Agitator vibration intermittent problem with unsteady liquid level

[plantprowler]

We have a new reactor fitted with a dual level agitator (4 bladed pitched turbine) that has been having an intermittent vibration problem. The operation in this vessel starts with only the bottom agitator covered by liquid & then as the batch proceeds the liquid level rises till eventually it goes approx. 3 ft above the top agitator.

The vessel has been tested under the full condition & shown no vibration. The shaft has a steady bearing at the bottom. The vessel has 4 baffles but only full width baffles (8% baffles) till half height after which they taper down.


Is it likely that there can be an intermediate liquid level which induces this vibration? Any theoretical basis to analyse this problem?

Would there be reason to suspect the point where the liquid level just barely reaches the top agitator? Could this contact initiation point between liquid & agitator be a vibration inducing cause?

 

[Compositepro]

Running agitator blades right at the liquid surface often causes problems because the level changes when liquid is splashed away from the blades. The blades should be full submerged or fully out of the liquid. One solution might be to use a variable speed drive to slow down the agitator when there is a problem, but this requires operator attention which is unreliable. Or, change the mixing procedure.

 

[plantprowler]

Thanks!

Is there an agitator design that will give good agitation irrespective of the liquid level (within limits, of course)?

I was thinking shorter bades more frequently on the shaft? Or perhaps a long baffle-like blade on the shaft?

Any ideas?

 

[geesamand]

Impellers will generate much higher lateral forces when the liquid level is at/slightly above the impeller blades. I don't believe the baffles have much to do with it.

Vibration is one negative issue - but you could also be taking big bites out of your shaft fatigue life if it's not designed for it mechanically. Contact your agitator supplier and see whether that's part of the design, and if so, whether it's suitable for constant operation at those liquid levels or only intermittently.

Yes, dividing the power draw over more impellers will lower the overload contribution of any one impeller. Multiple impellers generally stabilize each other in a way that reduce the bending moment at the gearbox, which also helps. But multiple smaller impellers could hurt mixing performance, so take care. Some may suggest you use impellers with stabilizer fins but in my experience they don't perform as advertised. (The concept of a stabilizer fin seems intuitively helpful, but the lateral velocity of the unstable impeller is so low, and the face of the fin is so small, that the fin provides effectively no stabilizing reaction force.)

Each impeller has a liquid level that will be most unstable - most operators will pump down through it quickly or just interlock operation of the mixer to avoid running during those levels. If the steady bearing is used you might replace it to restore the original clearances and minimize the vibration.

David