Thats actually made me remember something i was busting to ask...
Last time i did a stainless pipe i did purge, i gave it a minute to fill with gas before i started and yet there was still a few parts that were sugary around the weld, I put it down to either i didn't give it long enough to fill or the fact that what i was welding was 4" pipe.....maybe it simply wasn't enough of a purge line to fill it up.
Also at some point you end up welding a nearly completed exhaust which can be 5-6 feet of pipe...how the hell do you get all the way around a weld while still keeping it angled and all the while by yourself and keeping a good torch angle. in the end i put it flat on the ground turned up the purge and just hoped
ONDGAS wrote:Thats actually made me remember something i was busting to ask...
Last time i did a stainless pipe i did purge, i gave it a minute to fill with gas before i started and yet there was still a few parts that were sugary around the weld, I put it down to either i didn't give it long enough to fill or the fact that what i was welding was 4" pipe.....maybe it simply wasn't enough of a purge line to fill it up.
Also at some point you end up welding a nearly completed exhaust which can be 5-6 feet of pipe...how the hell do you get all the way around a weld while still keeping it angled and all the while by yourself and keeping a good torch angle. in the end i put it flat on the ground turned up the purge and just hoped
Argon is heavier than air so you have to position your pipe so the argon fills from the bottom and forces the air out the top. It takes longer to purge something than you might think. You need to tape them up good and let the gas keep running as you're welding and that's tricky because your burning the tape off with the heat. Time to get creative. Maybe a flex-head torch or some other style....or just mig them.
exnailpounder wrote:Argon is heavier than air so you have to position your pipe so the argon fills from the bottom and forces the air out the top. It takes longer to purge something than you might think. .
The reason it takes longer than one might think, is that the argon while initially denser than the air, it is really just a matter of time as it inter-mixes with the air. Argon cannot stay separated from the air it is displacing, for if it could, it would separate on it's own in our own atmosphere, yet it doesn't. It is the same reason argon does not "settle" in a tank of 75/25 argon/CO2 mig gas. All gas molecules are always shooting around randomly at all times, when it comes to welding gases.
The reason it takes longer than one might think, is that the argon while initially denser than the air, it is really just a matter of time as it inter-mixes with the air. Argon cannot stay separated from the air it is displacing, for if it could, it would separate on it's own in our own atmosphere, yet it doesn't. It is the same reason argon does not "settle" in a tank of 75/25 argon/CO2 mig gas. All gas molecules are always shooting around randomly at all times, when it comes to welding gases.[/quote]
Sooooooo then one may ask how do you properly purge pipe if there is still air in it?
ONDGAS wrote:The reason it takes longer than one might think, is that the argon while initially denser than the air, it is really just a matter of time as it inter-mixes with the air. Argon cannot stay separated from the air it is displacing, for if it could, it would separate on it's own in our own atmosphere, yet it doesn't. It is the same reason argon does not "settle" in a tank of 75/25 argon/CO2 mig gas. All gas molecules are always shooting around randomly at all times, when it comes to welding gases.
Sooooooo then one may ask how do you properly purge pipe if there is still air in it?[/quote]
Gotta keep that gas flowing all the time you're welding.
We've discussed this a bit before. You do need to fill whatever your purging at a slow rate to keep the gasses from swirling togeather. But Argon will stratify even in an open top vessel. This is why purge boxes work, and why some people suffocate while welding in open trenches or tanks.
exnailpounder wrote:Argon is heavier than air so you have to position your pipe so the argon fills from the bottom and forces the air out the top. It takes longer to purge something than you might think. .
The reason it takes longer than one might think, is that the argon while initially denser than the air, it is really just a matter of time as it inter-mixes with the air. Argon cannot stay separated from the air it is displacing, for if it could, it would separate on it's own in our own atmosphere, yet it doesn't. It is the same reason argon does not "settle" in a tank of 75/25 argon/CO2 mig gas. All gas molecules are always shooting around randomly at all times, when it comes to welding gases.
Back in my cave diving days...guys diving tri-mix, and even nitrox would roll their bottles on the ground with the notion that they were re-mixing the gases that had separated. One guy went so far as to build a turntable for double tanks. My friend was a nuke tech on a submarine and he told me there was a little something to that whole gas separation thing but nothing to worry about. Guys would re-analyze their tri-mix bottles if they had been sitting for awhile with the notion that helium atoms would permeate the steel tank walls. I was never sure if I was getting my leg pulled or not.
ONDGAS wrote:The reason it takes longer than one might think, is that the argon while initially denser than the air, it is really just a matter of time as it inter-mixes with the air. Argon cannot stay separated from the air it is displacing, for if it could, it would separate on it's own in our own atmosphere, yet it doesn't. It is the same reason argon does not "settle" in a tank of 75/25 argon/CO2 mig gas. All gas molecules are always shooting around randomly at all times, when it comes to welding gases.
Sooooooo then one may ask how do you properly purge pipe if there is still air in it?
Eventually you do end up pushing out most all the air with a proper purge setup, so it's just a matter of time. Of course you lose some of your purge argon at the same time, but that's the nature of the beast. You have to envision that while yes your argon purge is pushing out air at the other end of your purge setup, the argon feeding into the space has already initially begun to mix just a little bit with the air as their is no invisible barrier that will prevent them from doing so. But you definitely can't calculate it solely based on the CFH flow of the purge flow and the internal volume of the space being purged. This is why there exists O2 purge monitors----you can see in real-time how much % or parts-per-million you still have of O2 within your purge volume. If it were as easy as a straight-forward calculation to know when you're at 100% argon, purge monitors wouldn't exist.
Poland308 wrote:We've discussed this a bit before. You do need to fill whatever your purging at a slow rate to keep the gasses from swirling togeather. But Argon will stratify even in an open top vessel. This is why purge boxes work, and why some people suffocate while welding in open trenches or tanks.
I do believe that to be correct, just not what I was getting at. The mixture will always be there initially, there will just be less and less of it as time goes by because the welding gas is being pumped in at a greater pressure than atmospheric pressure. I was just trying to emphasize the "time" factor. but perhaps I worded it incorrectly. Sorry if I confused anyone. I should stop posting right before my bedtime or during my lunch break, lol.
There is truth to the helium leaking through the steel tank walls. I once worked on an old tank that was used to store helium. They were decommissioning it. We had to grind off the tags and stamp info. When they did the paper work associated with the vessel i was talking to an engineer about it. He said in there operation they monitor the loss rate through the tank walls. But they measured the loss rate in grams over a year period.
Poland308 wrote:There is truth to the helium leaking through the steel tank walls. I once worked on an old tank that was used to store helium. They were decommissioning it. We had to grind off the tags and stamp info. When they did the paper work associated with the vessel i was talking to an engineer about it. He said in there operation they monitor the loss rate through the tank walls. But they measured the loss rate in grams over a year period.
Grams of of weight for gas can translate to a huge amount of loss. People wonder why their tires go low with no holes or damage...the oxygen molecules are permeating the rubber...nitrogen too but not as fast. Fascinating stuff.
ONDGAS wrote:Sooooooo then one may ask how do you properly purge pipe if there is still air in it?
You can make a purge plug that will allow you to purge a much smaller area then a 5ft length of pipe/tube.
Make a plug that is a little bigger then the ID so it will go in with a little force, this way you know it will stay in place and not fall over. You can push the plug to within 10" or so of the weld, thus greatly reducing the volume you need to purge
Image1.jpg (20.72 KiB) Viewed 743 times
Here is a simple plug I made recently to purge an internal volume using aluminum foil.
I never put a brass filter on my purge line like you did In the above pic, would this be a problem? I would think it's going to fill the enclosed space your purging regardless so is it needed?
ONDGAS wrote:I never put a brass filter on my purge line like you did In the above pic, would this be a problem? I would think it's going to fill the enclosed space your purging regardless so is it needed?
A brass muffler like this does a wonderful job of diffusing the purge gas, rather than injecting it. Yes, there's a difference.
I should stop posting right before my bedtime or during my lunch break, lol.
