High Vibration in a Three Lobe Rotary Air Blower Motor

I'm no expert but I'd be more worried about why the blower is vibrating so much. The only 3 lobe rotary blower I worked on spun smoothly with no vibration and I'd be worried if it did vibrate because of the heavy lobes (can't be doing the bearings much good).

Some years ago, two motor-pumps were installed, attached to a single skid frame and isolated from the floor by a resilient mounting. After some weeks of running one pump, the process changed over to the second pump instead, and although the second pump had only run-up briefly for testing previously, it produced a lot more vibration and noise than the first pump. The cause was traced to no.1 pump shaking the skid frame and the bearings of no.2 pump knocking flats into the shaft of the no.2 pump where it had been standing idle for so long.

Is this equipment new or used/rebuilt? It goes without saying that new equipment or newly-rebuilt equipment should have the supplier working with you to sort out and repair the problem.

I would check the vibration with the motor uncoupled from the blower to ensure all or some of the problem is not with the motor balance. It is not clear from your brief description if you had sensors on both the motor and blower bearings even though the vibration signature is suggesting the blower is your culprit.

The blower is pretty simple, so there are only a few things that can be wrong: lobes out of balance (a balance issue would be a 1x RPM thing), the timing gearing between lobes is not correct, or there are problems with the bearings in the blower. I would take a look at the timing of the gearing as it is one item that can be easily be assembled incorrectly, especially by labor unfamiliar with blowers, if this is a rebuilt blower.

Jon.

GA. thanks for your detailed comment. The two 3 lobe blowers installed are only 18 months old, still not handed over by supplier in perfect condition as per contract. Both blowers have more or less similar problems. Earlier there were issues of abnormally high noise, it was corrected (by supplier) by extending the noise enclosure and increasing the length of discharge silencer. Then high vibration of blower, its bearings were damaged, recently replaced with new one (by supplier). Assembly was done by supplier's technician. But direct coupled motor vibration is high as explained earlier. Motor bearing is healthy. At decoupled condition motor vibration is almost negligible. There is no on-line sencers for vibration. These bowers are for process requirement which became highly unreliable. Supplier is not interested (or not able to) to solve the problem. We need to fix it urgently.

Now, I hope you can comment further.

Doing a bit of thinking out loud,

1. If the blower lobes were unbalanced, the vibration would be 1x RPM. So it is likely the blower lobes are not out of balance, because vibration is 3X RPM

2. If the motor shaft coupling had significant misalignment, vibration would probably also be 1X RPM.

3. Bearings are close to new, so they are probably ok.

4. This leaves the possibility of the timing gears being misaligned. It seems to me that most blowers do not have the gears keyed to the shaft but at least one gear is held by a friction coupling like a Ringfeder or arranged so that there can be some fine adjustment to allow tuning of the mesh of the lobes. If the lobes were mis-meshed a little you may be seeing the mesh become tight 3 times per revolution. This might show up as wear spots on the lobes--I'm not sure. Maybe a simple test would be to turn the blower shaft with the motor off. It should turn freely all through the rotation without any tight spots.

5. Are both blower lobes 3-lobed? I'm not sure, but I think there are also blowers where a 3-lobed blower element meshes with a 2-lobed element. Although, in your case you may see some 3/2x RPM vibration peaks if this was the case. Additionally, this should not change the logic of your troubleshooting.

In conclusion, I still come back to the suggestion of (4) the timing of the two rotors in the blower, followed by (1) checking of the balance of each rotor. Checking for tight spots is easy, followed by opening the end cover to access the rotor gears is also straightforward. You will need the blower manufacturer's service instructions for checking and setting the blower gear timing, though.

Hope this helps,

Jon.

PS-- In doing a bit more thinking, are the blower rotors symmetrical or do they have to be mounted to mesh a certain way? If a rotor was accidentally turned end for end upon assembly, would that could cause a problem?

GA sawmilleng.

Yes, both the blowers are identical and 3x3 lobed. Blower lobe rotors are not symmetrical they have to be mounted to mesh in a certain way. While changing the bearing, end plates were taken out by the supplier.

Actually bearings were damaged for both the blowers. Now after replacing the bearing, one was put in to service and facing this problem. For second blower, not only bearing, shaft journal was also scored. Shaft was built-up and machined but supplier not yet decided to put in to service.

To replace the bearings, the lobes probably had to be removed from the blower. That makes for a chance that they were put in differently from original construction, when the unit was balanced. Look carefully for original construction marks, they may be no more than center punch marks.

Why did the units fail, did someone forget to put oil in the cases? Are you absolutely sure the units are rotating in the correct direction, and the exhaust and intake are correct?

Get the name of the manufacturer, write to them explaining the fault and that the supplier has more or less said that he is not interested anymore and ask if that is their view too, demand that they do something about this and start writing to trade magazines about the suppliers service and attitude,if this hasn't been handed over yet refuse to accept it, I would also tell the manufacturer that you are looking to source a replacement from another supplier, once again trade magazines also trade associations are a good way of forcing them to do the right thing, wright to as many as possible.

Bazzer

Thanks Bazzer for suggestions.

It's got to be that the rotors are out of balance, they need to be removed and properly balanced, I've worked with these pumps in the high vacuum plating industry and the ones that I've come across have absolutely no vibrations at all, you need to fix it asap so as not to wreck the bearings,& then damage the casing which are a very tight tolerance.

Bazzer

I'd keep looking for resonance. Disconnect the intake and exhaust connections and run the blower with just the motor connected, then run the motor alone. Start a test with just the motor, if no vibrations, add the blower. If no vibrations add a connection, intake or exhaust, separately. This should isolate the problem.

Thanks Bazzer & Mike.

Pl see OP, we did it because we suspected resonance, but there is no change in vibration amplitude. Please see my reply at #6 too.

You said the supplier is not interested (or able) to fix the problem, that's sad. Can you return them, and shop elsewhere? There is a chance they have taken balanced machines apart, and gotten the parts confused. Finding the right combination would be next to impossible, the chance that the original maker marked the parts is possible, but figuring the marks out is a real problem.

Once you take the blowers apart, the supplier can claim the warranty is void. Return them.

Yes Mike, they are about 2000 km away from installation at our plant. So, they avoid coming frequently. We have to fix this problem urgently, because production getting affected in process plant.

These two blowers are add on units to enhance the production. We have 3 centrifugal blowers in operation since last 25 years without much problem. Going for new units take lot of time in our organization.

The 3x vibration may be due to a parasitic oscillation (flutter) inside the blower.

The flutter is an artefact of the blade pass frequency.

Feedback (adjustable) from the exhaust back to the intake can eliminate the vibration if this is the cause.

Thanks Wall, I will look in to this aspect.

I suggest the following:

1.If the base is mounted on cushy pads, please replace the complete set into new set of resilient pads to maintain uniform compressed loading on all pads. Possibly few pads have become rigid and lost its resilience; otherwise the machine vibration should have been passed on to the foundation, without amplification.

2.If the baseframe is directly mounted over the foundation, then lift the motor alone and blue-match the motor leg base with the machined pad onthe base frame on to which the motor base is fastened.

One of the above should solve your problem and this needs not much time or effort

Best of luck

King cow Ptotector

More of a question on my part. If you run each blower by itself, do they run smoothly? But together, the vibration occurs? Yes! = You likely have a harmonic issue whereby by the two forcing funtions feed each other. Do a modal analysis of the system, the results should point to any weaknesses in the design.

Dear Kingscow Protector, Ours is the first case (blower+motor both on pads in a single frame) as described earlier. Can we check the efficacy of vibration pads without removing it from position?

Dear tcoulson, Please see #11.

Some more information for everyone:

Blowers of same model and capacity were supplied to other customer, running smoothly. Only difference is low discharge pressure, 0.14 barg. We tried to run with vent open and it runs smoothly. But our requirement is 0.6 barg. So, is this a wrong selection/design by manufacturer?

Maybe....do the blower's specs ay that 0.6 barg is within the realm of possibilities?

There maybe some required tweeks at higher discharge pressures.

Don't let the supplier off the hook. Keep at them.

Dear Pritam,

1. From your additional information, it is clear that the vibration is pronounced on loaded operation and hence could be connected to thermal loads imposed by the expansion of the piping. Or the coupling faces(due to the least gap) are touching due to thermal expansion of the rotor on the floating bearing end? Can be checked.

2. What is the corresponding blower vibration, when the motor vibration is 16mm/sec on full load?

3. Considering to the ambient of 40degC and the compression ratio of 0.6bar, I expect the discharge temp to be around 100degC. Whether thermal gaskets are provided between the casing connecting flange to the piping? If you find the temperature of the inlet piping increasing much above the ambient, it could impose extra nozzle loads on the machine. Can be checked.

4. Generally, when the equipment gets mounted over Vibration isolators, by default, the machine hook up with upstream and downstream piping are to be through flexible connections. It is noted from your responses, that the flexible connection has been provided later as a part of modifications.

5. Common base frame is always preferred to suppress residual equipment vibration due to its rigidity. Hence cutting it could not have helped.

6. Lobe Rotors are never dynamically balanced by manufacturers (only static balancing done, due to low speeds. Out of balance of the rotors is ruled out from measured frequency.

7. As a second possible reason, I would still believe, that taking out the motor and blue matching its feet bearing area vis-à-vis machined pad plate of the base frame ,on which motor leg is fastened, to ensure matched bearing area of at least 85% and also inspecting the vibration isolators for damage or inaction. It is worth taking out an equipment and placing it back properly, while fighting a vibration issue, when you could also undo many of the hidden erection mistakes and come out of the problem. This is from my extensive experience.

Bye for now,

King cow Protector

Wall, Yes blower spec says, it is for 0.6 barg disch. pressure.

King cow Protector,

1. Thermal expansion in shaft and piping are taken care.

2. corresponding blower vibration was 13 mm/s.

3. it is designed for ambient temp of maximum 45 degC. At present it is around 30-35 degC. Discharge silencer is directly connected vertically down wards, with a sharp 90 deg bend (usual practice in such blowers),which act as insulation also.

4. Flexible connection and non return valve are provided in the downstream of silencer in horizontal pipe. The compensator and single flap NRV are frequently getting damaged. What I mentioned was to disconnect silencer near the blower discharge nozzle itself, but it did not give any favourable result.

5. Yes you are right.

6. That is our experience too, but here we wanted to involve the supplier for it, who says these points were taken care while erection.

V-belt drive could have beem a better option.

Now we're getting somewhere. I wonder just what the nrv does, and is it really necessary? This type of blower will not allow back-flow when it is off. A check just accentuates the pulses in the line, an accumulator would smooth them out, and might help with noise reduction, too, if it were insulated.

I wonder if you have created a pulse-(jet)engine here, and it is beating itself up trying to take off!

Thanks & GA, Wall & Mike, for thinking differently.

But we tried running the machine without NRV, vibration level is unchanged. In fact when run without NRV, while stopping it suddenly rotates in reverse direction. So, I feel NRV has its necessity.

Where is the backpressure coming from?

Wal,

It has to work against a liquid dead, so discharge pressure of 0.6 barg is needed. As other blowers are also feeding, there is back flow after stopping if there is no NRV, until isolation valve is manually closed..

sawmilleng,

GA, It is not for conveying material as explained above. What we noticed here is when loaded, makes lot of noise. It quietens when opening a blow off which reduces discharge pressure & so the load. We tries several way to break the resonance, but in vain.

Which ways have you tried so far?

You have to go through my earlier comments.

OK, seems that all you have tried is a flexible coupling.

Not much of an effort there or even a resonance mitigating action, isolation attempt more like it.

Have you tried negative feedback yet?

If you go back to your original original posting (another thread) I gave you some citations on this. Go back and read them.

Dear Wall, I reproduce from my original post here:

"As a trial we provided a flexible connection at blower discharge pipe (with silencer) which was vibrating heavily, but no change in motor vibration. Thinking that combined base frame is transmitting vibration/ causing resonance, it was cut in between but still no change in motor vibration."

I'm very much open to negative feedback too and try to the best possible.

I could not locate your citations in my another thread, will be thankful if you can help.

try Googling "blade pass frequency", "vane pass frequency", "turbine parasitic oscillation", "turbine oscillation mitigation"

Pritam,

You have mentioned you have silencer(s) in the discharge piping. Presumably you also have a flexible coupling in the discharge pipe of the blower....?

I'm running out of suggestions here!

Have you talked to the blower manufacturer? (Not the installing representative, who you suggest isn't that interested in helping you...) The manufacturer may have seen resonance issues and might have simple suggestions to help.

What kind of discharge silencer do you have? Is it reactive or absorptive? Have you talked to silencer suppliers to see what could be done?

Are you able to beg, borrow or steal any datalogging equipment to record the pressure waves that are generated at the blower discharge?

Are the multiple blowers clustered close together? Does this blower quiet down if you shut down another blower?

This is a topic that you have to become more knowlegable than your installing supplier so you can TELL them what is wrong and how to fix it!!! As an engineer, the quest for knowlege can be enjoyable and fun, but the stress of trying to get the fix done by an uncooperative supplier can be frustrating and stressful!!

Jon.

GA sawmilleng. Yes we also have a flexible coupling in the discharge pipe of the blower, after the silencer.

In fact, the supplier so far I was talking about is manufacturer only.

Silencers are absorptive types.

I really don't know what is "datalogging equipment to record the pressure waves". Can you elaborate it and explain how we can use its findings?

Yes, the multiple blowers clustered close together at two locations. At location 1, three centrifugal blowers are installed and at location 2, two lobe blowers. I don't know whether this blower quiet down if we shut down another blower, as not tried (not easily possible to stop other blowers).

Maybe a compressor is more suited for this job, since you have this blower dead headed and operating at max pressure. It's designed to operate with a certain flow.

There is another blower type that might do better, it's shaped like a Nautilus sea creature inside, forget the name. Only one moving part, an impeller.

please advise me as well krub, now we still get the same problem at DE motor side and sign vibration on 18x . And we perform discoupling and no load test, clean suction fillter, confirm discharge pressure then alignment and vibration measurement and found that vibration still high. while we found motor rise up around 0.1 mm after no-load test, then we realignment with in torrelance then test run around 4 hour then stop to perform check hot alignment found that blower rise up around 0.6 mm, please advise how to fix it? thank you very much krub