Separation

Separation as a means of removing impurities from a fuel can be undertaken by means of gravity in a settling tank or by means of centrifuging the fuel. Both methods work on the same principles that by subjecting the fuel to a constant force, the denser components of the fuel i.e  water and dirt will be separated from the lighter components i.e. the fuel itself.

Gravity acting on the fuel as it passes slowly through the tank will separate the denser components from the fuel where they will accumulate at the bottom of the tank. The contaminants can then be remove by sludging the tank.

Centrifuging

Centrifuging is the process by which the effects of gravity can be amplified by the use of centrifugal force to the extent that the separation process becomes rapid and continuous. Centrifuges work by rapidly spinning a bowl containing the liquid, thus producing the required centrifugal force to produce separation

The principle of operation of the centrifuge is simple. When a bowl containing impure fuel is rotated, centrifugal forces will throw any item with density greater than the fuel oil density (solids and free water) to the periphery of the bowl

Centrifugal separators used for the separation of two liquids of different densities (fuel and water) are known as purifiers and those used for separating solid impurities are known as clarifiers. Purifiers will also remove some solids and clarifiers will also remove small quantities of water.

Clarifier

The addition of an inlet and an outlet connection forms a simple clarifier. Rotational speeds vary according to designs and are of the order of 7,000 to 9,000 rpm. Efficiency is increased by the inclusion of a number of discs (up to 150) that increase the surface area and thus help separation. Discs are separated at a distance of 0.5-0.6 mm. After passing down the central passage, the untreated oil is carried by centrifugal forces towards the periphery of the bowl and then passes up through the disc stack. Here is where the actual separation takes place, in the channel formed between two discs. Two forces act on each solid or liquid particle. The particle is pushed upwards with the oil stream towards the centre while the centrifugal force directs it to the periphery. The residual force on denser particles (impurities) will drive them towards the periphery, while the less dense particles (oil) will be directed towards the centre of the bowl and raise to the outlet connection.

Purifier

When a centrifuge is set up as a purifier, a second outlet pipe is used for discharging water as shown. In the fuel oil purifier, the untreated fuel contains a mixture of oil, solids and water, which the centrifuge separates into three layers. While in operation, a quantity of oil remains in the bowl to form a complete seal around the underside of the top disc and, because of the density difference, confines the oil within the outside diameter of the top disc. As marine fuel oil normally contains a small quantity of water, it is necessary to prime the bowl each time that it is run, otherwise all the oil will pass over the water outlet side to waste. The water outlet is at greater radius than that of the fuel. Within the water outlet there is a gravity disc, which controls the radial position of the fuel water interface

A set of gravity discs is supplied with each machine and the optimum size to be fitted depends on the density of the untreated oil. When the fuel centrifuge is operating, particulate matter will accumulate on the walls of the bowl. If the centrifuge is set as a clarifier, the particulate matter will be a combination of water and solid material. If it is set as a purifier, the free water is continuously discharged, therefore, the particulate matter will consist of solid material. In older machines it is necessary to stop the centrifuge to manually clean the bowl and disc stack, however, the majority of machines today can discharge the bowl contents while the centrifuge is running.