ME1289
Mechanical
- Jul 7, 2020
- 20
Hey guys,
Looking to see if anyone has any input on a head scratcher pump problem.
Background:
We're working on a deepwell LPG cavern pump. The pump itself is basically a 13 stage vertical turbine pump with a 500 ft shaft. We have bronze bushings every 10 ft and the shaft is inside an isolated enclosing tube. The tube is fed oil by an external system. The motor is a very old but reliable GE vertical hollow shaft motor. It's a standard hollow shaft design with 3 large thrust bearings on the top designed to handle the weight of the shaft + impellers and the downward hydraulic thrust of the pump. Estimated weight of the shaft + impellers is 6k lbs and hydraulic thrust is anywhere from 6 - 9k lbs. The motor also has a radial lower bearing and internal bronze "whip" bushing that restricts radial movement of the pump drive shaft once inserted into the hollow shaft.
The pump has failed 3 times in the last 4 years when it hadn't failed once in the previous 20+ years. We have an identical sister pump on the same cavern that is in the middle of a 20+ year run. We believe we've identified the root cause of the previous failures and hope to have that behind us.
Current issue:
We're trying to return the pump to service after it's most recent rebuild. The pump discharge head has a "steady bearing" located between the hollow shaft motor and discharge head. It is a fixed (not expansion) spherical roller bearing (Rexnord FCB22431H to be exact). Every time we start the pump, this bearing runs fine at first, but after a couple hours, the temperature takes off. Failure analysis indicates that the bearing is overloaded with downward thrust. However, we're not sure where this thrust is coming from. We've tried several new bearings so it's unlikely to be a factory defect issue.
When the pump and motor are assembled, we pull lift on the pump with the steady bearing loose on the shaft. In effect, this puts ~6000lbs of preload on the main thrust bearings on top of the hollow shaft motor. The final step after lift is pulled is to tighten the set screws on the steady bearing to mate it to the shaft. At that point, we figure the main thrust bearings are already preloaded and have zero axial clearance. Any incremental thrust (ie hydraulic thrust after startup) will be handled by the main motor thrust bearings. Even through the steady rest bearing is "fixed" is has some small axial float. Obviously, something isn't working right because this setup, as designed, has run previously.
We haven't inspected the motor since our last failure so that is next on our to-do list but it seems to run fine in the few hours before the steady bearing overheats.
I am suspicious of a thermal growth issue due to the fact that the bearing temperatures takes off after an hour. Not sure where/why the thermal growth would be any different than in the past.
Anyone have thoughts on what could be loading this steady bearing? I realize this bearing seems like it should be expansion rather than fixed but I don't want to introduce another variable because I know it's capable of working based on past performance.
Attached is a very basic sketch of our setup. The motor looks a little weird but it's supposed to be a standard hollow shaft motor.
I'm sure I'm missing info so I'll gladly answer any questions.
Looking to see if anyone has any input on a head scratcher pump problem.
Background:
We're working on a deepwell LPG cavern pump. The pump itself is basically a 13 stage vertical turbine pump with a 500 ft shaft. We have bronze bushings every 10 ft and the shaft is inside an isolated enclosing tube. The tube is fed oil by an external system. The motor is a very old but reliable GE vertical hollow shaft motor. It's a standard hollow shaft design with 3 large thrust bearings on the top designed to handle the weight of the shaft + impellers and the downward hydraulic thrust of the pump. Estimated weight of the shaft + impellers is 6k lbs and hydraulic thrust is anywhere from 6 - 9k lbs. The motor also has a radial lower bearing and internal bronze "whip" bushing that restricts radial movement of the pump drive shaft once inserted into the hollow shaft.
The pump has failed 3 times in the last 4 years when it hadn't failed once in the previous 20+ years. We have an identical sister pump on the same cavern that is in the middle of a 20+ year run. We believe we've identified the root cause of the previous failures and hope to have that behind us.
Current issue:
We're trying to return the pump to service after it's most recent rebuild. The pump discharge head has a "steady bearing" located between the hollow shaft motor and discharge head. It is a fixed (not expansion) spherical roller bearing (Rexnord FCB22431H to be exact). Every time we start the pump, this bearing runs fine at first, but after a couple hours, the temperature takes off. Failure analysis indicates that the bearing is overloaded with downward thrust. However, we're not sure where this thrust is coming from. We've tried several new bearings so it's unlikely to be a factory defect issue.
When the pump and motor are assembled, we pull lift on the pump with the steady bearing loose on the shaft. In effect, this puts ~6000lbs of preload on the main thrust bearings on top of the hollow shaft motor. The final step after lift is pulled is to tighten the set screws on the steady bearing to mate it to the shaft. At that point, we figure the main thrust bearings are already preloaded and have zero axial clearance. Any incremental thrust (ie hydraulic thrust after startup) will be handled by the main motor thrust bearings. Even through the steady rest bearing is "fixed" is has some small axial float. Obviously, something isn't working right because this setup, as designed, has run previously.
We haven't inspected the motor since our last failure so that is next on our to-do list but it seems to run fine in the few hours before the steady bearing overheats.
I am suspicious of a thermal growth issue due to the fact that the bearing temperatures takes off after an hour. Not sure where/why the thermal growth would be any different than in the past.
Anyone have thoughts on what could be loading this steady bearing? I realize this bearing seems like it should be expansion rather than fixed but I don't want to introduce another variable because I know it's capable of working based on past performance.
Attached is a very basic sketch of our setup. The motor looks a little weird but it's supposed to be a standard hollow shaft motor.
I'm sure I'm missing info so I'll gladly answer any questions.
