forces to separate the water which then leaves through the water outlet.
b)
i) The oil water interface is displaced too near to the bowl periphery. Resulting in a collapse of the liquid seal, oil may now pass out the water outlet. Escaping oil via the water outlet could potentially drain the sump of a machine, overfill the sludge tank or cause substantial damage within the machinery spaces
ii) The oil water interface is too near the bowl centre, not creating an effective water-oil seal and poor solids removal. Water seal may therefore pass out via the oil outlet risking extreme contamination of the oil. Water within oil reduces and inhibits its ability to lubricate efficiently, causing machinery damage; or in the case of fuel oil emulsification of the fuel resulting in poor injection and combustion

Question 2

2. a) State two factors that have an influence on the separating process. (8)
   b) Give two reasons for a purifier not attaining full running speed. (8)

Answer 2

a) The oil density. 
The oil temperature. 
The oil viscosity. 
b) Shaft misalignment. 
Damaged motor.

Question 3

3. With reference to the treatment of lubricating or fuel oil:
   a) State function of a purifier, (4)
   b) State function of a clarifier, (4)
   c) State TWO constructional differences found in the bowls of purifiers and clarifiers. (8)

Answer 3

a) To remove solid particulate and water from the fuel.
b) To remove solid particulate only, slower throughput due to longer dwell time. c) No gravity disc in clarifiers as there’s no oil water interface to maintain, larger discs and bowl size than purifiers for a longer dwell time. A clarifier also has a blind disc and a purifier doesn’t.

Question 4

4. a) Outline the separation process that occurs within centrifugal oil purifiers (12) b) Describe the change required to the purifier if it is to be used for an oil of lower density (4)

Answer 4

a) When a fluid such as oil is subjected to gravity, substance of different densities separates. Before the process beings a small amount of water is needed to prime the machine and
John Willison IAMI Revision 37
prevent the oil carrying over into the water discharge. Centrifuging amplifies the effects of gravity using centrifugal force to make the separation happen much quicker, this is due to the gravitational force being increased. When oil is inside the bowl, the high speed rotation causes impurities such as water to be forced to outside of the bowl leaving the much clearer oil near the centre. This is due to the water and solids being less dense than the oil. Separation is aided by the addition of a disk stack which enlarges the bowls surface area. Within the oil outlet there is a gravity disc which controls the oil water interface during operation which help prevent cross contamination. Periodically the purifier will automatically de-sludge to discharge any solids that are collected and the bowl.
b) For fuel of a lower density, a smaller gravity disc will be needed in order to maintain the oil water interface. If the gravity disc wasn’t changed oil would be lost in the water outlet, which could drain a sump, fill the bilge sludge tank or even cause damage to a machine.

Question 5

5. With reference to a centrifugal fuel oil separator:
   a) Explain the importance of using the correct size of “Gravity Disc”. (12)
   b) State the TWO factors that determine the size of “Gravity Disc” to be used. (4)

Answer 5

a) Using the correct size of gravity disc in a fuel oil purifier is important in several ways. If the disc was too large the oil water interface is displaced too near to the bowl periphery. Resulting in a collapse of the liquid seal, oil may now pass out the water outlet. Escaping oil via the water outlet could potentially overfill the sludge tank or cause substantial damage within the machinery spaces. If the disc was too small the oil water interface is too near the bowl centre, not creating an effective water-oil seal and poor solids removal. Water seal may therefore pass out via the oil outlet risking extreme contamination of the oil. Water within fuel oil causes emulsification of the fuel resulting in poor injection and combustion. It can also cause corrosion and etching; corrosion is the most common cause of fuel injector failure, water in the fuel rapidly speeds the corrosion process resulting in significantly shorter life of a fuel injector. Etching is caused by a water-induced fuel which produces sulphuric acid that eats the metal surfaces.
b) The fuel oil separation temperature.
The fuel oil density.
The feed rate required for the fuel oil.