We are prestigious manufacturer and supplier of High Precision Balance Drum Bush in Thane, Maharashtra, India. In any centrifugal pump, each impeller tends to produces some amount of thrust because of different pressures and different geometries on the two sides of the impeller. In a high pressure multi-stage pump (such as BFW) the number of impellers is high, thus the net thrust would be large unless something is done to balance it out. The two main ways to reduce the net thrust are to oppose the impellers or to use a balance disk/drum. For axial split pumps, it is usually most economic to oppose the impellers. About half of the stages are oriented with the suction pointing toward the coupling and the rest are oriented with the suction toward the thrust bearing. The thrust of the stages pointed in opposite directions tend to cancel out. The net thrust that the thrust bearing must take is much smaller than it would be if they all pointed in the same direction.
But axial split cases tend to have an upper pressure limit. At very high pressures, barrel pumps are used since they can handle the very high pressures better. With a barrel pump, it is much more difficult to find a good way to direct the flow path through a set of opposed impellers. So instead, they point all the impellers in the same direction and use a balance disk or drum on the end.