Fan-motor system resonance problem

[Lnewqban]

Yes the impeller has been replaced with a completely new one. Both have been balanced, statically and dynamically.
.....
There is most certainly some deposits on the vanes. But the problem was already there before the build-up of these.

Dynamic balancing is the key in this case, but it should be done as an assembly (all the rotating masses), including shaft and key.
If not possible, testing the dynamic balance of the shaft alone could eliminate the possibility of a bent shaft (which I suspect).

Besides any fluid pulsing cause (if any), the resonance problem must have its root in masses that rotate out of balance, even if in a very small magnitude.
If that lack of balance can't be located and measured and corrected, modifying things to increase the natural frequency of the vibrating system is the next step.
Evidently, those added angles did not do much for that.

What is the meaning of your following statement?

I'm kind of rethinking the weights on the impeller to balance it out though.

... it's not that easy to show pictures of bearings and couplings since the place has to go in shutdown to turn the fan off. (It runs on 30 Hz now)

No problem, just trying to understand the arrangement of motor, coupling, shaft, air seal (if any) and bearings.
If not possible, pictures from different angles around the fan could help us.

It seems to be a cantilever impeller arrangement, which can make vibrations worse.
One of your pictures seem to show a solid bridge by-passing the isolation of duct-fan provided by the canvas connection.

Any luck with a video via YouTube?

 

[ArnoVonck]

Any luck with a video via YouTube?

I shared some links to these video's in a reply above

Video 1:
Video 2:

Here they are

 

[ArnoVonck]

What is the meaning of your following statement?

The fan has been balanced out by adding some weights to the impellers. This has been done when I wasn't on internship here yet. When I looked at the fan spinning without media through it, it seemed these weights were actually Off-balancing the impeller. I'm thinking about taking the weights off and then seeing how the system does when rotating.

 

[Tom.G]

Be sure to mark which weight goes where. That will save you a LOT of time if things get worse without them!

Also was the original balancing a static balance or a dynamic balance in 3 dimensions?
i.e. the usual out-of-round or off-center mounting problem
versus
also balancing for wobble parallel to the spin axis. (important for centrifugal fans)

 

[ArnoVonck]

No problem, just trying to understand the arrangement of motor, coupling, shaft, air seal (if any) and bearings.
If not possible, pictures from different angles around the fan could help us.

I'm sorry for the late answer. I've been on a holiday. I've went and took some pictures of the entire unit to maybe get you a better idea.

 

[erobz]

When you run it a rated design rpm (1470 rpm), are you getting rated static pressure?

Are there other systems tied into the duct? I assume it's going to a scrubber?

 

[jrmichler]

Based on Video #2 in Post #17, it appears that the blower housing has a mechanical vibration at 18.2 Hz. Mechanical vibrations are analyzed as spring - mass systems. The mass in this system is the blower housing, the spring is the side of the housing in the photos, and the motor base is fixed. That's why the request for the test in Post #27. Sometimes, your fingers are all the test equipment that you need. This might be one of those times, but you need to run the test to find out.

If so, the remedy is simple. Weld on a pair of stiffeners as shown by the heavy black lines in the image below.

You can leave the angled braces where they are, or make new ones that connect the same location on the base to the top of the stiffeners.

The theory is as follows: Natural frequency = $\sqrt{\left( \frac K M \right)}$, where:
K = Stiffness
M = Mass
Since we want to increase natural frequency from 18 Hz to a value above the maximum operating speed of 30(?) Hz, it is necessary to increase the ratio of K to M by about a factor of 4. Reducing the vibrating mass that much is not practical, but it is practical to increase the stiffness. The stiffness of a thin plate is very low, and it is easy to increase that stiffness by a factor of several times by adding the stiffeners shown.

 

[Nik_2213]

A thought: Those diagonal stiffeners ? Do they resonate ?? If so, can you add masses to 'quench' them ??

 

[ArnoVonck]

That's why the request for the test in Post #27.

The 3rd of May we'll do another run-up test. I'll do as you said in Post #27

 

[ArnoVonck]

When you run it a rated design rpm (1470 rpm), are you getting rated static pressure?

Are there other systems tied into the duct? I assume it's going to a scrubber?

No we don't get rated static pressure. Indeed, it's going to a scrubber.

 

[erobz]

No we don't get rated static pressure. Indeed, it's going to a scrubber.

I'm assuming your s.p. was too high, which was why it is currently being run at 1086 rpm? Or is it the vibrations just passing through 1086 rpm on the way to 1470 rpm the issue?