Pump Drive Shaft Binding

[cbabz10]

I currently have an issue with a centrifugal sump pump with a ~13 ft vertical drive shaft. The pump is used to move water and plastic pellets (3mm in D) up approximately 40 ft into a filtering system. The past repair/replacement of this pump has been every 8 months. The problem generally is that the pellets get into the drive shaft and cause it to bind(high amps) and trip out. The last time this pump was taken out of service there was over 5 ft of pellets along the shaft. Water is currently used to cool the bearings but these inlets have filters, the last repair we made we ensured the filters were all sealed. I cannot see how else the pellets would enter the system. And I cannot see them entering past the seals etc. Any ideas?

Thanks!