Vibrating steel beams carrying Agitator 4

[TLycan]

Dear All,

A client asked me to check a problem of a vibrating steel structure. The steel structure is two beam 12.00 meters installed over a tank.
these beams carry a Fan and a motor used to agitate a solution. The problem is these beams vibrate vertically as shown in the attached video. also the system is attached in an attached files.

Does anyone has an idea of the procedure of investigating such a problem?. Is there any material I can read about this subject ?

Thanks in advance

 

[TLycan]

Sorry here is the Vid

 

[Ron]

Not sure where you are located nor what your resources might be, but a vibration analysis of the system should be done. This consists of the placement of orthogonal transducers (accelerometers) at various locations on the beam and on the equipment housings to look for anomalies that can explain the vertical displacement.

It might be something as simple as the fan blades are out of balance, a bearing is worn, a shaft is worn or similar. Have someone do the work who is familiar with testing machine vibrations for such conditions.

The other issue is that it might be another example of designing for strength and neglecting serviceability!

You will likely need to change the natural frequency of the beams with stiffeners and/or bracing. Even if this is only a machine issue (bearings, shafts, mounts, balancing, etc.), it would still be a good idea to stiffen the beam system.

 

[TLycan]

Dear Ron
Thank you for prompt response,

It will be difficult to get accelerometers.

I was thinking more about Finite Element Analysis.

And As per the client this vibration was from the beginning. when operated the agitator for This first time , these vibrations were obvious.

 

[Ron]

If they were from the beginning then I would look toward underdesign for serviceability or improper mounts. Fan blades can also be out of balance when installed.

You can use FEA for part of it, but will need to do further investigation to tie it down.

 

[thaidavid40]

I noticed that there is a walkway associated with the agitator support. I am curious, when someone walks out on to the walkway when the agitator is running, do the vibrations lessen? If so, then you may simply need more mass to re-tune the structural system's natural frequency, and dampen the vibrations. This additional mass may require additional beam strength to support it, so you may have to supplement or change the beams supporting the whole system. You could inexpensively first test whether or not this will work by simply stacking some scrap weights on the walkway, near the center of the span (but not to the point of exceeding the beams' strength limits!). This would tell you a lot about the dynamics of the existing system.
Good luck,
Dave

Thaidavid

 

[TLycan]

Is there any source , reading material I mean, on this subject . I mean Structural design/analysis for dynamics/vibration of machines

 

[SlideRuleEra]

I'm familiar with this type of platform/tank/motor/agitator installation. They are used to keep the solids of viscous liquids in suspension while in the tank. An example is thickened FGD sludge at coal fired electric generating stations.

The electric motor is geared down (gearmotor) to turn the vertical agitator shaft at a rotational speed much less than the motor's rotational speed. As shown on your drawing, the agitator (somewhat like a ship's propeller) forces the viscous liquid "down". The fluid level in the tank and the (possibly changing) viscosity of the fluid likely will effect the vibration.

Make the vibration measurements per Ron's recommendations, but do so at various fluid levels.

You don't mention the size of the beams. From the video, beam span / beam depth ratio looks very high. No doubt the beams are structurally adequate for the loads. IMHO, you will find that the beams are too "flexible" to adequately dampen motor/agitator vibration. One solution may be to replace the beam with deeper (higher moment of inertia) members.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[TLycan]

SlideruleEra

It is exactly as you described
Vibration measurement as per Ron is not that available

For the beam size its HEA300 and I checked it under the static load provided in the first pdf file I uploaded found safe . the Deflection is 4.8cm

Attached is the structural system of the frame carrying the motor

 

[TLycan]

SlideruleEra

Forget to add, is the vibration shown in the Vid is a phenomenon that happens often? I mean Did you see such vibration often?

Thanks In advance

 

[JStephen]

In the cases I've been involved in, the mixer manufacturers specified equivalent static loads and maximum deflections for the support design. The equivalent static loads included some extra allowance for dynamic action. The maximum deflections are intended to provide a stiff enough system that you don't have vibration problems. Generally, the equipment suppliers are going to be the ones that have familiarity with these kinds of problems, and it would be worth checking with them in this case.

If the beams have adequate load capacity plus extra, and if they are hitting a natural frequency, it might be possible to add dead weight to them to change the frequency. Otherwise, it may be possible to attach additional beams or joists under the existing beams or beside the existing beams.

 

[SlideRuleEra]

TLycan - The vibration could be a symptom of simple problem. Suggest that you check the following:

1. Get elevations of the perimeter of tank foundation to see if there has been any differential settlement. This settlement would cause the agitator shaft to not be plumb, which could create vibration.

2. See if solids from the fluid have come out of suspension and built up in the bottom of the tank. They could be interfering with the agitator's rotation.

3. See if solids have built up on the agitator blades or shaft. This would put the rotating parts out of balance. This would be especially true if these parts are rubber lined (like some of the other parts) and the liner is damaged.

4. Ask if the vibration started just after any maintenance on the mechanical equipment. If so, the equipment may not have been properly realigned when the work was completed. Check to see if the shaft is plumb. See if the agitator blades are bent or damaged.

The HEA 300 beams are only 290 mm high. For a span of 12 meters that is a span/depth ratio = 41. A very high value. I would expect to see a ratio of 20 or 25. Check the simple things first, but alteration or replacement of the beams may be needed. Static deflection of 4.8 cm for a span of 12 meters is way too much, L/250. Static deflection of, say 1.5 cm, should be better, L/800.

Equipment of this type should give many years of reliable service, without serious vibration, but proper maintenance is essential.

http://www.SlideRuleEra.net

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[Ron]

SRE....nicely stated!

 

[SlideRuleEra]

Thank you, Ron.

http://www.SlideRuleEra.net

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