2. Centrifugal Separators

If we first consider gravity separation which occurs in a settling tank, solids such as sludge, dirt, etc. will settle out at the bottom of the tank. The heavy liquids, such as water, will settle out above the solids and the lighter liquids, such as oil, will be at the top of the tank. Separation occurs due to the different specific gravities or densities of the liquids and solids. The application of heat will speed up the separation process.

The centrifugal separator consists of a bowl containing a set of discs stacked one on top of the other. The dirty oil flows down the distributor and up through the holes in the discs. This is shown diagrammatically in Figure 1.

It is here that the actual separation takes place in the channel formed between two discs. In this channel the velocity of the oil is greater at the centre and zero at the disc surface. Each particle, solid or liquid, is acted on by two forces (the third, gravitational force, has little effect). The particle is forced upwards with the oil stream towards the centre due to its velocity, while the centrifugal force directs it to the periphery. The particle is considered to have separated out on reaching the disk stack surface, then it is forced and deposited at the bowl periphery.

Separators can be operated as either a purifier or clarifier. The former separates water and impurities from the fuel; the latter separates impurities from the fuel.

In the purifier mode it is important that the oil/water interface is in the correct position to allow efficient separation. This is controlled by means of a gravity disc or a regulating ring. Separator manufacturers provide nomograms to determine the correct gravity disc, based upon oil density and separating temperature. The purifier comes supplied with a number of gravity discs with different inside diameters. The optimum position of the interface is between the edge of the disk stack and the outside edge of the top disc. This can be checked visually by looking at the underside of the top disc. (See Figure 2)

In the clarifier mode the gravity disc is replaced by a clarifier disc with a smaller inside diameter. A clarifier only separates sludge and consequently no water seal is required.

Factors Affecting Separator Efficiency

The generally accepted density limit of the fuel is 991kg/m³ at 15°C. If the fuel density is greater than the value, the density difference between the fuel oil and fresh water will be so small that the hydraulic equilibrium in the bowl will become instable. This means that any small change in oil temperature, fuel rate or viscosity (the lower the viscosity the lower the drag force on dirt particles) will cause the oil/water interface to fluctuate, leading to a potential failure of the water seal, that results in impaired separation efficiency.

However, some companies have developed alternative methods for treating high density fuels. One such arrangement is the Unitrol system by Westfalia. The centrifuge with the Unitrol system is provided with two basic monitoring systems for sludge space or water content monitoring. Another system for treating fuels up to a density of 1010kg/m³ at 15^oC is the Alcap system where the centrifuge basically operates as a clarifier. In the Alcap system changes in the water content are sensed in the clean oil outlet by the water transducer.