wobbling agit. shaft due to vortex in tank 3

[micjk]

What do you call the wobbling effect on an agitator shaft caused by a vortex in a mixing vessel?

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

 

[unclesyd]

Shaft Failure!

You need to install side mounted baffle plates, add a bottom support/bearing to the agitator, change blade design, or slow the agitator down.
There are several other things you might be able to do but these are the most common.

You have got to stop the wobble in the shaft.

A quick visit by a tech rep for agitator drive could be of some value.

 

[micjk]

excellently put unclesyd, unexpected, but still excellently put. I am well aware of iminent shaft, bearing, gearbox, etc. failure if this is not corrected, but thought there might be a technical term for this. I found this during vibe data collection. I could see this vessel "shaking" and first instinct was bent shaft or debris built up on the prop. So I opened the vessel and used a digital camera with video capture to prove it. No debris nor bent shaft, but one heck of a vortex, that when it got to a certain position relative to the shaft the shaft started whipping. It is now in the hands of my "Higher-Ups" as to a solution. Oh by the way I'll have to give you another star for your answer.


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

 

[quark]

Did you check runout and float in the shaft? Generally propellers do create vortex(a big one) in the center and that may not be a problem. If you have a flush bottom in the bottom center then extra support at the bottom may not be a possibility.

I suggest you to check the design fluid parameters(density and viscosity) for any abnormal deviation.

Secondly check for shaft inclination.

Regards,

 

[unclesyd]

While I'm trying to get in touch with my old "agitator" buddy several things came to mind inline with quark's post.
We had 2 instances that I was involved in with a similar problem where the bottom hub on top supported agitator was bent. This caused the blades to be skewed which in turn caused a vibration and shaking as you describe. The problem was corrected by straightening the mounting hub to restore the correct pitch and angle of attack of the blades.
The second one was more involved. Besides straightening the hub the blades were modified by drilling one or two holes in the blade.
Both these agitators were center mounted and top supported.

Come back ith a few more details of your setup.

 

[micjk]

This is a top, off center mount set up. A philidelphia PMG-2 mixer. The gearbox has a hollow shaft that the agitator shaft (@1.5" dia.) slides into and is secured with 1 through bolt and two set screws. We are mixing a low viscosity slurry. The shaft is @ 5 ft long with (2) three blade propellers the bottom one having a stabilizer ring. We have three of these vessels on a skid. I recently caught one of these about to lose a shaft. The through bolt was sheared and the holes for this were elongated horizontally. Caught it with vibration and a strobe light. I have the video clip if you are interested. Send your e-mail address to roy.gariepy@dorlastan.com
Kind regards,

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

 

[quark]

Can you send the photographs to builblock1@yahoo.com ?

 

[StoneCold]

Sometimes you can get into a situation where the liquid level in the tank when it is vortexing is the same level as the agitator blades. This causes the blades to be partially exposed, depending on fluctuations in the vortex level. This causes huge stresses on the shaft. You can see the shaft wobble a lot in this situation even if the upper mechanicals are in good shape. This condition causes a majority of our seal failures. However convincing the operators that is the cause is another story.

 

[unclesyd]

I got a call back from my agitator buddy this morning.

Normally if the mixer is mounted off-center it needs to be inclined so it faces into the direction of the swirl/vortex. If this is a fixed mount on a horizontal/flat flange face then a baffle or two would be in order. He also noted that all mfg’s now have a CFM analysis that should be able to jump right on your problem whereas he worked from notebooks and tables. He also stated that it has been his experience that the connection you have between the drive and shaft is not his first choice.
He asked was this a change in service from the original design?

 

[DMAB]

We are designing ASME Sec VIII flat head vessels and need an agitator shaft to protrude out through head. ASME VIII does not address any material requirements for shafts etc.

Is Shaft an integral part of a pressure containment system or not . As per general engineering practices shafts design criteria is different than those of pressure containment system (Pressure vessels). To comploy with the ASME Code, what material shall be suitable for use as ASME SEC VIII applications?

Thanks
M. A. Bhutta

 

[unclesyd]

DMAB,
The shaft itself isn't considered part of to the vessel. You are to use the best available practice and compatible materials for it's construction. If possible the mixer shaft seal should be of a standard design like ANSI or API this is not a requirement it just saves a little design verification work.
The work comes in the form of the design of the vessel penetration where the shaft is located. This penetration has to meet the requirements of the jurisdictional vessel codes as a minimum and needs additional consideration given to the mechanical loads imposed by the mixer.
Most of the mixer suppliers will work with you on the mixer and seal if you supply the compatibility of materials to your process. Most will not verify the vessel penetration design this is up to the vessel designer.

 

[unclesyd]

micjk,
There is somewhat of a consensus, more like a plurality, on this end that a description of the your shaft during operation is called "Whipping". The problem encountered in the discussion was whether the shaft had a circular orbit or was following a figure 8.

 

[DMAB]

For agitator in flathead vessels, we are considering use of ASTM A108 material. Agitator shaft is used for low speed and low torque service. Can any one comment the suitability of this material. If this is not appropriate material, then suggest a suitable material.

DMAB

 

[quark]

micjk!

The renewed yahoo mail facility grants 10mb of single file transfer and 100mb of mail space. Can you remail it?

Regards,