Calculating Individual Flows of 2 Gasses Mixed in a Manifold. 2

[tc7]

This question relates to a welding problem, but more accurately, it is a gas flow problem that I ask for advice on.

I am connecting an argon tank and a helium tank together with a Y-pipe connection and at this Y-pipe the gasses mix and flow to the welding torch. Both the argon tank and the helium tank have their own pressure regulator and flow meter. I am required to weld with a combination of 75% argon and 25% helium (i.e., 75/25 ratio). I am also required to operate the torch with a total flow of 30 CFH, which is checked at the torch outlet with a handheld flow meter.

The pressure regulator on each bottle is preset and drops the tank pressure from ~1500-2000 psi to 25 psi. Therefore both regulators are feeding 25 psi into their respective flow meters. Both flow meters are ball type - and the argon flow meter is specifically calibrated for argon and the helium flow meter is specifically calibrated for helium.

We always thought if I set the Argon flowmeter at 22-23CFH and set the helium flowmeter at 7-8CFH, I would end up with the required 75/25 ratio and deliver the required 30CFH at the torch.

My inspector says I am wrong because the helium is so much lighter than argon, I am NOT achieving the required ratio.

Can some one kindly outline the calculations I would need to make so that I can properly set each flow meter to achieve the required 75/25 ratio.

Thank you if you can help.

 

[DLite30]

You may want to post this question in the welding/bonding/fastening forum...I've never heard of shielding gas being regulated by mass. Sounds like the inspector is looking for fights.

 

[zdas04]

It all depends on what the 75/25 ratio is based on. If it is percent by volume, then you are doing it right. If it is percent by mass then you are not doing it right. You might check your spec and see if it is volume or mass. If it is mass, post back here and someone will go through that math for you.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[tc7]

Thanks David-
According to our resident project engineer, I was told that it is always mass flow.

So then, hopefully the calculations are straight forward, but even if they aren't we should be able to handle them in the shop.
Looking forwrd.

 

[zdas04]

I hope your resident project engineer knows what he's talking about. I can't remember ever putting a ball-type flow meter (which cannot be pressure or temperature compensated) on anything and then asking the field guys to calculate a mass flow rate from it (which is what they are doing by giving you CFM and asking for mass percent flow rates).

Your resident project engineer should be able to provide you with the details of the derivation, but if I assume your atmospheric pressure is 14.5 psia, the pressure of both regulators is 25 psig, and both gases are at 60[°]F, then the Argon flow meter should read 30% of the Helium flow meter. That is if you are running 24 CFM of Helium, the Argon should be 7.2 CFM. Just about perfectly backwards from your current settings.

If the two streams are at different pressures then the ratio is different. If they are at different temperatures then the ratio is different. If your atmospheric pressure is considerably different than 14.5 psia then the ratios are different.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[tc7]

David-

Project "engineers" have there purposes I suppose but providing guidance and inspiration is not amongst them

Can you perhaps outline the calculation ?

Your stated assumptions:
>atmospheric pressure is 14.5 psia, >pressure of both regulators is 25 psig,
>both gases are at 60°F (say 80°F)
Are all correct. If you think my settings are backwards , I really need your help.

Hoping you can.
Thankyou

 

[zdas04]

You start by calculating the density of each gas:

[ρ]=(P*SG)/(R_air*T)

Both the pressure and temperature need to be in absolute terms (psia and Rankine). When you do that calc with the numbers I mentioned above (i.e., 25 psig becomes 39.5 psia and 60F becomes 520R) and look up the specific gas constant for air and the molecular weight of Argon and Helium you find that the density of Argon is ten times the density of Helium.

Then:

(q_argon*[ρ]_argon)/((q_argon*[ρ]_argon)+(q_helium*[ρ]_helium))-0.75

Substitute the ratio of densities equals 0.1 and solve for either volume flow rate and the answer is q_argon=0.3*q_helium

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[tc7]

Thank you David,
This should get me started in the right direction.
Best regards to you.

 

[tc7]

One last question on this David - can you cite a reference on this so that I can learn more about what I am doing here? Thanks again.

 

[zdas04]

I don't "cite references" on this site, I reserve that exercise for paying clients. If you want references, my hourly rate is $180. For free you get opinions.

I did realize one thing this morning, as long as the two gases are at the same (as each other) pressure and temperature, the ratio of densities resolves to the ratio of molecular weights of the two gases

[ρ]_mixture = [ρ]_he/[ρ]_ar=MW_he/MW_ar=0.1

That says that the temperature of the gases or the setting of the pressure regulators is immaterial as long as both are the same.


David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

I, as does Delite30, refuse to believe that it is mass based!

IMO it would be totally rediculous to shield with that 75% Ar (with a MW of 40) plus 25% He (with a MW of 4). If comprised of mass ratios would not result in any kind of an appropriate weld shielding mixture. All that Ar (97% by volume) would give you a really hot arc and really flatten the weld metal puddle with all that weight pounding down on it. What would 3% He by volume do for you in that situation anyway. Might as well leave it out of the mix totally.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[zdas04]

Yeah, but "the resident project engineer" said so. Surely you don't think the he just pulled an answer out of his back pocket without having a clue?

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

Oh, I'm doing it again. A mix ratio by weight would be 97% He and 3% Ar by volume. Real low temp, and short arc. In any case, not a good welding mixture.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[tc7]

Hi Fellas'
lots to consider with all of your comments. I may post over on the welding forum to get their point of view.

Big Inch - we have applications where we weld with straight helium, and we have other applications and processes (such as GMAW) where we run straight argon. The helium with it's higher ionization potential (as compard to argon) creates an arc of greater heat input and is by no means a low temperature short arc process. The mix of Ar & He will offer different arc characteristics that may or may not suit the joints and positions in production. Nevertheless, this series of comments has brought me to the opinion first suggested by David that settings should be based on volumetric flow and not mass flow - although I am being told either amounts to the same thing.
Thanks for your time.

 

[BigInch]

I probably am remembering the effects of each gas' contribution in reverse as well. In any case I do know it is volume based and just now checking for any other result, I cannot find ONE SINGLE web reference that is in any measure other than by volume. Every He mix I saw was at least 20% minimum by volume.

If I were you I'd tell your inspector that he's paying for any weld that needs to be replaced because of any problem eventually traced to a bad shielding mix and check his qualifications while your at it.

When they specially calibrated your meters, did they do it by volumetric flow or by weight flow.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[tc7]

"When they specially calibrated your meters, did they do it by volumetric flow or by weight flow."

Brilliant question Big Inch !! I never thought to call the manufacturer on that. That should settle my question once and for all.
A star goes to you buddy!
Thanks.

 

[BigInch]

I'd rather be known for my brilliant answers, but since it was really more of a rhetorical question, I'll accept.
Cheers.

If it ain't broke, don't fix it. If it's not safe ... make it that way.