When a centrifugal pump operates below a certain flow point, flow recirculation in the impeller eye begins. This depends on several design factors, such as suction specific speed, but generally recirculation begins at less than 80 to 60 percent flow. It becomes quite severe at less than 40 to 20 percent. At even lower flows, recirculation may become especially severe and is known as surge-violent, low-frequency sound, accompanied by strong low-frequency vibration of the pump and piping (see Figure 4).
Figure 4. Problems arise when a pump operates at flows that are too low.
In addition to obvious mechanical problems with recirculation, the flow experiences a complex vortexing motion at the impeller eye with localized high velocities of the vortex causing horseshoe-looking cavitation damage, usually on the blind side of the blade, as compared to high-flow cavitation.
Troubleshooting methods and failure analysis techniques can help pinpoint a cavitation problem with a particular pump. The indications of high-flow cavitation are different from low-flow recirculation damage. The side of the blades and the extent and shape of the cavitation trough can be helpful in determining the causes of each problem.
At the next Pump School, I will cover specific examples that compare static-head-dominated, friction and combined systems. To register, visit www.pumpingmachinery.com.