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Sr. Member / Supporter
Posts: 599
Reply with quote  #31 
This problem sounds so familiar... I wonder if its my company [confused]

Its only the 1x that is in question, so that really narrows down what it could be, though that is generally the case with these VS pumps where the main hurdle is taking the rotor weight and avoiding resonance. You spend a lot of time talking about the alignment, so I assume it has something to do with that. The VS line shafts are balanced only in segments, never as a full rotor because they are so long and flexible, its impossible to balance as a single piece. We therefore always plan for a large amount of unbalance.

The line shaft bearings in the column also often have large clearances and they can be susceptible to instability. This is actually made BETTER by introducing some slight misalignment between consecutive line shaft bearings. When it comes to VS pumps, having a perfectly aligned machine means there is no radial preload and therefore high chance of rotor instability.

I imagine if the structural natural frequencies didn't change, 3 weeks would be too short to send parts to get balanced... They probably introduced some misalignment in the LSBs or changed the clearances?
Danny Harvey

Sr. Member / Moderator / Supporter
Posts: 1,403
Reply with quote  #32 

Unfortunately, it's not only the 1 x that's in question.  It's the .145 ips at 1146 that puts them over the overall limit of .28 ips rms.

Since there's not much interest, I'll go ahead and spill the beans.

In order to cut the amplitudes at running speed and the higher resonant frequency and the overall amplitude, the manufacturer torqued the bolts holding the pedestal to the pump to twice the value that they were torqued to as assembly in the factory. I really don't know how it was determined that the bolts were not properly torqued. Maybe a review of the manufacturing process? 

I guess the added torque added stiffness. I didn't do bump tests this time because the plant is in operation and of course, the problem pump is the lead. There likely was a slight increase in the natural frequency (to almost exactly max speed) but I suppose that was more than offset by the increased stiffness so it resulted in lower amplitudes from the same exciting force which, as Oli alluded to, is likely imbalance or even more likely, angular misalignment. It's unfortunate that in the municipal pump world, specs call for us to meet a number not fix a problem, though.  I expect that this pump will some way be accepted as is.
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