A classic case is when the pump is filling the lines from empty. The pump will start and ramp to minimum frequency. The line fill function takes over and 'manages' the pump from there. Unfortunately if the pump is operating at too lower pressure (system pressure too low) the pump will upthrust and damage the motor thrust bearing and pump components. I have attached a pump curve showing the hydraulic limits (of that particular model). If the pump operates any where below the shaded area, it is at risk of upthrusting. If this is a multi-stage pump, then up-thrust can be a serious problem. The VFD will (I assume) slow the pump down to some minimum speed. But, at any speed, a multi-stage vertical turbine pump can produce up-thrust if it is operated at high flow (low head). Perhaps in most applications, this up-thrust is tolerable and will not result in failure. The pump manufacturer may be able to provide a thrust curve at the VFD minimum speed. We typically have these curves for our deep well water pumps. At very high flow, the thrust goes negative and can result in buckled shafts or thrust bearing failures. At lower speed, the up-thrust will be less than it would be at higher speed. So, determine the maximum up-thrust for the given pump at the minimum speed and compare this to the thrust load capability of the inactive thrust bearing in the motor. If the shaft is long and flexible enough to buckle, then the thrust bearing might not be the weak link. But your post describes a thrust bearing failure, so I assume that shaft buckling is not possible.

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