LPG Deepwell Pump Steady Bearing Failure

[ME1289]

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.

 

[Artisi]

Have you considered a parallel roller bearing in place of the spherical roller bearing, this will be sufficient as a steady bearing and won't be subject to any thrust load?

 

[ME1289]

Definitely considered it but I'm hesitant to make a change without understanding why all the sudden this bearing doesn't work.

 

[Artisi]

Can understand the hesitation.
Have you had the failed bearing/s looked at by the bearing manufacturer for their analysis.
How about the pump manufacturer, might be worth a discussion.
Why is this lower bearing spherical roller, is it to account for any intermittent up-thrust of start up or shut down, otherwise I can't see any benefit in this bearing type.

 

[ME1289]

Initial visual inspection indicates excessive downward thrust which is the head scratcher because there really shouldn't be any net loading on this bearing. A more detailed analysis is ongoing.

Downward thrust should be handled by the hollow shaft upper bearings. These are preloaded with the weight of the shaft and impellers before the steady bearing is locked to the shaft. I would think any incremental thrust (the 6-10k lbs of hydraulic thrust) should transferred to these upper bearings.

The pump is a 1960ish design so the pump manufacturer doesn't have a good understanding why this specific bearing was originally spec'd. Our internal records show that this bearing has been used since at least the early 90s.

Motor inspection is scheduled to take place in the next few days too. Maybe these is something internal to the motor causing thrust to not be transfer to the upper bearings of that is causing excessive thermal growth?

 

[mfgenggear]

OP
are there sensors in place for vibration , temperature? is there balancing issue?
are there thrust bearings because of friction , and or mass of the motor and pump?

 

[mfgenggear]

also how is the preload on the large nut and the bolts clamping the bearing. is it distorting the bearing in some way.
not location properly or over torqued?

 

[mfgenggear]

the temperature sensor is for the bearing.
to verify there is not unusual high temp.
consult with the manufacture for available options.