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Oil is the blood of any type of ship and is an important ingredient of the engine room. When I say oil I mean heavy fuel oil, diesel oil and lubricating oil. Each of these oils has their separate systems and need to be cleaned before they are used for various purposes. For example lube oil is used for cylinder lubrication, while diesel oil is used for diesel marine engines. Centrifuge oil cleaning is the best option available on ships to clean large quantities of oil in an economical manner. Of course centrigues have a use in other places apart from ships as well such as offshore & mobile platforms, and on shore industries. In this article we will learn the basics of this cleaning process.
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Centrifuge Oil Cleaning
Purifiers are used to clean the oil used on ships and there are heavy oil purifiers, diesel oil purifiers and lubricating oil purifiers. These purifiers come in various sizes and makes, but their general operating principles remain the same as well shall see now. Just take a look at the picture below and see what happens when oil and water (plus other impurities) are supplied to a tank which has the typical structure shown in the picture. In this case the oil will flow from one side, while water will come out at the other side and any solid particles would settle at the bottom.
This method is not very practical but it shows the basic principle of oil-impurities separation. Purifiers and clarifiers work on this principle but make use of the centrifuge force created as a result of fast spinning and you can see this in the next picture. The image shows the construction of a typical purifier which consists of number of perforated discs of inverted bowl shape, stacked one over the other (known as disc stack). Dirty oil enters from top and gets separated into clean oil and dirty component which come out of the respective places as seen in the figure. The centripetal force shown in the picture is created by spinning the entire arrangement along the vertical axis with the help of a powerful motor.
The exact process which is going on between two adjacent discs is shown more clearly in the diagram below which shows the various forces acting on the oil and other impurities which makes the separation possible. Hence it can be noted that reducing the rate of flow will improve quality of purified oil but will make the process slower hence an optimum speed should be used.
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The Circuit for Oil Cleaning
The entire circuit for oil cleaning for the main engine oil sump is shown below which shows the various components including a heat exchanger, three way valves, delivery pump and the lube oil line in yellow colour. It can be seen that the oil comes from the sump via the delivery pump, passes through the heater and gets purified and goes back to the main engine sump. Similar circuits exist for cleaning heavy oil and diesel oil as well.
Just in case you would be interested, there is another method to separate oil and water in small quantities using an Oily water seperator.
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Images with permission of Brian Beattie of Marine Engineering Knowledge