getting a clean purge on big stainless pipe

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I have been experimenting with different things to try and get a good clean purge on 8" and up stainless pipe but i just can't seem to get rid of the dark smoky look. I have had the best luck with 75%argon 25% helium but i still can't get it to be a nice bright silver root like I get with smaller pipe. Any suggestions? Thanks!
Reply:Try using purge dams or blocks to try to keep your purge from just trying to fill the whole length of pipe. I've used rice paper or if I was able to pull it out I've used thick foam cut to match I.d. of pipe.
Reply:Purge dams help speed this process up.  You need an O2 meter to really confirm that you're getting down to low O2 levels.   Leaks in your purge dams will kill you, and without an O2 meter you'll never know it's happening.I've not heard of using Ar/He; which sounds really expensive.Argon-hydrogen purge gas mixtures can also help.  The hydrogen can help with root bead cleanliness.  It actively scavenges O2 and other contaminants and keeps them from reacting with the molten weld pool.  But hydrogen can also promote cracking, so you need to carefully test/qualify the welding  procedure to ensure this isn't happening.  1% or 2% Hyrdogen mixtures are safe to work with.  H2 concentrations greater than 2% begin to create fire hazards, which you need to deal with.  I suggest sticking with 1% H2, if you're going to experiment with this gas mixture.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Argon helium mix is only like $10 a bottle more than argon so it isn't bad. Purge balloons also don't work for a lot of the stuff we do. Around here the argon helium mix is very popular. The other day I let a 21' length of 10" purge for 14 hours overnight at 20 cfh and it was still dark. 4" and under always comes out bright silver and no staining around it. There is nothing wrong with the bigger welds, they shoot 100% clean. I am just picky and want them to look perfect. A lot of what i do is code welding and i will have to write a procedure and send in samples to be bent and x rayed and then i will have to re-test if i change gas so I would like to avoid that if at all possible. I think i am going to try bigger holes in the tape on the outlet side of the pipe and try bumping my purge up to around 40 cfh and see if that helps. I am wondering if it is not enough gas flowing over the weld. I usually run it around 20 cfh now.
Reply:I work out 21' of 10" pipe to about 55 cf. At 20 cfh, it would take almost 3 hours to simply fill the pipe if it had nothing in it. Then figure in dilution over that large a volume, and it doesn't really surprise me that you aren't getting 100% purge. My guess is you need to increase your flow volume fairly significantly or cut down your volume that needs to be purged to get the results you want..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Originally Posted by DSWI work out 21' of 10" pipe to about 55 cf. At 20 cfh, it would take almost 3 hours to simply fill the pipe if it had nothing in it. Then figure in dilution over that large a volume, and it doesn't really surprise me that you aren't getting 100% purge. My guess is you need to increase your flow volume fairly significantly or cut down your volume that needs to be purged to get the results you want.
Reply:Try inclining the tube so that the vent hole is elevated and run a lower than normal flow rate.  This ensures the Ar settles to the bottom evenly and pushes up on the atmosphere.  Once its purged you should be able to level it back out again.Also, if you are not running open roots you could switch to a nitrogen purge for cost savings.  It will break down and create brittle nitrides if exposed to an open arc but for closed butt welding it works just fine.Have we all gone mad?
Reply:From a practical experience standpoint, using O2 metermonitoring purge gas outflow prior to fuel tank repair; toget to 0-2% O2 content:-requires purge volume of 5 X vessel volume---minimum-ramping up the purge flow doesn't necessarily speed things upAs others have mentioned--an O2 meter is definitiveBlackbird
Reply:Originally Posted by DSWI work out 21' of 10" pipe to about 55 cf. At 20 cfh, it would take almost 3 hours to simply fill the pipe if it had nothing in it. Then figure in dilution over that large a volume, and it doesn't really surprise me that you aren't getting 100% purge. My guess is you need to increase your flow volume fairly significantly or cut down your volume that needs to be purged to get the results you want.
Reply:Thank you moonrise, that is more the answer I am looking for. Lately most of the stuff I have been doing has been sch 10. The outside of the pipe is very clean and a light brushing and it is silver. I have done a ton of stainless and my welds look very nice. I always tape the joints and purge in the correct direction. I always elevate the vent end if possible. I generally try to do a very slow purge so the gas doesn't mix with the air. I have been wondering if there just isn't enough flow of gas after it is purged out. Next time I am going to try a slow purge like you said until the air is out and then enlarge the hole in the vent end and turn the flowmeter up.
Reply:Forgot to add that i use a miller maxstar inverter style welder with a ck17 air cooled torch, gas lens and #8 cup.
Reply:you are taping your roots prior to purging right?  not just letting all the gas blow right out of them?Vantage 500's LN-25's, VI-400's, cobramatics, Miller migs, synch 350 LX, Powcon inverters, XMT's, 250 Ton Acurrpress 12' brake, 1/4" 10' Atlantic shear,Koikie plasma table W/ esab plasmas. marvel & hyd-mech saws, pirrana & metal muncher punches.
Reply:Originally Posted by Dualieyou are taping your roots prior to purging right?  not just letting all the gas blow right out of them?
Reply:Also it isn't just me, I did a little looking at some stuff that other contracters did and it looks the same plus all the guys I work with look just like mine. It is dark and a little smoky by the sides of the root on the inside. The root is smooth, not rough by anymeans just not silver. I do it with or without a j bevel with the same outcome.
Reply:I've not heard of using Ar/He; which sounds really expensive.http://www.actualtests.com/exam-156-215-75.htmhttps://www.facebook.com/pages/C%C3%...36694099684017http://en.wikipedia.org/wiki/Interna...iness_Analysishttp://www.youtube.com/watch?v=sWc-XJKidho&feature=kp
Reply:We've had good luck purging with Nitrogen.  Much cheaper than Argon, and we still get x-ray quality welds for ASME piping.
Reply:If the beads are uniform and X-ray quality then what is the problem. I've had similar problems on big pipe. I finally said screw it and went about my business. Mind you this was 321 s.s. severe shot. As long is it is a good root who cares if it ain't silver.  Sounds like alot of wasted gas and time.