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Resonance Problem in Multistage Pump

Problem Detection:

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  • The Resonance Institute team visited the factory with the following equipment for a vibration measurement job:

  • Hot Rolling Mill Descale Pumps

  • critical to production

  • Working at 3000RPM constant speed, load variable

  • Excessive vibration at 1X frequency in the horizontal direction of the pump. Almost 7-8 times the 1X vibration observed in the vertical direction

  • Due to high vibration, a breakdown occurs every 3-4 months.

  • The fact that the vibration in the horizontal direction is quite high compared to the vertical is among the symptoms of the mechanical looseness problem. 

  • In addition, the presence of a bump-noise floor under 1X frequency in the velocity spectrum taken from the horizontal brought the resonance problem to the analyst's mind.

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Modal Test:

  • Based on the suspicion of resonance, it was decided to measure the Modal Test. 

  • Thanks to this measurement, the existence of a possible natural frequency state in the machine and/or its base can be proved.

  • The equipment is stopped during the measurement and shaken from certain areas with a "modal hammer" containing a force sensor. At this stage, simultaneous vibration data are collected with the help of accelerometers.

  • The force input is given with the modal hammer, and the vibration output is measured with the accelerometers. The in-between transfer function provides information about the structure dynamics (natural frequency domains, damping amount, etc.)

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Pump

Motor

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  • As a result of the Modal Test study, an effective natural frequency in the horizontal direction was determined just below the 1X frequency (48Hz).

  • In addition, the existence of another effective natural frequency in the 84Hz region has been proven. 

  • It has been determined that the natural frequencies observed at 48Hz and 84Hz are the 1st and 2nd modes of the pump in the horizontal. The mode shapes of these modes are as seen in the animations below.

Mod Shapes:

  • The mode shapes of the first mode (48Hz) and the second mode (84Hz) are shared on the right.

  • By looking at these mode shapes, it is clearly seen that natural frequency regions should be supported from suitable places in the horizontal direction in order to be able to move to other safe frequency ranges.

  • The proposed support work in the horizontal direction and the actual support structures are as shown below.

isdemir_descale_pumps_50Hz_natural_freq.gif

Mode-1: 48Hz

2nd-Global-Mode-Pump-84Hz.gif

Mode-2: 84Hz

Corrective Studies:

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Conclusion:

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Before

After

  • As a result of supporting the pump from the horizontal direction, there was some shift (still not enough) in the natural frequency regions, and large reductions in vibration levels were observed (85% improvement).

  • It has been observed that the natural frequency in the 48Hz region, which affects the 1X and causes the resonance state, shifts to the 54Hz frequency, therefore the vibrations are reduced to some extent.

  • Ideally, the natural frequency should be about 20% away from the forced vibration frequency (in this case above 60Hz or below 40Hz).

  • Lower vibration levels dramatically reduce the chance of malfunction.

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