I have a motor for vertical application coupled with a pump. The reed frequency of the motor is 12 Hz. The motor operates at 594 RPM at 60 Hz and has extremely high vibrations. I tried conical washers between the motor and motor stand at hold down bolt location. This did not help and the washers flattened because the motor weighs over 10,000 lbs.

Your participation will be highly appriciated.

It certainly could be resonance, but it may be helpful to fill in some details.

Where did the 12hz reed frequency come from (manufacturer?) Have you done bump test? Coastdown test? Is the vib directional? (I assume it is running speed? Any harmonics?) Have you tried temporary bracing to check for effect on vibration level?

Attempted balance? Did the problem develop over time or appeared immediately after installation/maintennace? Does the vib continue uncoupled? Does the vib appear immediately upon startup or takes awhile to appear?

Riyaz,

9.9 Hz at 1xSS is resonably well below the natural frequency at 12 Hz. Greater separation would be better, but this is not too bad. Did you conduct test to determine the 12 Hz frequency? Did you test in both directions (inline and perpendicular to discharge? Did you test if the motor on its pedestal is the dominant vibration mode or is the pump column active in the same vibration mode as the motor? Did you run motor uncoupled to test for motor unbalance? Did you measure shaft alignment? Did you index the shaft coupling and run again? Do you understand that using conical (Belville) washers would reduce stiffness and lower natural frequency and move it closer to running speed? If this is an old installation and past vibrations were lower, then you have to consider whether the natural frequency lowered over time (foundation deterioration) or whether there is shaft misalignment or a balance issue with motor or pump (most likely). Worn pump bearings and wear rings may also show as a drop in pump performance.

Walt

Bump test was done. It is resonance. How do I get rid of it? 12 Hz + / - 10 is the reed frequency from the manufacturer. Customer is not accepting stiffeners for cosmetic reasons. Equipment has less than 5 hours on it. Vibration appears only at 60 Hz. Ronald Eshleman, President of Vibration Institute thought I am trying to acheave impossible. If that is the case why the motor manufacturer should build the motor for this speed with this reed frequency!

Please Help!

"...extremely high vibrations" Can you quantify just how high they are and where they are being measured?

John from PA

1.0 inch per second RMS at motor upper bearing in radial direction

Yep that is extremely high.

Now to clear things up. Is the vibration at running speed (594 RPM, 9.9 Hz) or at 60 Hz?
This makes a Big difference.

If it is at 60 Hz then I do not think the reed frequency at 12Hz is in the equation. If the vibration is at running speed then it might.

Many vertical pumps have the discharge piping coming out of the side of the upper pump housing. The reed frequency is usually much stiffer in this direction and therefore the reed frequency is higher. If your pump has the discharge pipe out the side of the pump housing, is the vibration amplitude much different in the direction parallel to the piping and perpendicular to the discharge piping?

Other questions.

Is this a new motor on an old pump or a new pump and motor? If this is a new motor on an old pump, is the new motor the same weight as the old one?

Does the vibration measured immediately at startup from cold differ from that recorded after 10 minutes (or 30 minutes or 1 hour) of loaded operation?

An uncoupled run would be useful.

Both the pump and motor are new. Motor speed at 60 Hz is 594 RPM
Actually my Manager is handling this case. All the above information is based on what I heared from him. I will talk to him tomorrow to make sure that the above information is correct. If there is any correction, I will post it tomorrow.
Look for my post tomorrow.
Thanks Guys!

Here is more info:

motor is new.
2000 HP / 594 RPM
Manufacturer's reed frequency most likely calculated is 12 Hz
Bump test parallel to discharge pipe 675 CPM
Bump test perpendicular to disch 705 CPM
Vibration is directional in 7:30 to 1:30 in regards to axis. 12:00 position is discharge
Motor is mounted on pump volute
Pump is mounted on 3 cement foundation piers
Motor is tripod mounted
Mounting leg positions 12:00 / 8:30 / 4:30

Question:
What is the reed frequency you all speak of. Is that the Natural freq or something?


Dave

Dave,

Yes. It is the natural frequency / resonant frequency.

Don't make that final payment quite yet. It helps keep the lines of communication "open."

New pump AND motor ? What vibration specs are part of the contract or PO? If both are from the same supplier, it's their problem for a while longer unless your purchasing contract was written by someone who never suffered through one of these ordeals before and did not make vibration part of the acceptance tests. The pump manufacturer would likely not have signed on for any vibration spec more than ANSI/Hydrulic Institute standards. Even then they may now try to foist "the problem" off on the installer.

If due to a holey contract, or some other mitigating circumstances, you guys choose to be or must become part of the solution, as others have said, there are some more consistent and powerful tricks than trying to detune the motor mount with 50%(!) softer/flexible mount details.

Does the 1X "heavy spot" phase and amplitude remain consistent thru several start/stop cycles? A few trim balance weights strategically placed on the fan (or ratchet or lift nut or rotating whatever) at the top of the motor can often make a startling and long lasting improvement.

A tuned dynamic absorber using a round (non-directional) spring elementnear the top of the motor may make a startling improvement. If the absorber is designed and mounted including details with an understanding of stress and stress concentrations then it should be quite long lasting too. After the appropriate prototype device were made and tested and proven, I would have a hard time resisting the temptation to embellish it with several star-wars or electonics gizmos stuck to the movable mass. A "large" loosely fitted, lightly centered weight living submerged in a pressurized oil bath canister might serve as a tuned damper and operate with less visible amplitude thus alarm the towns people less.

quote:

If that is the case why the motor manufacturer should build the motor for this speed with this reed frequency!

Many vibration specs would require operating speed (frequency) be separated by at least 20% from resonance. This is concession by practical men that it is sometimes really, really tough to ensure resonant frequencies will be far from operating speed, and in the simplest form 20% separation would limit vibration amplitude amplification to about 2X.
http://www.gmt-international.com/images/Graph_1.gif

A pump with a calculated reed freq of 12 Hz/720 cpm on a perfect base could easily shrink to 670 cpm mointed on a "real" base.

Was the bedplate leveled with nuts on the hold down studs, then grouted, so tightening the anchor nuts really just jams the bed plate against the leveling nuts? That's a popular abd convenient detail that can easily bring on vibration issues.
Are the hold down bolts within a few bolt diameters of the pump legs? No? I didn't think so.
Is the bed plate dead steady, even a the pump legs, with no discernible 10 Hz vibration?

Because your measured reed frequencies are more like 675 cpm, the amplification is something like 4.5 to 4.8, so your 1.0 ips rms would be more like 0.2 ips rms if the reed freqs were tweaked above 14 Hz.

).2 ips rms is still ~ 9 MILS pk-pk, where ANSI etc would probably allow about 6 MILS (overall) on a 5 foot tall pump pumping clear liquid. see chart page 10 here
http://www.sulzerpumps.com/Portaldata/9/Resources/broch...eshooting_E00669.pdf.

For many years I was the 3rd party or referee making the vibration measurements that are rightfully part of thoughtful acceptance tests. I am sympathetic to pump and motor manufacturers because a well made, carefully balanced, beautifully aligned motor WILL have higher 1X vibration when mounted vertically. If the pump, or base, or installation, or facility floor are less than excellent the vibration will be higher still.

Dan,

Can you please tell me how to tweak the reed frequency above 14 Hz without affecting the cosmetics?