I am in a debate with my friends. My point is that in bubbler level measurement, we only need a pressure regulator, a needle valve and a pressure gauge. Through pressure regulator we set the value a little above the head pressure of measuring liquid in an open tank. Needle valve, which is installed just after pressure regulator, is used to set the optimum rate of bubbles. Once the desired bubble rate is achieved through calibration, there is no need to change the opening of needle valve.

My friend says that we have to have a rotameter which should maintain the flow rate of bubbles through some sort of built-in control valve. Can anyone decide through equations who is right?

 

You need a differential regulator which can produce a constant gas flow rate under varying head pressures. The common everyday regulator is not a constant flow regulator but a constant pressure regulator.

The needle valve on a rotameter will perform the same flow regulating function as a needle valve with no rotameter. The rotometer provides only visual indication of the gas flow.

David

 

No, you don't need a rotameter, in fact you don't even need a regulator or needle valve. You just need some method of establishing a small flow. I have used the gauge snubber fittings that have a sintered metal flow restrictor. Merium make a nice device, it has a needle valve and a glass bowl. You fill the glass bowl with light oil or glycol and set the needle valve to give you a bubble every second or so, the flow seems to stay constant for months on end. If your transmitter is far away from the bubble tube it's usual to introduce the bubble gas near or at the bubbler to avoid pressure drop due to changes in flow.

I saw an installation once where they had a gauge and selector valve in the control room connected to bubblers in several tanks about 100 meters away. To measure the level the operator selected the tank and then puffed up the line with a squeeze pump similar to the old fashion blood pressure monitor. once the gauge reached max the operator allowed it to stabilize then read the level.

Bubble tubes are very low tech, but the firm I work for still uses them for level, interface and density measurement for a very nasty application in glass lined vessels (Tantalum bubble tubes). They are also still quite common for flotation level in mining.

Your friend is referring to a constant flow regulator, some people spec them but not really necessary.

Regards,
Roy

 

David,

That's what I was taught also, but in reality if you connect the bubble tube to the transmitter and the gas supply to the bubble tube it makes no difference if you supply .01 lpm or 10 lpm of gas, the level reading will stay the same. If the tubing between bubble tube and transmitter is reasonably short, it's convenient to inject the bubble supply right at the transmitter that is purging any moisture or corrosive gas.

We normally use a 0-1 l/m rotameter supplied at line pressure (typically 80# N2) once the flow is set the rate hardly changes for empty or full tank. The change in bacpressure is insignificant compared to the drop across the needle valve.

Regards
Roy

 

The company I work for uses Siemans constant flow regulators (rotameters) to ensure a constant amount of air through both tubes of our DP bubblers. I can't think of any needle valve combination that would be able to ensure accurate readings between both diptubes (we use Rosemount DP transmitters). If you want, check us out at www.kingmech.com, the boss is currently updating the website for our Righton Instrument Bubblers.

 

All your asumptions are WRONG. First of all you need to know the relationship between pressure, density, friction loss, line length, air pressure, tube size etc.

The case the operator puffs air-- here the the density may not be changing and so is the level. Next accuracy may be +/- 10 % acceptable and the volume of the tank may be low! I do not know what you are measuring in the name of level.One needs to have a good understanding of bubbler type level measurements and the fundamentals.

Level and density can be measured by either by 1 dip tube or 2 dip tubes.

More air flow as a result of more air pressure will create losses resulting in errors in the measurement. You need constant air flow depending upon the head of a fluid that has to be overcome. If the level is constant most of the time and accuracy is of no consequence, a simple restriction will do---but this is a particular situation and is not a panacea!

A level measurement can have a liquid to be measured whose density is constant but only the level varies. I am not considering level measurement with variable density her.The bubbler air flow controller (not a rotameter-- since its valve has a fixed restriction) is used to send in constant air flow/bubbles to overcome the head/level in the tank. For a certain period of time the rotameter would appear to work but to be more precise, you need a flow controller to adjust the flow for varying pressure at the downstream side of the rotameter or shall I say bubbler.

You need to consider the tubing length, tubing size,friction losses,location of the transmitter before declaring the system is okay.

Simple restrictions and bubblers appear to be okay but in the real sense when density does not change and the accuracy of is of no consequence, to the untrained eye these systems seem to perform okay!

A good source of information is -- from Foxboro Instruments "Bubble tube installation- MI-020-328" PUBLISHED IN 1988. The contents are still valid and are good.

There is lot more to be covered, I am stopping at this!

 

Hi there roger,

I'm glad to have found someone who knows this topic well. I have a few questions which i hope you can help me out.

Quoting your previous post,

> ...The bubbler air flow controller (not a rotameter-- since its valve has a fixed restriction) is used to send in constant air flow/bubbles to overcome the head/level in the tank. For a certain period of time the rotameter would appear to work but to be more precise, you need a flow controller to adjust the flow for varying pressure at the downstream side of the rotameter or shall I say bubbler. <

Can you explain what you mean by the requirement of a constant air flow, and yet the need of a flow controller to adjust the flow? I'm taking the case where density is constant and level is varying.

I also wish to ask of the calibration requirements of a bubbler system during operation. There are systems which i saw that claimed self-calibration by timed-controlled purges to "ensure accuracy" and "auto-compensation".

What do they really do?

Thanks in advance!

 

> Can you explain what you mean by the requirement of a constant air flow, and yet the need of a flow controller to adjust the flow? I'm taking the case where density is constant and level is varying. <

This is what a 'differential' regulator does, it supplies a constant flow, by regulating the pressure. Hence the bubble flow is held constant against the variable, the head pressure of the water at the bottom of the bubbler tube. Monitoring the changing pressure reflects a changing level.

If the Foxboro note was too complex, here's a power point from Siemens, who now handles the former Moore "constant differential relay" used for decades on level bubblers.

http://www.lesman.com/unleashd/catalog/level/level_siemens_air-bubbler-system.html
(take out any spaces that the forum inserts in the URL)

 

The bubbler is one of the oldest forms of level sensing. The basic principal is that the air pressure required to push the tank fluid out of the bubbler tube corresponds to the liquid head above the end of the bubbler tube. You don't actually need to generate any bubbles, but bubbles indicate that you have more than enough air in the bubbler tube to measure the tank head. If you know the liquid density, you can calculate the liquid level from the air pressure required to generate bubbles. The rotameter in the air line is only there to show that air is flowing enough to generate bubbles, since it is generally not convenient or even safe to observe the tank to see if any bubbles are rising. However, looking at the rotameter is a manual operation convenient during startup, but not at any other time.

Air flow through the bubbler may have been adjusted when the tank is not full. As the level in the tank rises it creates greater force at the bubbler tip that eventually will cause the air flow to stop and tank fluid to displace air in the bubbler tube. The automation systems you mention are designed to keep the air flowing at all time so you do not need to go back to the needle valve at the rotameter to adjust airflow. Another solution is to always set the air flow when the tank is full. There is no inaccuracy due to excess air flow, but know that you are typically using dried instrument air to do the bubbling, and that is not exactly a "free" resource. Maybe the automatic airflow compensation would pay for itself, but somehow, I don't think so.

Dick Caro
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"What do they really do?"

Pretty simple really, in fact the simplicity of these devices is what made them an attractive and INEXPENSIVE way to accurately measure level in open vessels. (There are now even cheaper and more reliable devices available, which has almost made the humble bubble-tube a thing of the past, such as ultrasonics).

Anyway, a bubble tube simply measures BACK PRESSURE in the system, which is pressurised by the supply pressure. Please note that AIR FLOW has NOTHING to do with the actual pressure measurement: The level indicated by the system's sensing device, such as a pressure sensing load cell with say a 4-20 mA output is merely the back pressure of the air which is effectively leaking out of the bottom of the 'bubbler' pipe. The higher the level in the (open to atmosphere) tank, the higher the back-pressure of the air. If there is nothing in the tank, there would be no back pressure at all because all of the air would be freely flowing out of the end of the pipe - nothing opposing it or nothing causing any back-pressure.

The other extreme of course is when the pipe is blocked (say if you placed your thumb over the bottom of the pipe), no air can leak out anywhere therefore you would read full output (full back-pressure) which would be whatever the air supply pressure is set at. (Typically 20 to 30 PSIG).

You only need a rotameter in the air supply to the bubble pipe to indicate that the pipe is clear - not blocked up. If it becomes blocked you can see that the air flow would be zero or very low.
Controlling air flow is not necessary, although some people like to put a rotameter in the line simply to keep a constant check on the condition of the bubble pipe (not as a way to accurately control air flow), as the bubble tube system is measuring BACK-PRESSURE, proportional to level and that's all.

Having (I hope) better explained "What do they really do" you should now realize one of the biggest draw-backs or disadvantages of the bubbler-tube (or bubbler-pipe) method is that they can easily become blocked thus producing an erroneous high level reading, hence the need for the air-flow meter. On many of the stock chest bubble-tubes I installed and maintained in a large pulp & paper making site, (many years ago they use mainly ultrasonic in the same vessels now), we also provided a 'water purge' which was also fitted with a rotameter in effort to keep the bubble-tube clear of pulp build-up inside the buubler-tubes, which often very quickly, fully blocked the bubble-tubes...

Note however, that it is important to ensure that the air supply pressure is greater than the maximum level you are measuring. I.e. if you are meauring the level of a tank that is say 4 metres of H2O (that equates to a back-pressure of approx. 40 KPa when full) your supply pressure would need to be at least 60 KPa, but typically would be set at say 150 KPa. Always must be a lot more than the max back-pressure (maximum level). Remember it is BACK-PRESSURE (not supply pressure) you are measuring, so the supply pressure can be as high as you like, but not too LOW!

 

What I have been trying to find out is what is the purpose of a pneumerstat in such a system and what is its principle of operation? I am assuming its purpose is as a flow regulator for the air/gas to the bubbler.

HELP!
John.

 

The word sounds like an internet dictionary translation of air (pneuma) regulator (stat).

Bubblers use a constant differential relay (regulator) to create a constant flow rate of bubbles. The resulting back pressure reflects the head pressure, which of course, depends on the specific gravity of the liquid.

As noted in previous posts, Foxboro's Bubbler app note is comprehensive in applying bubblers.
It is here:

http://resource.invensys.com/instrumentation/documentation/eib/mi/mi020328.pdf
(delete any spaces inserted by the forum into the URL) or try the tiny URL:
http://tinyurl.com/2fgbcgz

Both constant pressure regulators and rotameters with a needle valve can limit air flow through a pipe and make bubbles, but a real bubbler measures back pressure created by a constant flow differential regulator, despite claims to the contrary.

There's a cutaway drawing of a constant differential relay on the last page of this pdf document (URL below), but its operation is not described. You'll probably have to find a description of operation in a older textbook from the pneumatics era, most of which were published on paper long before the .pdf era.

http://www.lesman.com/unleashd/catalog/level/Siemens_62-Relays_man_SD62_r10.pdf
(delete any spaces inserted by the forum into the URL) or try the tiny URL:
http://tinyurl.com/2dj9k4n

 

I take exception to this

> Both constant pressure regulators and rotameters with a needle valve can limit air flow through a pipe and make bubbles, but a real bubbler measures back pressure created by a constant flow differential regulator, despite claims to the contrary. <

Oh, so what makes you the authority or Foxboro for that matter?

We do many bubble tube installations for level, interface and density using just a simple rotameter and needle valve, no regulator or flow regulator.
Dropping down from full line pressure e.g. 70# there is an insignificant change in flow from empty to full in a vessel 10 feet tall. Adding a constant flow regulator is overkill.

And of course you don't just connect the purge gas just anywhere.

 

Roy is correct. There is no mystery about a bubble tube level sensor. The only thing that counts is that there is enough air/gas flowing to become a bubble. The measurement has nothing to do with the amount of gas flowing. The level above the bottom of the bubble tube is proportional to the pressure of the gas in the tube which is also related to the density of the liquid. The Rotameter is only there to allow YOU to observe that there is a flow of gas to the tube. How much means nothing. I wish all instrumentation was as simple as a bubble tube.

Dick Caro

 

Any technical reason why bubbler system only works in an open tank or vented tank and not in a close tank?

Regards,
MD

 

Hi there,

a Vessel is normally enclosed because it needs to be at a pressure higher than atmospheric pressure so a bubbler will not work in a pressurized vessel. If the tank is enclosed only for the purpose of corrosive product inside and it is vented to atmosphere the bubbler can still be used providing the product inside will not attack the tube.

It is basically a atmospheric pressure based system and measures the pressure needed to overcome a specific head of pressure so if you ad additional vessel pressure to that, the system will not measure only the atmospheric head pressure.

Regarding previous posts about the use of a constant flow regulator or not, it depend purely on yourself to use it or not. The system will still work without it but it is one of those thing you would rather use and be sure nothing will go wrong rather than not use it and get inconsistent and unreliability issues due to flow changes. Flow changes can ocur either through lower levels in the vessel or main supply changes.

Ok so what if the flow changes you might say but how about the fact that the instrument will become more sensitive to level changes if the flow increases therefore giving you a more sensitive and erratic response at lower levels than at higher levels?
Or that dried instrument air is expensive to make and higher flows than what is needed is a waste of instrument air.
Not serious you might say but it just goes to show that it will be better to use a constant flow regulator rather than to say it's working just as well without it.

 

Sure - Although the amount of air/gas needed to bubble is small, it is cumulative. That means that the pressure in the vapor space pressure above the liquid level will increase due to the bubbles. This will have the effect of increasing the pressure reading at the level transmitter that accounts for the pressure of the liquid level plus the pressure in the tank above the liquid. The reading will be wrong if the transmitter is to read liquid level. Eventually, the pressure in the vapor space will rise enough to cause bubbling to stop, and when that happens, the liquid level reading will be in error unless the air/gas flow is manually increased.

So the limitation: only use bubblers for open tank level transmission.

Dick Caro

 

One would be that the pressure is referenced to atmosphere. And the injection would change the pressures. Actually, the bubbler method should always work, provided that you reference the pressure to expel the liquid from the tube correctly. But that could be very difficult to do in a closed system.

Regards
cww

 

Yes - unless the tank is vented you run the risk of over pressurizing it in time. And you also need to use a differential pressure measurement between the bubbler line and the space above the surface.

 

I refer to Foxboro manual on bubbler system hook-up. Still wondering if in closed tank, can we hook-up the low side to the tank as well (similar like dP application) to get accurate reading as for open/vented tank?

Regards,
MD