Abstract: Mixed oil quantity should be reduced so as to reduce the cost of product pipeline and investment of slop tanks. The main method of reducing mixed oil quantity is to utilize seperating spheres. A seperating sphere is easy to show wear due to the friction on the internal pipe wall, the following parameters should change according to the length of the pipeline: the diameter of the sphere, the size of the contact rings between the pipe wall and the sphere. A differential pressure usually occurs due to the friction force between the pipe wall and the sphere. Acted on by the condition of the differential pressure, influenced by vescosity and pits (in the internal pipe wall caused by corrosion), seepage oil (before/after the sphere) from between the sphere and the pipe wall forms the mixed oil, the differential value causes the sphere and the liquid flow at different speeds, thus enlarging the quantity of mixed oil. It is hard for the seperating sphere we have been using to reach the satisfactory seperating effect. The new type seperating unit is designed to have a simple structure for regulating the width of contact rings, which basically keep one set value. Besides, a negative structure compensating seepage oil before the sphere is also designed, which makes the seepage ahead equal to what follows afterwards. If the two measures mentioned above are taken, the quantity of seepage oil ahead can be reduced to a minimum. the sphere can be made to travel as identically as possible with the average liquid speed thus reducing the mixed oil quantity to a minimum.