The radial force is an important parameter, even if it can be one magnitude lower than the axial force, as it affects the reliability of the pump. For example, it governs the bending of the shaft that strongly affects the performance of the seals in the pump and the internal strains of the rolling bearings. Physically, the radial loads are coming from the pressure build-up between the impeller and the single volute housing.
A pump string test, with the now commercially-available FOS ACBBs, performed in close collaboration with a major pump OEM, demonstrated this effect for magnitudes for axial and radial forces and radial force angle (see graph below). It could be seen that the angle of the radial force changes in direction with a 180-degree switch close to the best efficiency point (BEP) of the pump, as predicted. The behaviour is due to the location of the main pressure change over the “cutwater position” in the single volute design. At the same time, the magnitude of the radial force sees a minimum just before the BEP, also in-line with the theories for this type of pump. Unwanted forces from mishandling e.g. from a closed valve (deadheading), wrong settings (cavitation) etc., can potentially be detected too.