How does oxygen get into a pressurised line at a bad joint? 3

[steve102]

How does oxygen get into a pressurised gas line at a leak?

When you can see leaks at a joint in the line between bottle to welding unit joint (after adding a soapy water solution), can anyone help provide a technical reference of how the air (oxygen) gets into the line to be one of the causes of porosity?

I figure:

Pressurised gas flow going past say a poorly coupled connection results in turbulence that allows gas out, and at the same time, sucks air in, yes?

The argon molecules are larger than oxygen ones. Also, the leak is via the microscopic spiral gap in threads as another factor.

I know there's the Venturi effect blah blah, but I've not been able to find anywhere that fully explains how the air gets in when the gas is at high pressure.

I think that basically the smaller air particles get sucked in as the high pressure gas passes the leak.

I'm having difficulty explaining how atmospheric pressure gets into a line where there's a positive pressure.

(The application is air into an argon supply for TIG welding, but I think this is the best forum for my query).

 

[LittleInch]

"I'm having difficulty explaining how atmospheric pressure gets into a line where there's a positive pressure. "

You and me both. What sort of pressure are we talking about here?
Do you have a sketch or photo?

What I know about welding isn't a lot, but a TIG weld is quite susceptible to wind, air disturbance, loss of backing gas, poor technique, etc. This page lists 22 possible causes for porosity so not sure why you've narrowed it down to apparent air in the inert gas supply??

http://www.thefabricator.com/article/arcwelding/22-possible-causes-of-weld-metal-porosity

Some aren't TIG related, but you get the drift...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

Why do you think it's air, why not just welding gas? The internal pressurized gas flow cannot "suck air in," it's flow velocity is too low for a Venturi effect to occur. Even then, it would still be mostly welding gas.

Seems to me you mixed two separate effects, leaky joints and porosity, into a single thing.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[btrueblood]

Diffusion.

Molecules move in response to gradients in partial pressure. The partial pressure of oxygen (and water vapor) in air relative to the pure argon in the hose (near zero oxygen and H2O partial pressures) is high, and will cause some diffusion. Good quality hoses with materials chosen for low permeabilities to the contaminants of interest will minimize the problem.

http://www.mathesongas.com/pdfs/pro...se Materials Affect Shielding Gas Quality.pdf

 

[PEDARRIN2]

thread378-122965
thread378-123404

 

[racookpe1978]

Bad weld gas (99% "pure Argon" will given my welders porosity in the past), you need to specify 99.96%.

Also, wind and too little flow at the TIG welder tip. But, if you are seeing "gas bubbles" at the hose connection, the expensive Argon is leaking OUT, never air leaking in.

 

[IRstuff]

Diffusion through the walls is one thing, but into a leak is different. The concentration gradient would partly offset by the molecular momentum transfer pushing air back out.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[EdStainless]

Diffusion back though a leak is real. I have an Ar line at 60psi, if I put an O2 meter near one end of it I can loosen a fitting slightly at the far end, not even enough to get a leak that will form bubbles, and the O2 value 800' away will change as fast as the meter can respond. When one gas is high purity the driving force for diffusion is huge, and pressure does not prevent it. Even if you have a real leak remember that there is a this static zone along the edges of the leak where there is no flow.
We are working from liquid Ar source, DP<-80C and O2<0.2ppm, we have to fix lines and replace hoses all of the time. We even see it through regulator diaphragms. Keeping gas pure takes care.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[weldstan]

As Ed stated, it is quite real and remember that when welding, the gas is at low pressure when flowing in the range 0f 15 to 20 CFH. And I can corroborate Ed's finding with O2 measurements with no leaks and small leaks at connections. Also have used dew point measurements to further corroborate.

 

[quark]

There is an old thread on this topic. thread378-123404

 

[steve102]

Many thanks for all your replies.

For clarity, the issue is that a soapy solution over joints in the line from the argon gas bottle to the welding unit is bubbling (at joints). Oxygen is getting into the pressurised argon line. This oxygen is a principle (but not the only) cause of porosity, which we are trying to eliminate. I'm trying to explain how air at atmospheric gets in against the pressurised argon.

I think the key factors are: velocity profile (velocity is very low at the pipe wall), diffusion, turbulence, partial pressures, and suction (venturi?).

I think the argon in the bottles is of very high purity, but not 100%, and this lends itself to partial pressure issues.

I’ve pulled together what I think are key lines, and have made some progress – It’s a bit disjointed, but is hopefully going in the right direction.
The boundry layer at the wall has an effective velocity of zero. This allows molecule migration and then through the diffusion into the flowing stream, getting swept up into. Dry air has a very nice transport phenomenon to "pulling" moisture into its flowing stream.

Diffusion back though a leak is real…When one gas is high purity the driving force for diffusion is huge, and pressure does not prevent it.

Diffusion through the walls is one thing, but into a leak is different. The concentration gradient would partly offset by the molecular momentum transfer pushing air back out.

Molecules move in response to gradients in partial pressure. The partial pressure of oxygen (and water vapor) in air relative to the pure argon in the hose (near zero oxygen and H2O partial pressures) is high, and will cause some diffusion.
"have you ever been sitting in a large auditorium, a church for example, and some one passes gas (farts) at some distance away from you, with absolutely no ventilation or other type of air movement in the room of any sort at the time, and within seconds you can smell it? That is difussion."
Dalton's Law of partial pressures,

http://nuclearpowertraining.tpub.com/h1012v3/css/Flow-Velocity-Profiles-40.htm

LAMINAR AND TURBULENT FLOW Fluid Flow Flow Velocity Profiles Not all fluid particles travel at the same velocity within a pipe. The shape of the velocity curve (the velocity profile across any given section of the pipe) depends upon whether the flow is laminar or turbulent. If the flow in a pipe is laminar, the velocity distribution at a cross section will be parabolic in shape with the maximum velocity at the center being about twice the average velocity in the pipe. In turbulent flow, a fairly flat velocity distribution exists across the section of pipe, with the result that the entire fluid flows at a given single value. Figure 5 helps illustrate the above ideas. The velocity of the fluid in contact with the pipe wall is essentially zero and increases the further away from the wall.

Anyone?

 

[moltenmetal]

Pressure doesn't prevent diffusion at all- if there is a concentration gradient, there is a driving force for diffusional flow. That flow can definitely be up a pressure gradient.

However, diffusion can be offset by the advective flow in a leak as IRStuff said. If you have substantial argon leaking out of a joint, the likelihood that oxygen is back-diffusing up the waterfall so to speak is pretty low.

Diffusion through soft goods such as tubing and seal materials is also possible.

 

[hacksaw]

To have a useful conversation you need to first specify:

1. Purity and composition of feed gas,
2. materials of construction in your pipe/tubing and how joined-up.
3. Steps taken to flush and purge the line prior to testing and final use.
4. Test methods identifying the change in stream composition from source to pipe discharge.
5. Finally, temperatures and pressures used in the installation.

Diffusion is important where the partial pressure gradients exceed the pressure differential, hardly significant otherwise as the responses suggest.

good luck with your project

 

[steve102]

OK, I hope this helps:

Argon bottles 99.997% pure & @ 2000psi

Connected to a regulator that turns it town to deliver 15cubic four per hour through a 6mm I.D hose to a welding unit

Hose is basic flexible clear plastic hose (garden hose spec).

The pressure in the gas bottle drops as the gas is used, but the regulator keeps it constant (ignoring nominal periodic manual adjustments).

Temperature? atmospheric approx 80F.

 

[btrueblood]

Get rid of as much of the vinyl hose as you can, and use rigid metal tubing to each weld station. Use a good quality welding hose "whip", as short as practical, at each weld station; and as Ed says, replace them regularly (or test each hose and replace when the 02 and H20 goes out of spec, or when all the stuff you weld starts falling apart...your choice)

 

[moltenmetal]

There's tubing with an oxygen barrier layer, and tubing without one.

You need tubing which has such a layer.

When purity problems are encountered in flowing systems under pressure, the usual source of the problem is the soft goods, not leaks in the metallic components.

 

[steve102]

Many thanks.

I can't change the hose. I'm trying to explain how when there is a loose joint that air gets in to the tube and increases the air content within the tube, as this air content is contributing to porosity problems.

I can tighten the seals etc, but what I'm trying to get is a scientific / engineering explanation of how the air gets 'sucked' in. Imagine having to explain to a 14 year old why when you pour a soapy solution on a joint and it bubbles, that even though the pressure in the line is greater than atmospheric, that somehow, the air gets in. I hope this helps.

 

[moltenmetal]

If a leaking joint "bubbles" under soap solution, NOTHING is getting "sucked in" to that joint by the venturi effect. It's just not going to happen, period.

However, if you direct a jet of argon out of a tube, it will entrain a lot of air into that stream. That's why you need to direct purge gas through a diffuser or "gas lense"- to minimize entrainment and maximize blanketing of the hot metal being welded with the inert gas, rather than entraining and mixing air into that purge gas.

It is also possible for oxygen to diffuse right through the intact walls of a soft goods hose, if it doesn't have an oxygen barrier layer. Whether that is a significant amount of oxygen or not depends on many things- the flow of the argon, the thickness and temperature of the hose material, and how little oxygen you can tolerate in your argon.

It is possible for oxygen to diffuse into a leaking joint- as long as the flow of argon out of the leak isn't too rapid.

Think of it as salmon swimming in a river. Salmon swim upstream to spawn- even though the flow is downstream, they have a driving force to swim upstream. They can even jump up a small waterfall. But if the current is too strong, they can't swim upstream, even though they have a driving force to go that way.

Replace "salmon" with "oxygen" and "stream" with "flow of argon". There's your explanation.

Want to know how much of a leak you need OUT of the joint to prevent argon from diffusing INTO the joint? For that you need to do calculations.

 

[EdStainless]

Even high quality barrier tube will only a year or less.
And if the system sits unused with air inside the tube you may never get the oxygen and water out of it.

And diffusion will drive oxygen back against high velocity flow, molecular forces are huge.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube