We have had a couple of failures where the impeller would get in the volute on start up and the shafts move in the locknuts. Flow is slow to get good suction through the inlet and we never replace the impellers or accuratly set the clearance at installation. Laser aligned and monitored on vibration routes monthly with no signs of any problems except consistantly high vibration at van pass frequencies .35-.4 in/sec. My question is how much axial vibration can be caused by pump running at below the BEP and can increased impeller clearance increase vanpass hpdraulic forces?

quote:

My question is how much axial vibration can be caused by pump running at below the BEP and can increased impeller clearance increase vanpass hpdraulic forces?

This is a good question.
But my questions would be:
Why is the clearances never set?
Why is the flow slow on startup?
How can the shaft move in the lobknuts?
Is the impellar to large for the application?
Why are they "failures" if the impellars are never changed?

What type pumps are these?
What is the bearing configuration?

Of course this doesn't even come close to answering the question you asked: "My question is how much axial vibration can be caused by pump running at below the BEP and can increased impeller clearance increase vanpass hpdraulic forces?"

Sorry.

Clearance set a rebuilds @.040 from back of impeller to endplate. Never knew or realized this should be done until this FA has started. During failures time always takes president over precision and on 7-24 schedule qualified technicians are rare and production is at a halt until the machine runs.

Operators pull 410f oil out of a large fryer to cool the machine and oil is put in a hold tank and on start up the pump pulls from the tank until it fills a large HEX then the fryer then a large trough at the end witch flows through a drum (turning wire mesh) filter and back to the pump for a continuous loop.

Axial forces forces moved the shaft which was .0002 undersized and broke the lockring then the impeller got into the volute. Bearings fell off the shaft instead of having to be pressed. Some small amount of rust was visible as the operators installed a vent plug witch it did not need according to manuf. New impellers are coming and training on suction inlet cone to impeller checks are to be incorporated to a extensive Maintenance standard forthcoming.

(G) 3180 2 bearings at inboard one at outboard 200hp 1780 rpm motor. We set motor .011 high for thermal expansion. Use synthetic 051 oil in bearing casing which is not cooled.

These failures always happen at start up conditions and not being a expert in this part of the operation I am wondering what might be acting forces there that are not there during normal operation.

Thanks for any replays and just to let all of you know I have been a lurker for many years and learned more here than all those manuals I never had time to read and still do the hands on stuff also.

Regards
Steve

Steve
From the way I see it, the cause of this problem needs to be identified first. It is either mechanical or hydraulic. By mechanical I will be referring to the machine itself and its associated components, ie bearings, clearances between moving and stationary parts, etc, and by hydraulic I mean the system in which the machine is working. This should include the suction and discharge conditions as well as whether there is another machine that runs parallel to the one that gives you problems. While working on the bearings and clearances, it will also be a good thing to just have a look at your suction conditions and see whether everything is alright. You say this only happens during start up and, according to my understanding, the suction conditions have a strong bearing. Can you please provide info on type of strainers, suction valves as well as suction piping.
Cheers.

Steve,
I made attached spreadsheet based on HI calculations. You can plug your data and see how much axial forces change.

Centrifugal_Pump_Axial_Forces.xls (419 Kb, 26 downloads)