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  • 1.  Unstable vibration in Air Fan @ Waste Water Plant

    Posted 01-09-2024 15:13


    I need your support in resolving an unstable vibration issue with air fans in a waste water treatment plant.

    Equipment: Air Fans 

    Number of Fans: 3

    Equipment type: belt-driven

    Motor Speed: 1470 RPM

    Fan Speed: 820 RPM (overhung fan)



    On December 11, 2023, I initially collected vibration data on all three fans; 2 out of 3 fans showed high vibration amplitudes of around 10 mm/s in the motor and 7 mm/s in the fan bearings.

    FFT showed a predominant 820 CPM in both motor and fan bearings. Suspected unbalance and performed balancing of the rotor.

    After balancing on December 12, 2023, the vibration amplitude was around 3 mm/s and 2 mm/s in both fans.

    But, within a day of operation, the vibration amplitude at both fans started increasing with a predominant frequency of 820 CPM again.

    The suction for this fan is from the atmosphere, and there are multiple filters. The client cleaned all filters, replaced the damaged filters, checked the mounting locations, and confirmed that the damper positions were okay, but still there was no improvement.

    I once again tested the equipment on 16 Dec 2024. Vibration amplitudes at both fans were around 16 mm/s (motor) and 8 mm/s (fan), with the same dominant frequency of 820 CPM.

    Client operated the fans intermittently and checked the vibration levels, but it was on higher side only. Since I was left with no other actions, I once again attempted balancing Air Fan 1 by adding a correction mass of 170 grams, and vibration dropped once again to 4 mm/s in the motor and 2 mm/s in the fan.

    Once again, vibration amplitudes started increasing within a day and reached around 12 mm/s in a couple of days.

    A couple of points:

    1. The suction for these fans is from the atmosphere; hence, no significant wear is expected.
    2. The discharge is to remove gases from waste water, but due to enclosed operation, the fan impellers are always wet, and there is some water in the housing during start up and the client is draining the same before starting the fans.

    Please find attached the data for these fans and share your thoughts.





    Data for AMP.xlsx   294 KB 1 version

  • 2.  RE: Unstable vibration in Air Fan @ Waste Water Plant

    Posted 01-10-2024 00:55

    Hi Shanmuga, for this overhung fan, the blade looks a little bit wider. If confirmed it is unbalance issue, does it exist couple-unbalance. I supposed you performed the single plane balancing, may consider two plane for both static and couple unbalance.

    kevin zhao

  • 3.  RE: Unstable vibration in Air Fan @ Waste Water Plant

    Posted 01-10-2024 01:55

    Dear Kevin

    Thank you for your response. Valid point.

    But, will the vibration ever drop to normal level if the unbalance is still present? Air Fan 1 was running normal for almost a day (11th December evening to 12th Dec evening). I feel that even couple balancing also (even if it brings the vibration to normal) will not resolve the issue.

    Is there any effect of wet conditions in blower rotor. Some water droplets are always found in the rotor due to humid conditions. Does anyone have any experience or idea on this?


  • 4.  RE: Unstable vibration in Air Fan @ Waste Water Plant

    Posted 01-11-2024 05:21


    some comments:

    If there is NO mass build-up or mass-loss on fan rotor, then it is some change(s) in geometry of the rotating-system and/or supporting-elements. 

    Q: What is fixating means for fan rotor on the shaft ? Is it some kind of 'Taper Lock Bushing'? Could it be that seating surfaces betwen rotor/busing/shaft, due to moisture, are corroding and corrosion products are 'cocking' / changing position of the rotor...?

    2nd Q: Also you did saw answer about variable-speed - is it VFD driven (modulated fan speed)? If so - definatelly check for resonance frequencies. You can not balance equipment close to those.

    Normunds Sarsuns

  • 5.  RE: Unstable vibration in Air Fan @ Waste Water Plant

    Posted 01-11-2024 08:12

    Had a similar issue with 2 identical fans operating about the same speed, similar in size, entire fan base mounted on springs with flexible duct connections on suction and discharge with a discharge damper.. The fans were fiberglass to handle waste water air stream. Both fans required a trim correction when first started. Within 30 days, one of the fans went out of balance. Balanced the fan again, within 30 days same fan was out of balance. Close inspection of the fan wheel found suspected crack in one of the fan blades allowing moisture to build up causing imbalance. Repairs were made to this suspected crack, have not had an issue since.

    Dave Reynolds

  • 6.  RE: Unstable vibration in Air Fan @ Waste Water Plant

    Posted 01-14-2024 01:57

    @@@ Shanmuga,

    Can you get another spectrum with high resolution to see if there are any sidebands surrounding the 1x rpm peak of the fan?

    See the 2 attached pictures of your data.



    Ralph Stewart

  • 7.  RE: Unstable vibration in Air Fan @ Waste Water Plant

    Posted 27 days ago

    Hi All
    Thank you for your kind responses.

    Three things:

    1. I confirmed that the dampers, filters, base locations, alignment, pulley (including any slip) and rotor condition, etc. All are intact.
    2. Checked the natural frequency of the system which is around 1200 RPM, not coinciding with anything and not dominant as well.
    3. Once again balanced the Fan 1 rotor on 18th Jan. This time, the mass needed to be kept opposite the second balancing mass of 170 grams. After balancing, the vibration level is around 2 mm/s for 8 days now.

    I am yet to find the cause for the fluctuating vibration phenomena. Let me keep under monitoring and post here if there is anything.

    Than you once again for sharing your comments.