Rotating water ring thickness measurement

[pomp]

We have a device which has a 400mm diameter drum rotating from 400-800 rpm. Inside the drum there is a water ring, which thickness varies depending on flow rate, drum rotational speed and pressure. We have measured the thickness of this ring (actually the air column) with a capacitive type sensor. We have had problems with this and would like to measure the thickness of the water ring by placing a measurement on the rotating drum. I have measured pressure on the drum with some success, but since the water ring has a different tangential speed than the rotating drum at different operating conditions, it has made it difficult.

Are there any suggestions on another measurement technique?

 

[CARF]

What is this equipment?

Looks like you want to measure the pond depth in a sort of decanter / centrifuge?
Maybe an ultrasonic level measurent could do the trick from the bowl centre?

MVD

 

[pomp]

Our device is called a POMp and is used for removing air from process water.

The thickness we wish to meassure is not a pond but the thickness of a water ring inside the rotating drum. It is basically 3 concentric cylinders. The outer is a 316L stainless steel drum, the second being a water ring of 50 mm thick and the third an air column in the center. The water ring rotates with the drum and has an axial flow velecity. Process flow is entering at one end and leaving at the other.

 

[CARF]

Hmmm,

Interesting piece of equipment. Still it looks like a centrifuge, but here you want to separate air from water. OK, first some silly questions, then some out of the box thinking... Why do you want to know this water level? How would you measure this with pressure? (If you put a pressure sensor on the rotating bowl, you only would measure the 'centrifugal pressure'?)

Let me guess, the lower the flowrate and the higher the RPM the better the separation? How do you get the separated air out?

If the water ring is 50 mm thick the water has a certain measurable weight. Why not put the whole thing on load cells? If not too many vibrations this might work.

I'll ask my decanter peers for advice,
MVD

 

[pomp]

Our device actual is a centrifuge, but with continuous flow through it.

We want to know the water level, because air removal efficiency is dependant on how thick the water ring is. Too large or too small a thickness and the air removal efficiency goes down.

The rotating pressure sensor's output is equal to the water's density times the thickness of the water ring times the acceleration (P = r*a*h). I know the pressure (P) from the sensor, the density (r) and can estimate the acceleration (a) because I know the rpms.

We have a tube that goes down the center of the device and into the air column. Air will level by itself or a vacuum can be added.

The load cell just came into mind yesterday when talking to another person. Can the load cell detect a 2kg change (1mm thickness change) when the total weight is 1000kg?

Thanks for your interest.

 

[nbucska]

You could remove air better by subjecting the water to vacuum.

Neutrons are specially good to measure water thickness.

Since the surface is cylindrical, couldn't you use the
concentration/dispersion of light to measire the radius ?



<nbucska@pcperipherals.com>

 

[CARF]

Hi pomp,

Well load cells probaply will not give you that high accuracy. But why do you want this accuracy? You say the systems works best when the level is not too low or not too high. So you only want to 'control' to a desired level for maximum separation performance. I cannot believe that efficiency will go down a lot when you change the level a few mm's? Otherwise it will not be very usefull in a real plant, where in-flow will change (despite in-flow control).

OK, if I'm correct and you just want to control the level to say 50 mm +/- 10 mm, here's another idea.

It's process water, so I may conduct electricity. Now just built a 'ladder' of 3 electrical contacts across the desired level. Now you have three options of flooded contacts.

- 60 mm = high level (lower in-flow, increase RPM)
- 50 mm = correct level (no action)
- 40 mm = low level (increase in-flow, lower RPM)

You may appply 5 contacts(or even more as well) which will give you more controllabilty.

Hope this helps,
MVD

PS. Is there any URL where I can find this equipment?

 

[pomp]

nbucska,

We would like to measure the thickness externally. For light, we would need a window. The process flow is basically water, but there is a small anoumt of wood fiber and other particals that would destroy the window.

MVD,

Thanks again for your comments.

Concerning the load cell, I also feel it will not be a good solution. There are also issues of load changing depending on outlet pressure, and how much load is transfered axially.

You are correct that the thickness accuracy does not have to be 1 mm. However our device is connected to a papermachine which demands high stability. The controls will have to be adapted.

The idea of a &quot;ladder&quot; came to mind also. However we do not want any thing to prodrude into the water ring. This could cause dirt buildup and release and possible turbolence which could mix are into the system.

At the outlet of our device, the impeller is sort of like a cover to the cylinder. I thought I could drill through the impeller and place insulated pins, (so the pins don't make contact with the impeller which is in constant contact with the water). One problem is what kind if insulating material I could use. The process water has 0.1% wood fiber and fillers which are abrasive and can wear out the insulation.

You can find our products at

http://www.pom.fi.

Under products go to POMp degasser.

 

[CARF]

You could try smoothing the contacts in the bowl, so not a 'ladder' of contacts, but a flat strip.

Please take a look at:

http://www.wsus.com/

Centrifugal decanters. Here the level is controlled with a &quot;Varipond&quot; an adjustable ring system at the outgoing flow.

Cheers,
MVD

 

[CARF]

In the Westfalia decanters the liquid radius can be changed automatically using the patented Varipond-P adjusting weir by adjusting the system air pressure. This system allows real-time pneumatic adjustment of the pond depth (liquid level) without the need to change mechanical parts.

http://www.westfalia-separator.com/de/idx_food.htm