Thrust bearing failure
Thrust bearing failure
(OP)
I recently came across a submersible pump where the thrust bearing had failed and decided to investigate the cause.
The response was below.
By far the majority of installations that I have seen rely on the VFD controlling the flow to fill the line with no artificial head applied while the line is empty and this does not, to my knowledge, seem to have been a problem in the past.
Any thoughts?
Best regards,
The response was below.
While this description does make sense, I have never seen/heard the warning being given.Quote:
This issue is driven by experts in drives, who have no understanding of hydraulics and pumps. With a bore hole pump it is critical to keep the pump operating within it's hydraulic limits. Just using the pipe line fill function does not necessarily do this. Each installation needs to be checked to ensure that there is sufficient system pressure to ensure that the pump is operating within the hydraulic envelope that the manufacturer advises, at all times. Typically we see situations where the drive sales person convinces the installer/owner that by utilising the line fill function they can do away with other safety devices such as pressure sustaining valves.
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 the system pressure is too low during the pipe fill function we recommend that a pressure sustaining valve be fitted at the bore head.
By far the majority of installations that I have seen rely on the VFD controlling the flow to fill the line with no artificial head applied while the line is empty and this does not, to my knowledge, seem to have been a problem in the past.
Any thoughts?
Best regards,
Mark Empson
L M Photonics Ltd





RE: Thrust bearing failure
Constant speed centrifugal pumps should ideally be run between -20% and +10% of the BEP flowrate to avert potential bearing failures. VFD operated pumps should attempt to duplicate those conditions for the corresponding flow and head outputs they are generating at any given time.
The VFD operated corresponding limits would thus be,
maintaining flow withinb -20 to + 10% of Qvfd = Qbep / RPMbep * RPMvfd
while head is held around,
Hvfd = Hbep / RPM2bep * RPM2vfd
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RE: Thrust bearing failure
Johnny Pellin
RE: Thrust bearing failure
RE: Thrust bearing failure
The motor manufacturer should advise a minimum thrust required to keep the thrust bearing from skidding (assume this is your mode of failure). Bigger the bearing (higher thrust capacity) the higher value for miminum thrust.
And of course, reduced speed will reduce the head ^2, which has a pretty direct effect on the pump thrust.
You might do better with higher speed during fill. Even if it shifts some of the abuse from the motor to the pump.
RE: Thrust bearing failure
We had a 22-stage vertical pump launch the thrust bearing housing after running in a low/no-flow condition for about six hours. This caused the pump to up-thrust and the bearings began to skid. Eventually the balls locked completely and the shaft actually snapped in a ductile mode due to the heat generated where the bearings had seized.