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Curran919

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Reply with quote  #16 

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Manufacturers are given way too much credit for their understanding of stiffness and resonance.

HEY! Our biggest problem is communicating the information to the 1000 people that have their finger in the design pie.

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Time and time again these large vertical motors are used in power generation industry on either seal oil or boiler feed water and they seem to be ripe for resonance issues, but still, they persevere using them.


VS pumps are great. Yes they are susceptible to resonance. Much of this is because while we can tune the natural frequency of the pump, it is still subject to the foundation stiffness, which has a large effect, and which cannot be quantified by the vendor. The customer generally gives us no information about this. It really is a crapchute on these pumps, but at the same time, if there is resonance, most of the vibration levels are fine. 5 mm/s may alarm, but thats only because of the bullshit outdated standards that people apply to motor NDE on VS pumps. Have you ever seen 5 mm/s from structural resonance affect the longevity of one of those motors? You could double that and probably still not see a problem.

electricpete

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Reply with quote  #17 

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ivibr8 wrote: If the impact was on the structure though, I wonder how it may have transmitted the energy to the rotor shaft if it was near a node??
I agree this thought process makes my scenario less likely.  For my scenario to be valid, we would have to presume the vibration of the center of the shaft during the bump is much higher than the bearing housing, maybe 10x as high as at the bearing housing.  BUT  of course center of the shaft was not measured so we can't rule that out.   Also my scenario might seem unlikely because the 900rpm peak shown in the bump test spectrum is the highest among multiple peaks. BUT the relative magnitudes of those bump test peaks are affected not only by proximity to a node / anti-node, but also by other factors (including modal damping and the direction of the impact/monitoring).  So, I don't consider my scenario to be implausible... 

Please note I didn't raise this as a likely scenario to begin with, (I said "sounds like a good diagnosis... I wonder...").   I was only looking for possible scenarios where stiffening of stationary components would not be successful.  Given the only thing we know about the system is that a long jackshaft was added that seems like a reasonable thing to consider.  The total system may certainly be acting much differently than I postulated.  Checking the shaft dimensions seemed like a quick easy check on the plausibility of my scenario...

An easier way to check my scenario now that I think about it: if the 900rpm only dominates when impacting/monitoring in one direction but not the orthagonal direction, that would tend to rule out my scenario.   And if the vibration  during operation is directional, that also tends to rule out my scenario.   Conversely, if the opposite holds (no directionality in bump or operating vibration), that would tend to steer us away from typical structural resonance and towards scenarios like mine.   So Rusty, is it directional? 

Danny Harvey

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Reply with quote  #18 
I have done start-up testing on dozens of these pumps and it really is like Curran says-a crapshoot.

There are so many things that have to go exactly right in the construction process that when I do a set of three "identical" pumps, I can almost always expect something different from one of them.

I have seen many that get through the HI test easily and some that have had problems. Understandably, most of the manufacturers want to control the process of fixing a problem pump but EVERYBODY wants their chance to make suggestions. I recently had one who took my bump tests and directional data and made modifications that worked first time out.

It's expensive and I don't know who winds up paying for it but I would bet that the pump makers suck it up. I'm not sure that's fair but I would guess that it is worked into the price up front if possible.  

So back to Rusty's point, yes, many designers don't know much about designing around natural frequencies. But it's a two way street and it's filled with obstacles on both sides.

Sorry for the drift, Rusty.
Walt Strong

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Reply with quote  #19 
A fellow engineer once said that specification/designer should target natural frequency for shaft speed. That method should produce no match at all!!

Walt
Curran919

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Reply with quote  #20 
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Originally Posted by Danny Harvey
It's expensive and I don't know who winds up paying for it but I would bet that the pump makers suck it up. I'm not sure that's fair but I would guess that it is worked into the price up front if possible.


Thats exactly it. We either get lucky with no resonance, or we get lucky and the customer doesn't care, or we get unlucky and have to make a modification. Most of the time, the motor stool has been designed such that modifications to open up or fortify the maintenance windows is simple enough to detune.
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