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Instrument Air and Humidity Back-Migration

01/28/2013 5:01 PM

I have heard that if there is a leak from a very dry air system to the atmosphere, then atmospheric humidity can back migrate, or backflow, into the dry air which is still in the pipe and raise the dew point of the air still in the pipe.

For example, I have heard this can occur even if the dry air (instrument air, in my case) is at 100 psig.

It sounds rather incredible but I have been asked to investigate this and I am looking for references, or even anecdotal evidence.

Anyone?

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#1

Re: Instrument Air and Humidity Back-Migration

01/28/2013 5:17 PM

There doesn't seem to be much work done here.

I'd be at a loss to understand how a systeem that is always at a higher pressure can possible be contaminated by air at a lower pressure.

It sounds more like a drying problem after the compresser.

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#2

Re: Instrument Air and Humidity Back-Migration

01/28/2013 5:48 PM

First time I've heard that one. Think of all those pneumatic positioners that bleed air. Each of those is a source of moisture back into the instrument air? I don't think so. I've seen capillary action wick moisture uphill, but I haven't witnessed moisture overcome positive pressure. If this were true, Z purging of enclosures for Cl 1, Div 1 areas would result in water filled enclosures from the migration of moisture through the leaks. I think this is a fanciful explanation to cover some failure to maintain a sufficiently low dew point of the instrument air; along the lines the dog ate my homework or the devil made me do it.

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#3

Re: Instrument Air and Humidity Back-Migration

01/29/2013 4:22 AM

<...looking for references, or even anecdotal evidence...>

As the concept is nonsensical, references and anectdotal evidence might be a bit difficult to find.

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#4

Re: Instrument Air and Humidity Back-Migration

01/29/2013 1:58 PM

I have not heard of this in regards to dewpoint but I have seen a similar issue with parts per million Oxygen measurements in pure Nitrogen or Argon.

If you have an analyzer on a pure gas with a reading of say 1 ppm O2.

Then you create a small leak in the sample line, watch as the sample gas leaks out your analyzer reading will increase. The theory being the sample gas leaking out of a pin hole leak creates a low pressure area right at the surface of the tubing. Imagine the low pressure causes the tinyest amount of atm. air to be dragged into the sample line.

I have seen this happen with low ppm measurements and I have been able to repeat it.

I see no reason why moisture would not be dragged into the sample as well.

I do not think it would be enough to alter the dewpoint in the system but it will be enough to alter the reading if the leak is in the sample line going to the dewpointer.

For extreme dry measurements it is recomended to back weld or braze the fittings so a possible leak is a major concern most likely to combat this very phenominum.

It is one of those things that appears to not make sense until you see it happen.

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#5

Re: Instrument Air and Humidity Back-Migration

01/29/2013 10:55 PM

Of course this happens. It's Charles's law of about 200 years ago and it's still works. Each species of gas applies pressure independent of other gasses. The moist air outside the pipe applies higher pressure of water vapor than the low partial pressure of water vapor inside the pipe. So the flow of water vapor is from the outside in.

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#9
In reply to #5

Re: Instrument Air and Humidity Back-Migration

01/30/2013 1:27 AM

Ofcourse, it is Dalton's Law of Partial pressure.

Nature is devious, almost uncanny. The reason for the back flow of moisture is hence pat!

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#10
In reply to #5

Re: Instrument Air and Humidity Back-Migration

01/30/2013 6:07 AM

One interesting example of Charles's law is with Latex balloons inflated with helium. Over a few days, the helium leaks out, but the balloons don't simply go flat. At the same time, air is leaking in because to the the oxygen and nitrogen molecules, the interior of the balloon is a vacuum. It all boils down to how many molecules of each species are on each side of the barrier.

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#6

Re: Instrument Air and Humidity Back-Migration

01/29/2013 11:06 PM

Sorry, it wasn't Charles, it was Dalton. But it's still correct.

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#7

Re: Instrument Air and Humidity Back-Migration

01/29/2013 11:42 PM

This reference question 2(c) has an interesting answer:

http://apcentral.collegeboard.com/apc/members/repository/ap03_sg_chemistry_26428.pdf

Because the molar mass of water vapor (~18) is less than the molar mass of nitrogen (~28) and of oxygen (~32), water vapor will effuse faster than either of the main component gases of the air. Hence the remaining amount of water vapour in the system will DECREASE faster than the other components. Depending upon the method of assessment, the water vapor proportion in the system SHOULD measure as being REDUCED over time.

And THIS reference http://www.chem.ufl.edu/~itl/2045/lectures/lec_d.html

explains that comparison of the effusion rates of two gases with different masses will follow the relation:

This effect was observed in the 19th century by Graham and is sometimes called Graham's Law.

Hence the effusion rate for water vapour SHOULD be around 1.26 times the rate for the main components of air. Another contributor says that Charles's Law explains it the other way around.

So, if you are seeing an INCREASE in system humidity, these classical laws just are disagreeable.

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#8

Re: Instrument Air and Humidity Back-Migration

01/29/2013 11:57 PM

I have seen this anecdotally....Our plant air dryers supply -40 C dew point air from the compressor house to the plant, as measured by a dew point meter. This was not instrument air, but pretty much meets the spec. It was dried to that level to keep equipment functioning in cold weather.

I had heard the same story about moisture migrating back into the air lines through leaks. We normally had a lot of problem with air valves freezing in exposed areas of our plant when the weather dipped below about -15C. This didn't make a lot of sense unless there truly was water in the lines. The solution was to keep the valves warm with little heaters.

We installed a spare meter at the extreme limit of the air system about 800 feet away in an outdoor area of the plant. Sure enough, the dew point meter supported our observations...I can't remember the exact reading but it was -10C or so.

We didn't figure the problem was so much the air leaks, of which we had our share of, but the moisture migrating into the system at the point of use. The majority of use was to cycle large air cylinders. Each time an air valve opened it was directly connected to the outside world through the air cylinder, which had just exausted the previously-cycled air to the atmosphere and was loaded with a bunch of moisture, waiting to get back into the dry air system when the valve opened again. Additionally, our tradesmen swore that the problem got worse when the winter humidity went up--which would seem to support the theory that the water in the ambient air migrated back into the air system.

I would suggest you talk to your compressor/air dryer equipment suppliers about this. Once you get past the salesman talk, they probably can come up with the scientific explanation if pressed a little. I think the poster who mentioned Dalton's law is on the right track.

HTH,

Jon.

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#11
In reply to #8

Re: Instrument Air and Humidity Back-Migration

01/30/2013 9:09 AM

Your anecdotal theory may be correct. However, you didn't mention what the pipe material was that carries your dry gas sample a distance of 800 feet. If the pipe material is plastic, water vapor will diffuse through the wall of the pipe, raising the dew point. Always use metal pipe when transporting dry gas.

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#13
In reply to #11

Re: Instrument Air and Humidity Back-Migration

01/30/2013 10:26 AM

The piping was all steel.

Jon.

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#12

Re: Instrument Air and Humidity Back-Migration

01/30/2013 9:29 AM

A valid experiment that demonstrates Dalton's law is to take a length of rubber or Tygon tube and put a valve at each end. Flow hydrogen through the tube to remove most of the air, and then close both valves. Go find a bottle of single malt and ponder what's going to happen for a few hours. Upon your return, you will find the tube, instead of being round, is as flat as a pancake. Open either valve and you will hear air rushing into the tube which returns to round. Without the aid of mathematics, you know that the very small hydrogen molecules diffused through the wall of the tube into the surrounding air. However, the air molecules were too large to diffuse back through the tube wall. So, you created a solid state vacuum pump. You can do this demonstration mathematically, but once you observe it, it becomes etched in your memory. *note- the single malt is optional.

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#14
In reply to #12

Re: Instrument Air and Humidity Back-Migration

01/30/2013 11:39 AM

I haven't done the experiment you describe, but I'm almost certain it will work. On the other hand, I believe the process is more closely related to osmosis than to partial pressures. The atmospheric atoms or molecules can't exert pressure inside the tube because, as you have indicated, most of them are too large to pass through the rubber or plastic.

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#17
In reply to #14

Re: Instrument Air and Humidity Back-Migration

01/30/2013 12:21 PM

What you say is absolutely correct. However, the hydrogen molecules are small enough to diffuse from the inside of the tube to the external atmosphere. The atmospheric molecules are too large to diffuse into the tube from the outside, so the internal pressure goes down and the tubing collapses.

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#15

Re: Instrument Air and Humidity Back-Migration

01/30/2013 12:02 PM

Thanks to all those who gave the helpful feedback.

My actual concern is that I wish to use a low flow of instrument air to purge out an acidic gas header. There were worries that if back-migration or diffusion of moisture could happen, then acids could also diffuse into the instrument air thereby contaminating it.

Clearly, the back migration is something that can't be dismissed and I have to either design around it or perform a risk assessment if don't.

Welderman: I was disappointed that the single malt was only optional!

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#16

Re: Instrument Air and Humidity Back-Migration

01/30/2013 12:03 PM

I haven't done any math regarding this, but one possible explanation is to think about atomic and molecular velocities. The speed of sound is a measure of the average velocity of atmospheric atoms and molecules; for moist air near sea level it is roughly 350m/s. The velocity of individual gasses varies inversely with the mass of the molecule, so the average velocity of the (lighter than nitrogen) water molecules is somewhat higher. Within the water molecules, there is a Gaussian (bell-shaped) distribution of velocities, so at least a few of the water molecules are traveling twice as fast as the average, and a few of those will be traveling in the opposite direction to that of the gas escaping through the leak orifice. If the average velocity of the leaking gas is lower than the velocity of those water molecules, then some can be expected to 'flow upstream' through the leak orifice into the slower gas inside the pipe.

Once they get past the orifice, they can easily diffuse through the entire system.

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