A pump rotor is subjected to axial thrust due to the pressure of the liquid acting axially on the radial surface area of the rotor elements - its impeller, wear rings, shaft sleeves, bushings, etc.
The axial thrust is directly proportional to the elements' radial surface areas and the pressure acting on those areas. The force acting on one side of the rotor elements are usually different in magnitude from the force acting on the opposite side. The summation of the opposing forces results in a net thrust imbalance on the rotor.
In a single stage unit the thrust imbalance is usually small enough that the thrust bearing is more than sufficient to handle the thrust load. In single stage overhang unit operating under very high suction pressure the thrust imbalance can be very high and should be reduced in magnitude by changing an area subjected to pressure such as by changing an impeller wear ring diameter (usually on the back side of a single suction impeller), or by reducing the shaft diameter under the seal to reduce the mechanical seal size.
In a multi-stage unit where the thrust imbalance can be very high due to the high differential pressure, reducing the thrust imbalance may require altering either (1) a pressure area, or (2) the amount of pressure acting on that area, or (3) both.
Item (1) can be done by changing the diameter of a balance disk specifically designed for this purpose. The disk can be a shaft sleeve located adjacent to the high pressure stuffing box, or at the center of the rotor, or both.
Item (2) can be done by providing a flow bleed-off line to reduce the pressure acting on a rotor element located near the high pressure stuffing box.