When pumps operate away from their BEP, the pumps will generate uneven pressure forces inside the volute casing causing a radial thrust load on the impeller. The radial thrust load has many detrimental effects on the life of the pump. The first and most obvious is a loading on the bearings. Increased radial loads cause higher bearing reaction forces decreasing the life of the bearing. The force applied to the impeller increases as we move further and further away from the best efficiency point. Another problem caused from radial loading is the slight bending of the pump shaft, causing the seal faces to wobble back in fourth to stay within close proximity to each other. This causes mechanical damage to the seal due to the oscillating loads, but also allows debris to enter the seal faces. This leads to scoring of the seal faces and/or deposits becoming lodged in the faces that hold them open an cause a leak path. Additionally contact between the impeller and wear ring surfaces adds to the efficiency losses in the pump and additional vibration adding to bearing and seal failures. Below is a diagram the shows the variation of radial load with flowrate for several casing styles. Most submersible designs use the single volute style casing. As you cans see the loading is at a minimum near the BEP but as we move to higher or lower flowrates the loading increases substantially.
Axial thrust is a very important part of pump reliability. The very large surfaces of the impeller allow pressure to build up on the shrouds of the impeller causing very high loading of the bearing in the axial direction. Different styles of impellers have very different loading characteristics. The open impeller with no shrouds, has very low axial thrust loading because there are no shrouds for the differential pressure to build upon. Semi open impellers actually have the worst axial thrust characteristics because they have one shroud that allows the discharge pressure to build up across the whole shroud surface whereas on the other side of the shroud the pressure goes from the section suction pressure and increases to the discharge pressure as you move out radially. This differential pressure can often be in the thousands of pounds which would be directly applied to the bearings. Closed impellers with two shrouds make it much easier to balance the axial thrust but still require some form of a thrust balancing device. Thrust balancing devices include vanes on the back shroud of the impeller. Often called back pump out vanes, these vanes pump the liquid from behind the impeller towards the outside diameter. This lowers the pressure near the shaft and increases the pressure towards the discharge diameter mimicking the thrust profile on the front of the impeller. This is often used on semi-open impeller designs. Another popular form of thrust balancing device is to drill balance holes through the rear shroud of the impeller allowing the high pressure at the back of the impeller to bleed through the balance holes back into the suction. Most forms of thrust balancing cause a loss in efficiency, ultimately costing more money to operate the pump. They are necessary, however, for the mechanical integrity of the pump and the reliability of the unit, so we have to live with them. It is imperative when pumps are rebuilt that the balance holes and back pump out mains are checked to make sure that they are clean clear and functioning as intended.