The density of a fuel tested at 15°C may approach, be equal to or greater than that of water. With high density fuels, the reduction in density differential between fuel and water can cause a problem with separation but not with the usual solid impurities. Heating of the fuel (Figure2.19) will reduce the density and this may be sufficient in itself to obviate the problem of water separation.
The Alfa-Laval design of centrifuge intended for dealing with high density residual fuels, is a self-sludging machine which has a flow control disc that makes it virtually a clarifier. There are no gravity discs to be changed to make the machine suitable for fuels of different specific gravity/density.
Heating is used to reduce the density (and viscosity) of the fuel so that water and sludge accumulate in the outer part of the bowl, as the result of the centrifugal effect. As the interface moves inwards, but before reaching the disc stack, water droplets flow through to reach a water sensing transducer. Via micro-processor circuitry, the transducer causes the bowl to self-sludge or the water to be discharged through the water drain valve. The system is said to be capable of handling fuels with densities as high as 1010 kg/m3 at 15°C