Upward thrust on vertical pump 1

[eyec]

what would cause this? Motor thrust upward taking out the bearing. Would assume if anything it would thrust in the downward direction.

TIA

 

[Artisi]

what type of vertical pump are you talking about and what is the configuration of pump/driver? this make a big difference to the discussion.

 

[eyec]

4650gpm 1780rpm
6stage
4160v motor
1500hp
rigid coupling

correction to previous post should have read motor shaft thrust upward.

 

[lowedogg]

it sounds like you are talking about a vertical turbine pump, like what goes in a sump pit.

are you talking about the motor bearings or the pump seal?

is the pump/system meeting the listed performance (ie flow and head)?

 

[BigInch]

Running at off BEP flows can cause shaft thrust imbalances.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[cb92]

It takes a lot of force to lift a 1500 hp motor. Are all the turbines lifting on the pump, or are they balanced for no axial thrust? Maybe a sudden water hammer affected the pump. Possibly an expansion happened, and up was the only way to go.

 

[eyec]

yes, this is a pit type pump.
took out the top bearing but not the bottom.
shaft show signs of scoring/gouging on only one side.

cavatation???

 

[Artisi]

" yes, this is a pit type pump.
took out the top bearing but not the bottom.
shaft show signs of scoring/gouging on only one side.

cavatation??? "

What bearing are you talking about, pump line shaft bearings, thrust assembly bearings (assuming it has a thrust assembly) or motor bearings.

You would be better served to write a clear description of what is going on, describe the pump configuration and a clear description of the failure, number of hours run etc .

Crystal ball gazing is a dying art!

 

[eyec]

motor upper guide bearing was wiped out
thrust bearing was smeared

sorry for short answers info is slow and confusing as it comes in

 

[JJPellin]

This is a common problem with vertical turbine pumps. It is more common with pumps that have a large number of stages. In order to allow for the downward thrust when running at normal flow rates, the manufacturers tend to add back wear rings and balance holes through the impeller. But, if the downward thrust is reduced when running at normal rates, it can easily reverse when running at very high flow rates. We see this with propane re-run pumps that have 25 stages. If they start the pump up into an empty line and run it beyond end of curve, the up-thrust will buckle the shaft and destroy the seal. For a six stage pump, I am surprised that the up-thrust is a problem. But, if the motor does not have any true thrust bearing for up-thrust, then it only takes enough to overcome the weight of the rotor. There was a very good article on up-thrust in this type of pump presented at the International Pump Users Symposium in 2005 or 2006. I don’t have a copy of the article handy.

Johnny Pellin

 

[Artisi]

Still not clear - is the pump thrust accommodated in a separate thrust assembly or is the motor carrying pump thrust and rotor load.

 

[chief]

If the pump rotation is wrong, you will still get a delivery pressure , but the upthrust will be excessive and wipe out your bearing. Indication of wrong motor rotation will be high motor current and low flowrate.

Offshore Engineering&Design

 

[eyec]

sorry for the dalay but turns out this was not a pump problem. after investigation of all mechanical possibilities we re-visited the electrical side of the equation.

long story short, stray voltage thru the lower motor bearing was found.

thanks to all for their responses.

 

[JJPellin]

Artisi,

He described this as a rigid coupling. This would suggest to me a pump with no thrust bearing. In fact, the pump would have no true bearings at all, only line shaft bushings. All thrust loads would be taken by the upper motor bearing, typically. We use this configuration commonly on deep well pumps and multi-stage vertical turbine pumps (sometimes called "can" pumps). I am still not convinced that the stay voltage problem is the root. The description of the shaft scored deeply on one side and the failure of the upper motor guide bearing sounds like up-thrust more than stray current. There may be more than one problem.


Johnny Pellin

 

[Artisi]

JJPelin,

1500 hp verticals?

 

[eyec]

jjpellin,

you last description is correct on the pump configuration
that is why i initially thought of cavitation

you may also be correct as to more than one event causing the failure of the motor but we have not found a mechanical cause. i have looked @ flow rates during the failure and am getting flow rates from prior "normal" operation to compare

last step is to pull the pump and look at it

thanks

 

[JJPellin]

Artisi,

I believe the largest pumps of this configuration in our plant are 1000 HP. But a 1500 HP example would not be beyond reason. I have seen papers written about pumps of this type up to 5000 HP and larger.


Johnny Pellin

 

[Artisi]

At least we now have confirmation that the motor bearings are carrying the pump thrust - up and down. Next it would be of interest to know if this is a new installation or an old unit with a new problem.

 

[eyec]

old unit with new problem
this is a B pump and the A pump has not had this problem
repair history for this pump included a water incursion (leak) and an oil leak

 

[Artisi]

Has the operating point changed ie, more or less head,has the start-up /shut-down proceedure changed? - although this seems unlikely seeing as how pump B doesn't appear to have a problem but worth checking.

Are the units on a common or separate discharge line/s?