We use bubbler systems to measure level in our scrubber's slurry tanks. Slurry is composed mostly of limestone and sulphate from a flue gas desulphurization process. Our problem is the constant plugging of the dip tubes; we have tried different combinations of air flows and pipe diameters but still need to clean the tubes every 3 or 4 days, or even more often in some cases.

Is there a guideline for selecting proper tube diameters and air flowrates to minimize plugging?

As additional info, plugging sometimes takes the form of a solid crust or ring of solidified slurry a few inches above the end of the dip tube, as if the bubbles dried up and plugged the tube. In other cases it is simply a semi-wet mass of slurry that plugs the tube but washes out easily; this could be due to a momentary pressure loss allowing slurry to get into the tube, but we have worked to eliminate pressure outages and personnel has been trained to never shut down air, so I would believe it has more to do with improper airflow.

Any suggestions or comments will be appreciated.

 

RD
I'm not a chemist but check out the reaction of the air with the fluid (we had to modify the Ph of a chlorine solution to prevent calcification at the diffuser).

You may have to use another gas for the bubbler.

Dennis

 

I have seen bubbler systems with clogging problems due to inproperly prepared/installed tubes. The end of the tube needs to be mitered so that it's opening is level thus forcing the bubble to seap out uniformly.

 

From the description of the problem, it would appear that you have two plugging phenomena occurring, which may or may not share common causes.

In the case of the solid ring (and the semi-wet solution if it is clogging up into the tube), it is apparent the the tank's material it getting up into the tube. It is doubtful given proper purge gas flow (see the next paragraph) that the material is "bucking" the flow "pressure" somehow to allow sufficient material to rise in the tube a few inches and cause the pluggage. Generally the only mechanisms for this are molecular motion and diffusion which I don't believe would cause a pluggage a few inches above the outlet. This could be, however, be caused by inadequate supply pressure allowing the material to rise in the tube, however, the constant flow regulators (typically a constant differential pressure relay), if set properly (for good even bubble formation at minimum flow back pressure) with adequate supply pressure (>10 psi over maximum tank pressure plus level head - this should include consideration of pressure swings in the tank), should be able to correct for this. Obviously, long term pressure outages were the tank material enters the dip tube could still lead to pluggage (if this is the case some people use a volume accumulator and check valve as a back up to provide a pressure reserve and to protect again low supply pressure variations). A second possibility and more likely is that you don't have uniform bubble generation. The end of the dip tube is typically of a "V" notch, a "L" shaped construction, or an ellipse (~45-60 degrees). If the bubbles are not uniform, it may be possible that you are getting some back flow up in the tube when the bubbles are being released. Think of a partial or irregular bubble that when the bubble is released forces slurry flow up on the back side of the bubble (sort of rolls up, forcing a small amount of slurry up the tube on top of the bubble). For a dip tube with a diameter of 2", a guesstimate that the back flow might reach about 1-2" above the dip tube outlet. The correct notch at the dip tube end may be determined based the slurry material, pressures, and flow rates and some testing or experimenting may be in order.

For the second problem of semi-wet slurry mass, this could still be caused in part by improper bubble formation but in addition there is probably a process problem (change in slurry properties or temperature) or a reaction with the bubbler gas. It would seem that the slurry would have to cause a build up with high surface tension to resist the flowing pressure so a higher purge flow rate may cure this problem (however, there is a limit to increasing the flow rate as you can get back pressure due to flow rate which can cause errors in the measurement). One way do this is to increase your purge flow when the tank is essentially full until you see a back pressure affect the measurement and then back off say 25-50%. If this works with no plugging, start backing off by 10% until you experience the problem again then raise the flow rate to 25% above this. This method should lead to an effective purge rate while minimizing the purge gas usage. The flow rate should not be less than 1 CFH.

As to solutions, the first is to get the correct notch at the end of the dip tube, with adequate tube size (typically 2" dia for a tank based on the necessary mechanical strength and good bubble formation in the fluid being measured) then work on the correct flow of the bubbler gas. As you can see this is somewhat of an art for difficult applications.

If you are still having problems, you might try an automatic blow down system that periodically blocks the pressure transmitter and blows the tube down to clear it.

A possible (though untried) solution for reducing the build up frequency is to polish the inside of the tube to a point about 2 inches above the build up. When things deposit out in a buildup, the available surface area is a factor. When the surface is "rough," the surface has a higher surface area with more sites for the build up to form on. While this probably won't cure the problem, it may reduce the frequency.

The below web site has some information on bubblers:
http://www.omega.com/literature/transactions/volume4/T9904-12-PRESS.htm<http://www.omega.com/literature/transactions/volume4/T9904-12-PRESS.htm>

William (Bill) L. Mostia, Jr.
Principle Engineer
WLM Engineering
www.wlmengineering.com<http://www.wlmengineering.com/>

This information is being provide on a Cravat Emptor basis.

 

Thanks a lot for your comment, actually our dip tubes run straight down vertically into the tank and thus the tube end is straight with no mitering. Should tubes in bubbler systems be inserted angularly into the tank rather than straight down? Also, what about dip tube diameter and airflow? If incorrectly chosen, can they also cause clogging, or as long as it is bubbling it makes no difference?

 

We have a number of applications using bubbler & pressure transmitter technology on slurry tanks. We also use similar bubbler & differential pressure transmitter technology for indirect measurement of Slurry density.

The only solution to plugging that work for us was a automated line purge system.

We use a constant flow (Moore) relay to provide the reference air flow through the bubbler tube, measuring the pressure. We installed a full flow solenoid valve between the transmitter, and the point where the flow relay connects to the bubbler tube. A similar solenoid is also connected from a high pressure instrument air source to the same point where the flow relay connects to the bubbler tube. The two (2) solenoids are controlled in such a way as to Block and Hold the Transmitter sense point, and then apply the high pressure air to the bubbler tube, "blowing" any collected material off the end of the tube.
Durations & Intervals were determined by test. The high pressure solenoid is closed, and after a duration long enough to allow the pressure to stabilize back to the measurement valve, the sense solenoid is re-opened. It works well. We've used this purge method on all our Flyash pressure Monitoring systems successfully as well.

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T.W.(Wade) Carlson, P.Eng.
Senior Instrumentation & Control Engineer
System Manager, Engineering Services
System Administrator, Shand Project Data Handling System
Instrumentation & Control Department
Engineering Services Division
Power Production Business Unit
SaskPower Corp.
2025 Victoria Ave.
Regina, Saskatchewan
Canada S4P 0S1
Tel# 306-566-3220
Fax# 306-566-3219
internet email [email protected]

 

We use Righton Instrument's auto blow down system. Never have a problem in slurry applications.
http://www.rightoninst.com

[email protected]

 

I once saw a solenoid valve driven air blow down system on a clay silo hopper in a cement plant. At preset intervals it would send a blast of air around the neck area of the hopper thereby clearing it of any plugging. The manufacturer called it a "cannon". In your case some such adaptation either canned or homebrew could be a solution.

Tomy Zacharia