b.Explain how the centrifuge described in is able to remove water from a fuel which has a density that is higher than that of water and state any factors that may assist the operation.
As Chief Engineer, write out the start up procedure for the centrifuge described in for the benefit of your staff.
State how the problem of catalytic fines in fuel oil may be dealt with.
a.
As the density of the oil approaches that of water (above 991kg/m3 ) the hydraulic equilibrium in the bowl becomes unstable, and a gravity disc will no longer maintain a water seal.
To overcome this problem, Alpha Laval has developed the Alcap separator, the principle of which is illustrated.
Oil is fed into the high speed rotating bowl which basically operates as a clarifier, but water and solids are separated and are thrown to the outside of the bowl by centrifugal force.
At regular intervals a sludge cycle will take place. Water is admitted into the bowl to soften the sludge and displace the oil in the bowl. When a transducer in the oil discharge line detects water the bowl is opened and the sludge and water discharged. The bowl opens and closes very rapidly and oil loss is minimal.
If the fuel contains water it will build up in the bowl and start to be discharged with the clean oil. The transducer in the discharge line will detect this and if this occurs after the minimum sludge cycle time, a sludge cycle will be initiated; if the water is detected before the minimum sludge cycle time, then water discharge valve will open.
b.
Density of fuel is quoted at 15ºC. This density will fall as the fuel is heated up. The density of water, although it does change a very small amount, stays fairly constant at 1000kg/m3 . By heating the oil to as high a temperature as possible (90 - 98ºC) the relative density is reduced to below that of water, allowing the water to be thrown to the outside of the bowl.
Other factors that assist separation are:
Viscosity of the oil: The lower the viscosity the lower the drag force on sludge particles. Viscosity of a fuel is reduced by heating.
Throughput: Should be as slow as possible to maintain fuel demand.
Interface: If the oil/water interface is within the disc stack separator efficiency is reduced as oil cannot flow along the full surface of the disc.
c.
Ensure purifier has been fully assembled, that the bowl cover locking dogs are in position, and the brake is off.
The purifier pump suction should be closed, the pump discharge valve open to the heaters, and the recirc valve open back to the purifier suction. The discharge from the purifier should be open to the settling tank.
The feed regulating valve to the purifier should be set to zero.
Start the purifier watching the ammeter which should fallback as the purifier speed increases. Ensure the purifier does not vibrate. In case of vibration or excessive current shut down immediately and investigate.
Open the fuel suction from the settling tank to the purifier pump.
Warm through and open heating steam to and from heaters. Ensure heater control set to 95ºC.
Continue to recirc fuel until purification temperature is reached.
Open operating water and set bowl operating water to close.
Open feed regulator to purifier and shut recirc valve. Set purifier oil feed to minimum.
Ensure sludge cycle is set to x hours.
Change discharge from settling to service tank, and adjust feed rate to match engine consumption.
d.
Catalytic fines comprise of small particles of silicon and aluminium carried over from the refining process. They cause abrasive wear in fuel pumps, injectors, liners and piston rings.
They are difficult to remove because they are often hollow which gives them a relative density close to that of the fuel. Experience has shown that by keeping the total contamination below 80mg/kg, efficient centrifuging could keep the fines down below a level when abrasive wear takes place. This means keeping the purification temp as high as possible and the throughput as low as possible.
