Vibration in an open topped agitated vessel

[PeteE]

I have an open topped, agitated vessel, taking a flow from an air stripper. At flows greater than 130m³/hr the vessel starts to "oscillate" and flex. This is rather worrying as it is a GRP vessel and the oscillations could damage it.I believe the vessel might be slightly scaled which might be emphasising the vibrating effect but the agitator motor doesn't appear to be out of balance or making any strange noise, which is what we would espect if it were scale.
Is the vessel undergoing harmonic vibration, or vibrating at its resonant frequency and possibly how do I combat the effects of this, or even how do I measure this if is resonating?

Thanks from
A rather worried Chemical Engineer

 

[cloche]

Do you have the opportunity to analyze it with a FEM model, doing a modal analysis? If so, you can extract eigenfrequencies and modes and compare to the freqs of the motor and the blades. If it's a resonance problem, it should reach a maximum in a relatively narrow freq range and then decay: do you have the opportunity to go farther than 130 m^3/s? If you already tried this and it always oscillate starting from 130 m^3/s and up, I'd tend to exclude resonance (or, several eigenfrequencies could exist one near the other from 130 m^3/s up to your max flow capacity). Could also be related with fluid flow instability starting at a certain flowrate: this is more difficult to predict (you need CFD) or to measure (you need pressure gages).
Vibro-acoustics measures do can find out resonating modes experimentally: contact some specialists in this field, it shouldn't be very difficult to find some near you.

 

[PeteE]

Thanks for the reply . Vibration continues above the 130 m3/hr flow rate so will start to look for expert help.

 

[m777182]

I would try to put some obstacle at the inflow point to spoil the smooth flow into a more randomly twisted flow with no predominant frequency.
m777182

 

[sms]

Since it is open topped I assume you can observe the fluid in the vessel. Is there a vortex pattern forming? Is the the fluid sloshing at that flow rate? Can you adjust the level in the vessel, sometimes these problems are easily solved by running at a lower or higher level in the tank, since the natural frequency is a function of mass, reducing or increasing the mass in the vessel may help.

 

[mtnengr]

If you think that you may have a problem, the easiset way to alter the natural frequency of the system is to install a bubbler at the wall. This will dramatically reduce the frequency of the system.

If you provide me with the dimensions of the tank, its wall thickness, material, properties, fluid density and bottom conditions, I can provide you with the theoretical frequencies of the tank. No charge.

Use

http://www.volcano.net/~d.citerley

to transmit the info.