increasing Vibrations 1

[bopsyseng]

We recently refurbished a 1750hp motor as part of Preventative Maintenance. After the installation of motor the vibs increased from 0.2ips to 0.63ips over 7 hour period. The problem was assiciated with the insulation plate. After fixing the insulation plate the uncoupled motor vibs were 0.065ips with no change over 2 hour period. After coupling the motor to a nine stage pump the vibs increased from 0.2ips to 0.42ips over 4 hour period. Before the refurbishment there was no problem with the motor and pump. Any ideas what is the problem? Thanks.

 

[electricpete]

You mention increase in vibration over time. Unless there was something else changing in the system (like load slowly increasing), this would normally suggest some type of thermal effect, either:

thermal growth causing misalignment
thermal bow of the rotor.

What is the frequency/frequencies? Is the vibration at running speed? Are pole-pass frequency sidebands visible? Horizontal or vertical motor? Uniform in both radial directions? Any axial vibration? Axial phase shift? Temperature of the fluid? Any indication of significant temperature change? What does the base look like?

My gut feel would be thermal bow of the rotor but you need to methodically work through it.

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[bopsyseng]

The vibration is at running speed. Vertical motor. Vibs are high in A2 direction, no axial shift, temperature of the fluid is 350'F. No change in temp. The pump is underground. Motor is mounted on a can 5-10ft over the pump. If you think it is thermal bow is there any other way to verify this? Thanks for help.

 

[byrdj]

Is the coupling grease packed? I had similar problem with excessive grease packing being throwing out.

 

[bopsyseng]

The coupling is not grease packed. In order to make sure the isulating plate is not coming loose we ran uncoupled and found the vibs are steady and in the range of 0.04-0.045ips. Also the coupling is not grease packed.

 

[bklauba]

If you are sure that nothing is lose, it would appear to be out of balance. Can you get the balancing data from the refurbisher? With this in hand, if you have to go so far as to tear the motor down and have the rotor checked on a balancing stand, any significant difference would have to be explained. (And the cost of the tear down credited off the refurb charge.)

BK

 

[byrdj]

It looks like you proved it is not the motor, since you seperated and the motor vibs were acceptable. However, this would not elimante the motor mounting from your investagation. Thus, with out benifit of seeing the installation, suggest now focus attention on mountings, couplings and the pump. I would first look at coupling. If solid type, check for crank between the bolted halfs. Also putting the high spot of each half of the couplings 180 apart may help (make new match marks). If one of the many different flex type couplings, each will have its own possible problem. Like worn teeth or coupling gap.

Good luck
JFB

 

[electricpete]

Maybe the orignal poster can clarify... if you stop it,allow it to cool several hours and restart it, does the vib again start low and increase over hours. If this is the case my personal opinion would be balance does not act this way and alignment does not act this way unless thermal-induced misalignment or gear coupling coupling lockup.

My personal opinion is still thermal bow of the motor most likely.

Yes, the motor ran ok uncoupled, but the motor does not run as hot during uncoupled run as during running loaded/coupled.

How to test it. One possibility of we want to narrow down to the motor would be to run the motor uncoupled, with ventilation intake choked off. Monitor stator winding rtd temperatures to ensure they are within limits - let's say less than 120C for class B insualtion. I have has Siemens recommend this to me and assure me it was safe. One issue is that vibration occuring during the test proves it's the motor, but lack of vibration does not disprove it 100%... the heating pattern of the rotor by reduced ventilation is not identical to the heating pattern of the rotor due to mechanical load on the motor.

Another possibility to check for thermal bow is to attempt to measure runout of the shaft exiting the motor during motor coastdown after it has been running loaded with high vib. Would require you set up near the motor ahead of time and give careful consideration to personnel safety.

Now that I am thinking of it: rotor problems within the motor (either thermal bow or rotor bar) should show up with pole pass frequency sidebands around 1x in vibration and around 2*LF in current. Use high resolution spectrum to look for these would be a logical first step if you haven't already done it.

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[bopsyseng]

Electricpete has a good suggesation. However, here is what we did. We found the spool piece (extension between motor and pump coupling) was out of alignment (20 thou or 0.020). The out of alignment was due to bolts that were used to connect the spool piece. This caused the spool piece to shift and hi vibs. Now my theory for increasing vibs is as the spool piece heated up the metal expanded and caused the vibs to increase. The spool piece is around 100-150lbs and will take few hours to heatup to a stable temperature. Do you think my theory holds? Electricpete thanks for your suggesation.

 

[electricpete]

When you mention a spoolpiece I think of a rigid steel piece. It doesn't flex so how can it heat up? Doesn't make sense to me but you never know.

Just for our info, can you clarify: have you restarted after tightening spoolpiece bolts and vibration went away?

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[byrdj]

How about this WAG. If the bolts in the spool piece were not fitted and if radial rabbet worn, the coupling faces would slip after few hours at load and put a crank in the coupling.

Please let us know the outcome.

 

[bopsyseng]

The spoolpiece is a rigid steel piece but it is bolted to the coupling. The process fluid through the pump is around 350-400'F. Eventhough the room has a air cooler, just above the pump where the coupling and the spoolpiece is situated (inside a 10ft canister) does get heated up not upto 300'F but >150'F. THe spool piece did not shift while running but while installing.
So far all we have done is ran the motor with the spool piece (after re-aligning) and vibs are reduced to 0.05ips. THe real test with coupled to the pump and loaded will be run in few days. I will keep you guys posted. By then please post your ideas. Thank you.

 

[byrdj]

Another though. I would assume the pump and motor have their own thrust. If the distance between thrust are a few feet,and experances a temp rise, then the coupling must allow for the axial thermal growth. For example a flex plate type coupling will need a cold pull when installed.