High vibration in vertical turbine pump 3

[Prajesh Mistry]

Hi,
We have a newly commissioned vertical turbine pump of KBL make which is having the details as below:
Power: 180 kW, Flow: 2500 m3/hr, Disch. head: 20 mtr, Service: Raw water, motor RPM: 984
Problem:
The pump is having high vibration of approx. 30 mm/s at motor NDE, However we did the following steps one by one to eliminate the source of vibration but stills the vibration persist.
1. Foundation strengthening 2. Blue matching of motor with motor stool & 3. we also did pump overhauling (based on hard rotation of pump) to check the bush clearances & found clearances are very less however after increasing the clearances to normal clearance still the vibration remains high.
- One of our observation is the pump structure frequency is around 900 which is very close to pump running speed. May be this is the cause of high vibration? If yes how can we eliminate the same?

 

[ewooden]

Desertfox is asking the right question. The #1 cause of 1X vibration is bad alignment. Have runout measurements been taken all the way down the shaft and above and below each flange during assembly? Is the shaft plumb? Are you certain the impeller is concentric to the shaft and seals? What about motor alignment? These must be answered first before digging for frankly unlikely harmonic effects.

 

[Tmoose]

From the OP's very first post - "One of our observation is the pump structure frequency is around 900 which is very close to pump running speed. "

From the OP's 2nd post, 5 days later -
1. The dominant frequency is 1X with side bands of 80 CPM. Vibration is fluctuating between 15 to 30 mm/s.
2. The vibration is higher in one direction(along the discharge line) only.
3. During overhauling of pump, impeller was balanced @G2.5.
4. After loosening the discharge flange, natural frequency came down to 720 CPM from 900 CPM.

IF ( huge leap of faith on my part ) the resonant frequency was measured in the direction "along the discharge line", I think this pump is going to always have 5X to 15X higher vibration at 1X rotation than it would if the foundation/pump 'structure" were properly designed and installed, which, if there is a confirmed resonance so close to 1X, they most assuredly are NOT.

As for the charts and tables that say the #1 cause of 1X vibration is bad alignment.
I respectfully submit that -
1 - Some of those charts-n-tables originate with suppliers of alignment gear.
1A -Some of those charts will mention that 2X and axial vibration > 50% of the radial vibration are "signs" of misalignment.
1B - Some members of this esteemed forum have said on other bulletin boards that their experience is the connection between misaligment and vibration has proven to be singularly equivocal at best.
2 - The Late great* Art Crawford used to say that something like 90% of all vibration problems are due to resonance.

*As might be expected, not everyone would describe Mr Crawford with this adjective.

 

[ldeem]

I am way late to post but for posterity I will add a situation I had that was similar. First of all I am a structural engineer and had designed a two vertical turbine pump platform. The platform cantilevered out over water. To make a long story short the platform worked well with one pump running but had terrible vibration problems when two pumps were running. The fix was adding concrete jersey barriers which changed the dynamic properties of the structure enough to solve the problem.