Agitator vibration problems during mixing

[Sorex]

Hi,

I have vibration problems using an agitator in a mixing tank. The tank (1000 litres) contains water and gelatine is added. After the gelatine has been added a vacuum is applied to improve the solving of the gelatine. When using small batches, say 200 litres, large agitator vibrations at high speeds are present. These problems are new, this has not been a problem for years.

Further info:
- material shaft and blade: Stainless Steel 316
- agitator is positioned in the middle of the tank; straight downwards
- max speed: 1750 RPM
- diameter blade: 10’’
- diameter shaft: 55 mm
- tank height: 1500 mm
- distance between blade and bottom: 200 mm
- tank diameter: 1000 mm
- 2 bevels
- Critical speed appr. 750 RPM

Action taken until now:
- balanced blade, shaft, cooling fan and motor
- Reduced total deflection of agitator to 0.13 mm
- Replaced bearings

Cavitation is also a possible cause we’ve looked at… Only thing we changed is the supplier of the shaft. Both shafts are exactly the same as far as we can determine. Anyone any other ideas??? Something I’m missing completely??

Thnx!!

Grtz, Sorex

 

[sms]

When mixing these small batches is the impeller completely submerged? Impellers that are partially submerged cause vibration problems. Is there a steady bearing at the bottom the shaft? Unsupported impellers are just a problem waiting to happen. We always use a steady bearing or bushing. Is it possible to observe the mixing process or is the vessel completely enclosed? Observing the impeller and mixer often gives excellent insight into the problem. Are the vibrations at impeller running speed frequency? If the vibration frequency is a fraction of running speed that may be associated with a fluid excitation of the rotor critical speed. Another reason for a steady bearing. If it is a mixing issue, the manufacturer should have CFD models that can help.

 

[Sorex]

Thnx for the reply. To answer your questions: the impeller is partly submerged during these small batches. The impeller is unsupported and the frequency of the vibration is not at the running speed, which should indicate the fluid excitates the 12 Hz critical shaft frequency. All mixing info I have read untill now indicates our mixer is far from ideal. For instance, the position is the impeller is way to low in the tank according to mixer-design rules. The strange thing is though.. it has been working fine for 10 years, and suddenly we're having problems with vibrations that cause the shaft to bend.....

 

[btrueblood]

Only the shaft is changed, or is the impeller/mixer blade a new design also?

 

[micjk]

Sorex,
As stated above can you look inside while mixing? Is the frquency constant or does it come and go? We had a similar phenomenom occur with one of our vessels. When going through rinse cycle, low fluid level we got some wierd vibration. I looked inside and there was a vortex. When this vortex got around the shaft it caused it to start whipping, at times as much as 1 1/2 inches.

Roy Gariepy
Maintenance and Reliability Dept.
Dorlastan Fibers LLC.
Goose Creek, South Carolina USA

 

[MikeHalloran]

Do you have the old shaft?

I'm wondering if maybe it's hollow, and has a loose slug or something similar inside it to dampen vibrations.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[Sorex]

The shaft is completely solid. The design didn't change and the same impeller blade was used.
A possible cause for the higher vibrations can be the following: the old shaft was completely forged and the new one has a welded flange. Does anyone know if this can have an influence on the process? The material of both flanges has been investigated and the old shaft had a smaller grain, which indicates a difference in plastic deformation between the two. The modulus of elasticity is the same though and that should be the only parameter that would influence the shaft's behaviour....
We are now investigating the possibility of using a high speed camera during tests in the vessel.

 

[APH]

Welded joint has to be looked carefully, especially when it is rotating. You could have crack(s) on weld joint that is not visible by naked eye. This would cost a little effect when there's no load on the joint, but when the you have additional load apply to it, there is a chance that you're reducing the local the joint stiffness, thus effecting the system natural frequency.

If it run smooth before, that means something has changed. Isolating problem might be a start ( cost time and money..lots of it). Is this a rigid joint (no shock mount installed)?

Look at the surface finish of the shaft arounf the bearing surface. Look at the bearing itself. Improper bearing installaLion cost huge vibration issues especially when the shaft is only supported at one end. Type of greese you use in the bearing unit can play a role. If you use ball bearing, if one of this ball bearing is just slightly dent, this would create un-balance system resulting in noise (energy). When this small energy transfer to a system, it could disrupt the system natural frequency.

my 2 cent.
APH