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Jigme Wangchuk

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Reply with quote  #1 

Hi All,

The Six Stage Vertical Turbine Pump is being used to deliver service water from the sump to the chiller of the hybrid cooling system of the hydro generating unit and for the ventilation system. It is being run by induction motor of synchronous speed of 1500 RPM and rated capacity of 125 kW.

Recently, it has been tripping frequently due to overloading (drawing of high current up to 240 Amps which is very high as compared to what it used to draw earlier which was between 160 to 180 Amps.  The vibration amplitudes were also observed to be very high.

The vibration readings were as follows:

Along discharge pipe axis (Radial: For Motor): Non Drive End- 22 mm/s; Drive End-17 mm/s

Along perpendicular to the discharge Pipe Axis (Radial: for Motor): Non Drive End- 12 mm/s; Drive End 8 mm/s

As a measure, the pump and shaft were changed and the aligned. Motor was also replaced by the spare one. But the problem could not be solved. The motor current and the vibration amplitudes were observed to be still high like before.  The only domination component in the vibration spectrum was 1X. The 1X was observed in with both the motors (original and the spare). So I am ruling our unbalance since the spare motor was used in another pump where it worked fine.

Just to see the vibration response I added about 20 kg of mass on top of the motor fan cover. After adding the mass it was observed that the vibrations were dropped by about 40 percent. But still it was at higher side.

  1. Could it be the case of structural resonance?
  2. Will it be appropriate to solve the problem by stiffening the discharge casing?

Any suggestions and recommendations would be very valuable.

Thank you

OLi

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Reply with quote  #2 
If this came suddenly I would primarily look for cracks and changes in the foundation, mounting, to find if any structural change made the tuning of the resonance possible or is the speed adjustable, changed?
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Jigme Wangchuk

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Reply with quote  #3 
Hi OLi,

The speed is not adjustable. The problem of high current and high vibration was experienced recently after it was used twice. However, as mentioned earlier even after changing the motor, pump and the shafts and aligning them, the problem persisted. Cracks were no observed as well. The pump has been running for 9 year. it was earlier operated for about 2 to 3 hrs daily until recently where it was required to run for 24 hrs after installing new ventilation system. The current in all the phases were also found to be balanced.  Currently standby pump is in operation

1.Could high amperage draw be the cause of high vibration? if so what do you think would be the best approve in solving this. 

2. Or could it be due to aging and may require to improve its stiffness?


OLi

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Reply with quote  #4 
It can be ageing of the foundation, anything can happen, motor is not synch is it so new vent system can give a higher load thus decreasing speed?
What is nameplate full load speed? What is your actual speed measured?
In that case stiffening can help, IRL stiffening w/o increasing weight that goes the other way is difficult, tubes have high stiffening/weight ratio.....
When you shut down, do the vibration decrease directly, instantly? When you start is it ok up to near running speed and then increase fast?
If so you may be at the peak or on the way up to the peak of the resonance and stiffening would help.
I doubt vibration would be only source for hi current. I think the new ventilation system give higher load than expected but that is just a guess.

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RustyCas

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Reply with quote  #5 
Very unlikely that the high vibration would cause high current draw.  I don't see how resonance would cause high current draw either.  Have any significant changes been made to the piping system, especially on the discharge side?  I'm thinking that "opening up" the discharge piping might allow the pump to move too much water and run out on the pump curve.  Does the "pump" operate normally?  Are pressures the same as before?  Is it providing adequate flow?  You say the "spare" pump is in service.  Is it set up the same (piping, that is)?  Have you taken any readings on the pump housing?  Is it possible you have a rubber bushing "wadded up" and trying to seize the shaft?  You would expect to see a "rub" spectrum, with many harmonics of turning speed.

Would it be possible to slowly close a discharge valve for this pump while monitoring the amps?  The amps should fall off as the valve closes (and flow, and thus load, decreases).  Was this motor replaced or refurbished recently (just before this started happening)?

I ask because I was once called to investigate a horizontal pump that was drawing high amps after a rebuild.  Closing the discharge valve had little effect on the high amps.  So I checked the rotation, and it was running backwards. He had wired it "with the same rotation as the other (companion) motor".... but the pumps were opposite hand with the motors mounted 'tail to tail' so opposite rotation required for the two motors.  You'd be amazed at how many machines I've found running backwards over the years, and not just on start-up, but actually in service.

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John from PA

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Reply with quote  #6 
Quote:
Originally Posted by Jigme Wangchuk

It is being run by induction motor of synchronous speed of 1500 RPM and rated capacity of 125 kW.

Recently, it has been tripping frequently due to overloading (drawing of high current up to 240 Amps which is very high as compared to what it used to draw earlier which was between 160 to 180 Amps.



Just a point of interest, what is the full load amperage rating of this motor?  Assuming 230 vac 3-phase isn't the rated current draw up around 400 amps? Or is this being operated at some voltage higher than 230 vac.

Also, I'm with Rusty when he said "Very unlikely that the high vibration would cause high current draw.  I don't see how resonance would cause high current draw either."  As I stated on the other forum I do see some possibilities where the reverse is true; i. e., high current draw could cause high vibration.  Shorted turns for instance can result in higher than normal amperage and result in higher than normal 1X due to unbalance.  An infrared camera might help.

Jigme Wangchuk

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Reply with quote  #7 

Here are the details of the motor (original and the spare)

1. Old (Original) 3 phases Motor: 125 KW, 204 Amps, 415 V (+- 10%), RPM: 1485, 50Hz.

2. New (Spare) 3 phases Motor:   150 KW, 255 Amps, 415 V (+- 10%), RPM: 1489, 50Hz

Winding resistance and IR for the second motor was measured.Resistance for all the phase is 0.2 Ohms. IR values were found to be in the acceptable limit as well.

(OLi) Since the motor is not run by VFD, I could not determine how vibration amplitude changes with respect  to the changing speed. But the amplitude was high.

(RustyCas) I agree with you and John that high current draw may not be due to the resonance or vibration. I agree that its very important to first resolve the issue of high current draw and then tackle with the vibration issue if it still exists.  To answer your queries, the motor was not replaced until the issue of high current surfaced. But the problem remained unsolved even with the replacement of the motor by the spare motor. There was no changes or little effects in amps even with closing of the discharge valve. The pump and the shafts were inspected and reassembled for two times after surfacing of the problem. Eeverything were apparently found normal during inspection. Even rubber bushings were found in good condition. There were no harmonic of 1X in the spectrum plot, rather only 1X component was observed.  As suggested I shall now check the the rotation of the pump. Thanks for that [smile].

Curran919

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Reply with quote  #8 
I'll agree with everyone else, step 1 is check correct rotation. This happens all... the... time... Usually the motor cables installed backwards, but can also be an impeller installed backwards (though far less likely).

You also have the slip speed for this motor at rated load. If the load is far above rated load, and its causing the over current, you may be able to see this in the real speed, if your equipment is precise enough. It could show that you are indeed running off the pump curve. I will assume you have no flow meter/indicator? Was the new ventilation system intended to change the load characteristic of the system? Who's the pump manufacturer?
OLi

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Reply with quote  #9 
Check vibration change during normal start/stop no VFD required for that. If problem in some way is electrical vibration stop directly as power is cut. If you have a resonance within operating speed range it drops quick at power down and increase quick near running speed at startup. One more off the chart is the case where a level switch float thingy was floating in a tank suction side, at every start it got sucked in to the pump creating havoc and at every stop it went back to the tank leaving the pump as original and nothing to be found........
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