WE HAVE 3 VERTICAL PUMPS INSTALLED SIDE BY SIDE IN A SINGLE SUMP WITH H=32MTR. AND Q=1315 M3/HR.
NORMALLY TWO PUMPS ARE RUNNING AND ONE IS STANDBY. WHEN ONLY MIDDLE PUMP IS RUNNING , VIBRATION OBSERVED ON THE ADJACENT PUMP(STATIONARY PUMP) WAS OBSERVED TO BE 4.5MM/SEC. ON MOTOR NDE BEARING . VIBRATION OF MIDDLE PUMP IS FLUCTUATING FROM 5 TO 7 MM/SEC WHEN IT IS RUNNING ALONE. WHEN TWO PUMPS ARE RUNNING IN PARALLEL , THE VIBRATION INCREASES BEYOND 8 MM/SEC. ALSO DURING SHUT OFF CONDITION, THE VIBRATION INCREASES. PRESENTLY , THE PUMPS HAVE BEEN PROVIDED WITH REDUCERS IN THE DISCHARGE. NORMAL PRACTICE IS TO INSTALL DIFFUSORS IN DISCHARGE. WILL INSTALLIONG A REDUCER IN PUMP DISCHARGE LEAD TO HIGHER VIBRATION DUE TO INCREASED VELOCITY? ALSO , THERTE IS A RUBBER BELLOW AFTER THE REDUCER. THE LENGTH OF THE REDUCER BETWEEN PUMP DISCHARGE HEAD AND BELLOW IS UNSUPPORTED.WE ARE SUSPECTING THIS TO BE THE PROBBLEM. CAN THIS LEAD TO HIGHER VIBRATION?
ALSO , WE ARE UNABLE TO FIND OUT THE REASON OF VERY HIGH VIBRATION(4.5 MM/SEC ON MOTOR NDE BEARING) ON ADJACENT STATIONARY PUMP. KINDLY ADVISE.

I suspect that all three pumps' operating speed is close to a natural frequency of the motor structure. This can be confirmed by conducting a natural frequency test with pumps off. Use the impact-response method and measure in both directions (inline and perpendicular to discharge pipe) at the top of the motor. If a resonant structure is confirmed and corrected, then the flow sensitivity should go away.

Walt

Hello Akhtar

Is this Sea Water Pump in Combined Cycle Power plant?

With vib on a standby pump almost as high as the running, Walt has a good point - it seems that resonance is likely playing some role. In addition to bump tests, you might check the effects of temporary bracing. Is the vibration directional?

Some other weird stuff going on.... flow depndence, fluctuating magnitudes. What does the spectrum look like ?

This message has been edited. Last edited by: electricpete,

A question for Walt Strong >>>
How is a "Natural Frequency Test of the Structures" with the pumps OFF conducted ?

Hello Akhtar,

Are the 3 pump motors VFD controlled ? If yes, are the inverters 3-, 6-, 12- or 18- pulse ? Are the excessive vibrations throughout the entire speeed range or only in certain critical speed ranges / bands ? Are the pumps long shaft submersible without their own thrust bearing, i.e. the motor NDE thrust bearing takes up the combined mass of the motor rotor, the coupling, the shaft and impeller9s) as well as the down force of the impeller(s) ?

A questions for all contributors in this connection :

There is a segment of experts in the industry who claim that VFD driven motors can produce torsional torque pulses which kick-start or contribute to a drive going into resonance with the structures ( combined set-up of motor, pump, support structures ). VFD manufacturers would dispute this claim, reasoning that the VFD inner speed loop controllers are not fast enough to react to any torque pulsations ( should they indeed exist ).

Your comments are greatly appreciated.

quote:

How is a "Natural Frequency Test of the Structures" with the pumps OFF conducted ?

A search of the Board will uncover a lot of discussion on natural frequency (impact) testing.

The best method is with an accelerometer, impulse hammer (load cell), and a 2-channel analyzer.

If you only have a single channel analyzer, then these two methods can be used:
1) Set an amplitude trigger level above the background vibration level; impact structure with hammer and capture about 4 individual impacts (averages)
2) Set analyzer to 4 or more averages and rapidly hit structure with hammer while trying to vary the rate/frequency of the hits. I like this method because it is very easy and pumps energy into natural frequencies better than single impacts.

In either case, you should acquire a background vibration spectrum to be sure the impact spectra are above the background in the frequency range of interest.

For a vertical motor-pump, two sets of impact-response data should be taken, in-line with pump discharge and perpendicular to pump discharge. In each case, the accelerometer and hammer hits are in the horizontal direction near the top of the motor. I prefer to use a real hammer (3-lb head or heavier) instead of a piece of wood or whatever. For low frequency test below 100 Hz, I would use a hard rubber impact pad; I prefer a Hockey Puck, but I have used a piece of tire and conveyor belt.

I have also found natural frequencies with machine running using both the negative (subtractive)-average method and the high-average method, but that's another story.

Walt

Walt
I've used a couple of different methods to chase down natural frequency problems. I use a small variable speed servomotor with half-moon shaped weight attached to the shaft (severe unbalance) as an exciter strapped to the unit under test. I just vary the speed while watching the spectrum for peaks. On smaller items where the mass of the motor would change the natural frequency significantly, I have constructed a rotor of plastic with four really strong magnets at it's edge. Bringing the edge of that spinning rotor near a metal object provides non contact excitement. A bit crude, but it works.
Steve

We saw one case where the floor (supporting the motor) was the culprit and had a mode that would cause flexing at (or close to) RPM.

I have a concern about the statement-"VIBRATION OBSERVED ON THE ADJACENT PUMP(STATIONARY PUMP) WAS OBSERVED TO BE 4.5MM/SEC. ON MOTOR NDE BEARING ".
I ahve observed many pump installations that have the pumps mounted on a commom support structure with poor or no pipe isolation between the pumps. When the vibration level is at 4.5mm/sec on the stationary pump, this leads me to believe that there is no isolation between the systems. It also causes premature bearing failures when the bearings are not rotating and the rolling elements are beating the race at this level.

I have recommended many times to install proper isolation under the pumps along with separating the the pumps from the common mounting platform.

As for the increase when (2) pumps are running, are the pumps running at the proper range of the flow curve? Is there a restriction in the discharge that pushes the flow from the pump out of the proper range of the flow curve? I havae seen all of these symptons cause elevated vibration levels on pumps mounted on a common structure. Where I used to wirk on this equipment we used the term "beat frequency" refering the same effect that fans can created when they are supplying air to a common plenum.

Hope this helps in diagnosing the problem.

Gary K.