How should the forces acting on the shaft of a centrifugal pump be analyzed when it operates at below the minimum continuous flow rate or in a vacuum condition?
Thread Content
When an RT horizontal centrifugal pump operates with empty suction or at a very low flow rate, its vibration increases and the bearing housing heats up. So, how do the stress conditions on the shaft and bearings (axial forces and radial forces) change as the pump moves from normal operation to a state of empty suction?
During normal operation of a horizontal centrifugal pump, when the flow rate remains stable, the axial force on the rotor is also relatively stable. Due to the difference in pressure before and after the impeller, the pressure at the inlet is low; as a result, the axial force acts in the direction of the inlet. The positioning bearings are used to counteract this axial force. Both the axial and radial forces remain within normal and stable ranges, and the vibration of the bearings is at its minimum level； When the pump is under vacuum or has a low flow rate, the fluid inside the impeller fails to fill the impeller’s flow channels, resulting in an uneven mass distribution within the impeller. This leads to loss of dynamic balance and increased vibration. Due to the vacuum condition, the pressure difference across the impeller may decrease; although the axial force also decreases, it becomes unstable. The radial force, on the other hand, increases and remains unstable as a result of the vibrations of the impeller. A simple analysis, for reference only.