Unstable vibration in Air Fan @ Waste Water Plant

Best Practices for Unstable vibration in Air Fan @ Waste Water Plant

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Hi Shanmuga, Considering original balance weights are still in place, c an you describe the shape and design of the fan impellers? Specifically, do the impellers have pockets or areas where dirt, debris, or water might accumulate?

Dear Harout The impeller is shaped like a typical fan rotor only. I dont think, the water could accumulate during normal operation. Impeller was clean enough - no any debris found. Another supporting point is that the equipment was getting nicely balanced after keeping the first correction mass itself and running healthy. One point that strikes me is that, both fan vibrations increased at the same moment on December 12th. There is something abnormal related to the process, I strongly believe and I could not locate it.

Hi Shanmuga, few more questions, 1. Process abnormalities: Have there been any abnormalities related to the process? Specifically, have you checked the suction and discharge pressures/flows, and temperatures? Do these parameters vary significantly during operation? 2. Damper Operation: Are the dampers in the system fixed or variable? Please measure the vibration while adjusting the damper settings to observe any effects on vibration levels. 3. Structural looseness: Have you conducted a thorough inspection of the entire structure for potential looseness or cracks? 4. Equipment History: Can you provide any failure history for both the motor and the fan? Have there been any previous corrective actions taken, such as machining or other repairs? 5. Past behavior: What was the condition of the machine in the past, particularly before your recent intervention? How was the fan behaving prior to the issues you are currently addressing? Thanks, Harout

I had a similar problem on a fin-fan cooler in Oman. We kept balancing the fan but vibration amplitudes increased very soon after - every time. Until eventually one of my colleagues put a bunch of filled sand bags on the structure to change the natural frequency and, hey presto, the vibration went away. I suggest you stop the fan and carry out a bump test at the motor support structure. I suspect you will find a natural frequency at fan speed. If that is the case you need to either increase the mass or change the stiffness.

Shanmuga, I have an idea; *_ Was the fan/motor frame isolated or locked when your original balance was made? From my experience dealing with fans mounted on isolation I have found that they often have failed and cause similar issues that you are finding. I would suggest that the frame be locked down and then checked for balance and corrected as needed. If the machine is at the ground floor the isolators really are not needed. They are generally used to isolate the machine vibration from the surrounding area. A simple straight forward test is to place wooden wedges next to the isolators and vibration is being monitored. I suspect you will find the the vibration level drops as the failed isolator is blocked with the wedge. Also realize that machine isolators do not have infinite life, they do fatigue with conditions, rusty springs, rubber degraded and hardened, ect. I would check these next and then if found to be bad replace them, or eliminate them altogether.

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► Check if the fan geometry is not deforming during spinning. In addition, test the vibration system by removing just the impeller to determine the part causing this rattle.

Your data sheet shows highest vibration at the motor....not at the fan bearings. So I assume the 820 RPM peak is the FAN speed right.? If so, why would the motor vibration data be higher than the fan bearings if the initially suspected cause is fan unbalance? One possibility you didnt mention is the condition of the pulleys. Make sure the drive and particularly the driven (fan) pulley is concentric and not bent. If there are any anomolies at this component, it could possibly be transferring to the motor. Also, ensure the motor foundation is tight and secure

Mr Jim The whole system is mounted in the same frame which is in the first floor of a building. To avoid vibration transmission, the system is mounted in vibration isolators. I am not surprised to see increased vibration in motor since both system are mounted in the same frame and the motor is not very nicely supported. Since the vibration frequency is 1X of Fan, I have ruled out any possible problems in motor. Even a solo run test taken by client also indicates negligible vibration level. I dont really suspect fan pulley because the vibration level dropped to normal level after balancing. Since both Fan 1 and Fan 3 had similar symptoms, I had no doubt about pulley eccentricity. Do you think that the rotor will get balanced out and shows normal vibration if the pulley is eccentric? Why does it subside and increase after sometime then?

OK, didn't realize the whole system is mounted on a single frame....that may explain higher vibration at motor (vs fan bearings) I think at this point I would monitor both magnitude AND PHASE. Seems like "something" is changing in the system, either the force itself or the response. When you completed the balancing, were the correction weights in same place or not? Does the phase change while collecting data? Does it change over time? Have you done a complete inspection of the impeller to ensure there are no cracks? Same with the frame support system If you have a strobe light, can you see abnormal/excessive motion of the shaft and pulleys and belts? What is the previous history of these fans? Did they run acceptably for a long period of time and then suddenly start to get worse? How confident are you that the snubbers (vibration isolators) supporting the whole system are not worn or possibly not adjusted correctly? Jim P

Shanmuga, You have supplied just enough information to generate many more questions. Considering the multiple balancing efforts and the presence of water. I might remove the belts and do a static fan balance check and correction. That should get you close with a 820 rpm wheel. You cannot balance away forcing functions that are not related to the mass center being coincident with the center of geometry. It may also be prudent to leave the housing drain slightly open during operation to avoid turning a fan into a pump.

Mike I have attempted a static unbalance check before the first balancing attempt itself by removing the belts and there is no static unbalance present in the system. The housing drain was kept open and the water in the housing was pumped out during start of the pump. I dont think the fan housing had any water (though its wet) during normal operation.

Hi Shanmuga, 1. Is the any debris in the wet airstream? 2. Did this problem just begin on December 11? 3. Are you able to vary the motor speed with a variable frequency drive? If so does the imbalance subside? Is it only at 820 fan speed? 4 Is it a hollow foil fan blade? Could it have holes, cracks, welds, allowing water or debris to colect in the blades?

Without knowing more, I would seriously consider either a loose foot or possibly sheeve is not secure. Do agree the entire frame could be vibrating at fan RPM, but more likely a loose foot on motor or pillow blocks.

Hi Shanmuga Reading all the comments and associated replies so far, it seems that you have already explored most of the options to troubleshoot the issue. Based on my experience with fan unbalance and related vibration, below are my comments In vibration analysis we need to keep it simple and straightforward. If we try to go too much into details it leads to analysis paralysis and usually take us down the rabbit hole. Keep it simple means. Trust and rely your vibration data. If vibration data is showing high 1x then we know it is due to unbalance (90% of the time). What we need to understand is what is driving this unbalance. Looking at your data, for overhung fans if it is pure unbalance issue then we should see highest readings in Axial direction on fan pedestal bearings. Which we are not seeing in this case. Hence, I would concentrate on factors leading to unbalance rather than keep balancing the fan. I do not think there is any issue with balancing per se Underlying causes could be i) Foundation ii) Resonance iii) temperature variation at maximum operation i) Look at foundation more closely. Replace those rubber mounts even if they are looking ok from outside. If possible carry out motion amplification to see the relative movement. ii) Do bump test to discount resonance issues as suggested by Ron Fred and Gary Kaiser iii) Investigate if temperature is increasing or process is changing. As the vibration/unbalance is increasing gradually (over one to two days). It won't be a bad idea to double check alignment as well. Pulley alignment. Regards Ahmad Kidwai

Dear Shanmuga In the past week I experienced something similar on our hood exhaust fan, I think in my opinion you have misdiagnosed the problem. On 11/12/2023 before the balancing, the axial reading on the fan was very low which is strange for an overhung fan if the main problem was an unbalanced force. Kindly note that you cannot aim to balance a fan and get good result if the forcing frequency isn't an unbalanced force, I have first-hand experience few weeks ago trying to balance a resonance problem, it gave me result but after 24 hrs. of operation the problem resurfaced in an amplified manner which is similar to your situation. In summary , do other test to eliminate structural looseness and resonance as i strongly believe one of those is the problem as a balanced fan wouldnt go out of balance unless the weight added drops off or additional weight in form of debris got added during operation,also did you notice that after balancing and operating for few days the resultant vibration amplitude increases To eliminate resonance and structural looseness 1 view the waveform of the spectrum , you might learn something from it 2 vary the speed if possible and check the resultant vibration , for unbalance the amplititude varies with the square of the speed (theoretically),,for resonance the amplitude would change drastically while if it is structural looseness there would be no significant change to amplitude. 3 Do a bump test to eliminate resonance if you have the equipment to 4 check the structure and stiffen the support especially around the motor Cheers

This one smacks of a resonance problem, Note that if that is the unit in the first image are the panels of the fan volute made from fibreglass/composite( being a water treatment) I would hazard a guess at yes due to the corrosive nature of the process? the repeated balance failures sound more and more like a natural frequency issue and as previously advised checking the condition of anti-vibration feet and also the structure of the veloute for flexing wouldn't be a bad course of action.

Shanmuga, Can you get a spectrum with high resolution to confirm there is or is not sidebands on both sides of the Fan's 1x rpm? See the attached picture copied from your original posting. Thanks, Ralph

SHANMUGA , After looking closer at your "before" and "after" balance files, I see you are working on Fan #1 and Fan #3. Is this correct? Do they both change from balanced to out-of-balance at about the same time or at different times? Did you say they (all 3 fans) are mounted on the same suspended (isolated) platform base? YOU MAY HAVE POSTED THIS ALREADY, BUT: What is the motor speed of each fan? Are they 50 Hz or 60 Hz Line Frequency? Are all 3 fans running the same speed, 820 rpm? Is there no problem with the fan #2 or do they not run it at all? Can you post a waveform and spectrum from both motors? Thanks, Ralph

Hello Shanmuga, As you mentioned both fans suddenly jumping back up following the balance, have you checked any common process factors. I'm thinking about dampers / actuators, have you asked the question specifically if everything is in the normal running condition during the balance. I've been caught out before with balancing and then they open the dampers down the line and everything changes! Just a thought... Regards Colin

Have you used a strobe light on all the structure(s)? That was one of my go to 'tools' when I had vibrations that I could not find or fix.

Clean fan wheel, remove all old balance weights ,rebalance fan and put a drain valve at the bottom of the fan housing attached to a pump to pull the water out of the housing and your issues should disappear.

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