Principle of threephase decanter centrifuges is as shown in the above figure. The substance to be separated is fed by the feeding tube into the accelerating cavity of the spiral conveyor, then enters high speed running drum wall. As the light liquid, heavy liquid and insoluble solid contained in the substance have different specific  gravities,  the three materials are subject to varied centrifugal forces: the insoluble solid settles into the inner wall (outside) due to its highest specific gravity and subject to maximum centrifugal force; the light liquid settles maximum far away from the inner wall (outside) due to its lowest specific gravity and subject to minimum centrifugal force; and heavy liquid settles in the middle place. The insoluble solid is discharged from the solid discharge port by a spiral feeder which has relative differential speed with the drum; and the light and heavy liquids are then separated by different structures inside the machine wherein the heavy liquid is discharged by the centripetal  pump, and the light liquid is discharged via the force of gravity, thus the purpose of three phase separatin is achieved.