# Air bubbler liquid level system



## erics37 (May 7, 2009)

The service call began as "The low air pressure alarm light is on and nothing will run in Auto mode" they had a guy standing there manually pumping the wet well down.

After I figured out how the system works I had to troubleshoot it. Turned out that the 4-20 signal that this bubble level thing produced was part of a series loop also including a digital display on the front of the cabinet. Analog signal was fine through all that, but it ran through a loop isolator which is what wound up being the problem. Somehow it was screwing up the signal to the PLC so the Low pressure alarm was false. Bypassed the isolator and it works for now, I think the loop impedance is low enough to survive a couple days.

This system also runs literally EVERYTHING through the PLC. I think that's kind of a dumb idea. If the PLC gets knocked out for some reason you wouldn't even be able to run anything manually without wiring some jumpers and crap in there. But that's just my opinion I 'spose.


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## sparky970 (Mar 19, 2008)

Bubblers work great unless the dip tube gets plugged


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## denny3992 (Jul 12, 2010)

so how does it work( i do alot of wwtp service work also) and would like to know


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## buddhakii (Jan 13, 2011)

I see them a lot around here but only used for level indication. Float switches are used to run the pumps. I don't quite understand how everything is run through the plc. How did they run the hand operation through the plc? 

A bubbler works by pushing air through a tube to the bottom of a tank. The amount of pressure required to push the air correlates to the level of liquid. The more pressure required the higher the level.


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## buddhakii (Jan 13, 2011)

Come to think of it I do know of a couple lift stations that use the bubbler to run the pumps. There is a level indicator on the front of the panel filled with mercury that has the controls wired to it on the back so when the pressure builds it pushes the mercury higher in the sight glass and closes the circuit wired to the back when it reaches the contact points.


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## JRaef (Mar 23, 2009)

I've built a lot of bubblers in my day, long before people were willing to trust PLCs on lift stations. The original method was to use multiple Magnehelic gauge/switches, usually Dwyer, with each one set to the different levels. Then E&G in Florida came up with the mercury tube idea in the 80s which was copied on the west coast by a company called Tesco Controls, and both of them became giants in the lift station industry for a while because it became difficult for custom panel builders to compete with that. But once the industry came up with reliable solid state pressure transducers, even the mercury guys had trouble competing against that, which made it so that any panel builder could make them again. 

Bubblers are used over floats because in some collection and lift station designs, there is no screening of the influent, they use what are called chopper pumps, grinder pumps or just non-clog pumps. That means rags, kotex, roots, diapers (yes, people still flush them) etc. can hang up on floats and cause them to not indicate, or falsely indicate. Bubblers are more reliable (though not perfect either), but more expensive. It's a trade off usually decided on at the Civil Engineer level. 

As to the issue of running everything through the PLC, that's a more recent development. But if you think about it, everything is dependent upon that little compressor too, and it is far more likely to fail than the PLC. I recently worked on one (made by Tesco interestingly enough) that used trapped air, so no compressor. It had to have an atmosphere compensation tube however (because changes in barometric pressure would affect it) and that tube plugged up with mold. So there is no perfect system.


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## erics37 (May 7, 2009)

buddhakii said:


> I see them a lot around here but only used for level indication. Float switches are used to run the pumps. I don't quite understand how everything is run through the plc. How did they run the hand operation through the plc?


This was being used for level indication AND control, and there was no backup floats or anything. I told them we'd work up a price to install a high-high and a low-low float, at the very least.

There were literally NO relays in the control cabinet (okay there were a couple for the auto-dialer). The HOA switches on the front went directly to PLC inputs, and the logic decides to run the pump controllers (or not) straight from the output card.



JRaef said:


> As to the issue of running everything through the PLC, that's a more recent development. But if you think about it, everything is dependent upon that little compressor too, and it is far more likely to fail than the PLC. I recently worked on one (made by Tesco interestingly enough) that used trapped air, so no compressor. It had to have an atmosphere compensation tube however (because changes in barometric pressure would affect it) and that tube plugged up with mold. So there is no perfect system.


If the compressor failed, you could still manually pump down the wet well. If the PLC failed, you couldn't unless you got creative and started running some jumpers and stuff.


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## John Valdes (May 17, 2007)

When I worked for SFWMD, every structure (dam) used this system to replicate the water level on the fresh water side and the salt water side.
Its function was to open the dam when water levels on the fresh side were high enough. It was also imperative that the dam gate never open if the salt water level exceeded the fresh level.
This was to prevent salt water intrusion.
The switch itself was a mercury switch actuated by a bladder.


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## micromind (Aug 11, 2007)

I don't think I've seen a bubbler system with only one compressor. Always two. 

Also, with as much lightning as we get around here, most of the manual controls bypass everything. Put the H-O-A switch in manual and the pump runs.


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## JRaef (Mar 23, 2009)

Yeah, it is odd that even the Hand mode goes through the PLC. It's often necessary to have a low-low level cutoff that over rides everything so that when in hand, they can't run the pumps dry, but that's usually a separate float switch or something as a last resort. There are some pumps that are OK with being run dry though, so it's not as big of a deal. Flygt pumps are like that, you can run them down to where they suck air and leave them like that for a long time. They may end up over heating, but they have a separate heat sensor in the motor to protect it.


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## micromind (Aug 11, 2007)

denny3992 said:


> so how does it work( i do alot of wwtp service work also) and would like to know


A bubbler system is a way to determine the level of a liquid in a vessel. 

It has a small air compressor that is connected to a tube, the end of the tube is near (but not at) the bottom of the vessel. 

There's a tee in the tube inside the control panel, and a pressure transducer attached to the tee. The output of the transducer is connected to some sort of control (usually a PLC) with pre-programmed setpoints which will start and stop the pumps. 

If there's no liquid in the vessel, there's very little pressure in the tube. As the liquid level rises, more pressure is needed to push the air down the tube. This pressure is read by the transducer in the control panel.

It's called a bubbler because there's always bubbles of air from the submerged end of the tube. 

The best backup I've seen so far (yeah, I'm biased; it was my idea......lol) is to have a high level float above the normal operating liquid level. This way, the float doesn't get any debris on it. 

If the liquid level trips the float, the pump starts; bypassing all other controls. It runs for a set amount of time then stops. It also trips an alarm. 

While a low float could be used to stop the pump, since it is always submerged, it can fail due to becoming entangled is debris in the liquid.


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## micromind (Aug 11, 2007)

JRaef said:


> Yeah, it is odd that even the Hand mode goes through the PLC. It's often necessary to have a low-low level cutoff that over rides everything so that when in hand, they can't run the pumps dry, but that's usually a separate float switch or something as a last resort. There are some pumps that are OK with being run dry though, so it's not as big of a deal. Flygt pumps are like that, you can run them down to where they suck air and leave them like that for a long time. They may end up over heating, but they have a separate heat sensor in the motor to protect it.


Another reason for the low-low cutoff is because some pumps must be completely submerged in order to be explosion proof.


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## dthurmond (Feb 7, 2011)

We have a lot of controls like this most if not all Are dywer magnohelics and photohelics . They are used in our process tanks and we also use them in return airflow tunnels . They are very reliable and don't fail much . The photohelics look at how much pressure it takes to push a air bubble though a tube . The magnohelics look at the pressure drop across filters .


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## Jlarson (Jun 28, 2009)

We build our own from scratch to fit the application. We do them for process and pump stations, water resource measurement in either active or research wells and for remote canal and stream measurement.

You can also set up a two level bubbler to measure specific gravity.


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## erics37 (May 7, 2009)

micromind said:


> A bubbler system is a way to determine the level of a liquid in a vessel.
> 
> It has a small air compressor that is connected to a tube, the end of the tube is near (but not at) the bottom of the vessel.
> 
> ...





micromind said:


> Another reason for the low-low cutoff is because some pumps must be completely submerged in order to be explosion proof.


I like the high-high float backup because it'll run the pumps come hell or high water (pun intended) :laughing:

In this case the station would need the low-low, these pumps won't suck air for long.


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## ScooterMcGavin (Jan 24, 2011)

We use bubblers with a warrick (conductivity probe) as a high high backup in case the bubbler plugs up. Are biggest concern is high level because of spill issues. The pumps are controlled by the Plc but each one has an emergency key override switch which bypasses everything except the motor overloads. If an operator activates the emergency override it alarms the central control room also.

As of late we have been moving away from bubblers and going with guided wave/non contact radar.


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## erics37 (May 7, 2009)

scameron81 said:


> As of late we have been moving away from bubblers and going with guided wave/non contact radar.


Please elaborate! How do those work?


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## sparky970 (Mar 19, 2008)

erics37 said:


> Please elaborate! How do those work?



Guided wave can be a probe or cable (attached at the bottom of the vessel). The transmitter is set up with information (typically probe length and a dead band area at the top). It then sends a pulse down the probe. When it hits the surface of the fluid, the signal is reflected back to the transmitter and it measures the amount of time it take for it to reflect back. This will correlate level. There's more to it, but that is the simple explanation. One advantage of guided wave is that it's not affected by specific gravity.


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## CTshockhazard (Aug 28, 2009)

Back in the day, we used a bubbler in between the official band rehearsal and the freeform jam!


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## JRaef (Mar 23, 2009)

CTshockhazard said:


> Back in the day, we used a bubbler in between the official band rehearsal and the freeform jam!


Yeah, but that one only had a high limit feature... when you got too high, you couldn't hold the bubbler tube any longer, thus limiting how high you got.


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## erics37 (May 7, 2009)

JRaef said:


> Yeah, but that one only had a high limit feature... when you got too high, you couldn't hold the bubbler tube any longer, thus limiting how high you got.


That's why you need to get a dual-channel bubbler.


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## D-Bo (Apr 15, 2012)

erics37 said:


> That's why you need to get a dual-channel bubbler.


That, some caramel sheesha, devils lettuce, and a jethro tull vinyl


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