Maritime transport utilizes the lowest grade of fuel oil to run its propelling plant. This is one of the major reasons for it being the cheapest mode for transporting goods across the globe. A mid-sized container ship consumes about 200 to 400 metric tonne of fuel on a daily basis depending upon the operation, speed, and engine efficiency.
Before using low-grade oil such as Heavy Fuel Oil for combustion within the engine it’s treated on the ship at numerous levels:-
- First, the fuel oil is internally transferred from bunker to settling tanks. Within the settling tank, it is heated up to 50 to 60 degrees Celcius to separate it from water, which is drained off from the bottom.
- The oil in the settling tank is drawn out by the feed pump which delivers the fuel to a thermostatically-controlled heater to raise the fuel temperature to about 80 to 90 degrees celsius before supplying it to the centrifugal separator.
- Then the oil is further treated in the purifier in which the water and solid particles are separated. The purified oil is then transferred into the service tank from wherever the oil is directly fed to the marine engines and the ship’s boiler.
- The centrifuge separator is an essential machinery in the Engine room used for purification of fuel oil and for efficient combustion. It is a bowl carrying the dirty fuel which is rotated at high speed using external force. The external force is generated by the electrical motor driving the bowl shaft either by gear or using a belt drive. The Centrifugal force developed by the motor exerts a force on all particles in the fuel. The heavy particles like solids and water are thrown to the periphery of the bowl from where they’re separated.
Working Principle of Centrifuge Separator
The separation in the purifier happens based on the principle of Difference in the specific gravity of various liquids and solids. If 2 different grades of liquid are mixed together in a tank and are left for some time, the denser fluid and solid particles will go down and the lighter fluid will float on the surface. Suppose there’s oil mixed with water and solid particles kept in a tank, as per the gravity separation the solids will settle in the bottom. Water being heavier than oil, will settle above the solids. The oil which is the lighter liquid will settle on the top.
Now if we rotate the whole suspension, then due to centrifugal force, the solid particles being heavier will experience maximum force and will gather around the periphery and the oil being lighter will experience the least force and shall remain in the center.
Now, if we add angled baffle plates to the tank, the underside of every parallel plate provides an additional surface for suspended oil droplets to coalesce into larger globules and come up. The solid particles will also slide down the surface of each parallel plate, this allows the denser liquid and solids to settle at the bottom while the lighter fluid rises up to separation.
The same concept is applied in a centrifugal separator, once the dirty oil enters the separator, it passes through plates and tends to be pushed upwards by the fluid flow.
Fluid moving between the 2 plates has a velocity greatest at the midpoint and minimum when approaching the plates. This is because the centrifugal force will retard the horizontal component of the movement for the heavier particles causing them to approach the underside of the top disc reducing its flow velocity. The lighter fluid rises because of the push of dirty liquid flow coming into the separator. For heavier fluids and solids the centrifugal force eventually overcomes the upward force acting on the particles due to fluid movement and that they start to move towards the outer rim and get collected near the sludge port. In this way, it can be seen that reducing the flow rate to a separator will tend to increase the quality of the output.