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Up Topic Welding Industry / General Welding Discussion / SS Purge Issue
- - By rangerod (**) Date 12-10-2010 07:07 Edited 12-10-2010 07:09

How would one go about purging an inner tube shape? I have introduced the argon in the into the vessel in both parallel & perpendicular directions in relation to the tube using a Hw-20 gas lans to help diffuse the gas better. We are using a  Argweld V purge monitor & the manual states when .1% is reached welding may commece. There is one exhaust port 90 degrees from argon input with a purge maintenance of 10 cfh. The monitor is located 180 degrees from argon input. When I started welding the monitor read .02% to .03%. Welding is being performed on the mid point of the inner diameter of the tube shape. I have not been able to get rid of internal oxidation. This is my only issue with the setup. Do I need to install a baffle to keep the purge gas from doing rounds around the track, turbulence? I can not track down the offending condition causing this problem. Am I missing something here? Any help with this issue would be greatly appreciated. If I left any essential detail please let me know & I will get it to you. Thanks

Parent - - By js55 (*****) Date 12-10-2010 13:30
Some stainless steels can be stubborn for whatever reason. Don't always believe the purge monitor. Purge longer and see what happens.
I was ona job oncewhenwe had to purge 2 cuft volume of space for several hours to get the root clean. I have no idea why. 
On the other hand, I also have no idea how much oxidation concerns you.
Parent - - By rangerod (**) Date 12-10-2010 23:08

Tha purge was on for 3 hrs & I was still getting a dull blue color on the root side. I am leary because I dont want to get any nasty "sugar". We have access to clean oxidation up both sides but "sugar" would be a diffeent story. My main  concern is an unsound weld. Thanks.

Parent - - By Tommyjoking (****) Date 12-11-2010 04:42 Edited 12-11-2010 04:58
Hi rangerod

My advice is to introduce (if you have not already), restrictions to the purge process.  You may be pulling O2 in if you do not have a sealed condition.  IF the welded area is reasonably distant from your gas inlet and outlet, simply tape the area with masking tape and get a good seal on both ends, leave a very small diameter hole on the outlet for gas to escape.  It will shorten your purge time greatly if you can configure the purge outlet to be higher then your inlet, make gravity your friend.  Purge only the area necessary for welding, not the whole vessel.  Provide enough CFM to bring your meter up into the .2 % range quickly, then taper off.   I know this may seem high but it depends on the amount of space you are trying to purge.   Personally I would run a very high flow until your meter reads over .2 and then cut it down and retest the welding. I have welded a lot of 100% only conditions and it can be very difficult to find the fine line, it will vary according to the structure, how fast you release it, and your flow vs. pressure.  I agree with jf55, the meters can be dodgy and will not read correct, especially dependent on the flow past the sensor.  I have seen them read wildly different dependent on which part of the chamber we had them in....even with what we know would be a full evac.  I would expect to have very high percentages with a three hour purge, just off a gas bottle and hose, even on a good size vessel.
my little  $.02

I hope that helps you
Parent - - By rangerod (**) Date 12-11-2010 05:47

The part is 4" "donut" shape 88" in diameter. Input gas is fed with a gas lens pointed in the direction of the circumference & sealed coming in at the bottom of the "donut", meter is sealed reading at the top of the "donut" with a 3/32" purge port 90 degrees from meter. I am trying to purge the entire tube so I can weld continuously. By the sounds of it my purge was not long enough because I relied on the purge meter. This is my first experience with a monitor & I was under the assumption that once the monitor said go it was a done deal. I was wondering if I installed a baffle would it keep the gas from running in circles. I am new to this type of sealed purging so please bear with me. Is there a way I can calculate how long I must purge a specific volume since purge monitor is not completely reliable? I don't have a reference point of experience to gage this from. Thanks for your help.

Parent - - By Tommyjoking (****) Date 12-11-2010 18:52 Edited 12-11-2010 18:55
Are you sealing your joint(s) off and removing as you weld?   If have many joints are a very long one...they should be taped off until welding.   A 4" pipe running in a 88" diameter should completely purge in say about 25 minutes...not hours.  If I am understanding you right that is.   This is important....your "purge port" or your outlet needs to be at the most vertical or highest point of your vessel.  If it is not more then likely your purge gas is just escaping and leaving the oxygen above it.   Turbulence can be an issue, causing the two gases to mix but overall gravity will take effect and work long as you know you don't have a "wind" going on in there.  The monitors do work I just don't trust the handhelds I have used closer then say a 15% tolerance.   I honestly could not give you a percentage of gas where you would get the results you want, ( even .2% seems very light to me) someone else here will have good advice on that.  But a full purge of that size would not be very much gas.

Hopefully someone with more experience will chime in with better advice.
Parent - - By MMyers (**) Date 12-14-2010 23:28
Just replying here because this seems to be the most logical place to do it.

What kind of argon bottle are you pulling off of?  High pressure cylinders can be compromised if they are not handled properly at the filling station.  We use liquid argon for all argon in our lab.  Purge rates are typically below 30 cfh during purging and ~10 cfh during welding, depending if we're doing a full pen root or other conditions.
Oxygen content for purge and gas quality is measured in parts per million.  Your meter can not measure that precisely, 1 ppm is 0.0000001%.  Our purge criteria is 30 ppm max on everything but Titanium, but under 20 is very common. 
Have you checked every single connector between the gas valve and the purge inlet?  We use Snoops brand leak check every time we make a gas connection. 


Just read the dimensions of this thing.  The size and number of openings are probably what's killing you.  That you're getting blue on this kind of part actually impresses me.  Tape every single nook and cranny that is open except for your vents.  I'd probably purge at about 8 locations around the diameter of the part if you want to do continuous welding. 
You don't need a gas lens for purging.  SOP here is just to stick an open tube into the ID of the part.  Back purging plates is a little different, but same general "not lensed" approach.  Silver roots are the acceptance criteria where I work, regardless of material.
Parent - By rangerod (**) Date 12-15-2010 04:17

I am using a single 330 bottle. "monitor read .02% to .03%" maybe I should have left out % the monitor reads .02 to .03 on the screen when I started welding & instructions state "welding may commence after .1 is achieved. Connections are good seems like I was pushing too much cfh during initial purge. Finished the part today & the best I could get was shinny gold with a slight shinny blue here & there. Silver roots are what I am striving for so I will implement any suggestions & see where I can get it. Thank you.

Parent - By js55 (*****) Date 12-13-2010 13:36
Unless you have an acceptance criteria that says otherwise dull blue is fine.
Parent - By Jim Hughes (***) Date 12-17-2010 14:11
With all the parameters you introduced us to, if your getting a dull blue you are there. That is not surgar, and unless you have some color ristriction you are fine. Remember when your dealing with stainless your goal is passive protection. Oxidation (black or surgar you don't have it), dull blue very little oxidation, silver no oxidation. Again the caveat is, what, if any color restrictions your under or what process will see the back side of that weld. Is it food service? then color matters. My last point is if you have access to the root side of the weld, even if it is sugared you can clean by light grinding to get rid of the sugar. Once surgar (oxidation) is removed then passivity is restored.

- - By commonarc (**) Date 12-11-2010 21:07 Edited 12-11-2010 21:43
You may want to try some solar flux.  It's a powder mixed with alcohol to make a paste and you paint the inside of the weld joint with it.  We've had good luck using this stuff on high pressure stainless boiler tubes. This, combined with your purge should do the trick.

As stated above, you should use metal tape on the whole seam except the small part you are welding on.  You'll never get a proper purge unless you do this.
Parent - By Lawrence (*****) Date 12-11-2010 22:12 Edited 12-11-2010 22:15
Solar Flux is a last resort for applications when inert purge is impossible.  Solar Flux is not an inert gas equivelant.

If your only getting a little "blue oxide color" for your current efforts. The solar flux will leave a nasty and often corrosive residue on the back side of the weld which must be removed in most applications, of course this depends on what your vessel is holding.  Your little blue oxide color on the root of those welds will not likely effect your mechanicals... Solar Flux will.

Tommy gave good advice..  Think about his words regarding a purge relief hole at the highest point of your part.. It may be worth your efforts to roll the part so that your purge outlet is at 12 O'clock..

Try Tommy's advice and let us know what happens one way or the other.
- - By rangerod (**) Date 12-13-2010 16:00
To all

There is no acceptance criteria for "color" just trying to get the best results possible. The purge port is at the highest verticle point & the current weld is back weld on the root so there are no longer any open seams. When I do 100% penetration welds with no access to the inside I do tape shut all seams & only remove only enough tape to perform one weld area at a time. I do have solar b but at this point my wps is set up for backing gas but I'll keep that in mind for the next project if it is not holding corrosive liquids. Seems like all bases are being covered just thought I would run it past everyone to see if I missed something. Thanks to everyone for the help it is greatly appreciated.

Parent - - By ZCat (***) Date 12-13-2010 22:04 Edited 12-13-2010 22:07
Turn up the purge! 10 cfh in unacceptable if you ask me. I have welded a metric sh!t ton of stainless and I never purge anything on 10. You need to be able to feel it coming out the bleed hole when you pull back the tape to start welding.
Parent - - By rangerod (**) Date 12-14-2010 02:47
The 10 cfh is the maintenance purge not the initial purge which is 50 to 70 cfh. The maintenance purge I have come to find out the hard way (internal blow out of puddle) must be 5cfh or less when there is no purge port other than the open root of the seam to purge out of. Is there an alternative to this method? On this part I have a purge port installed & today I bumped the maintenance purge to 15cfh which helped alot. I debate which is the lesser of two evils, the color or bump it up & chance that the puddle will blow out. I am learning but I wish I could pick up my learining curve without bringing any of the vessels any harm. Thank you for help.

Parent - - By js55 (*****) Date 12-14-2010 13:40
Actually, the 50 to 70cfh could be your problem.
Note, when you purge slow the argon will fill the bottom of the cavity and then push the air out of the top, theidea being to eliminate turbulence.
If your argon flow is too high the air will mix with the argon and it will take longer to evacuate the air.
This is why I always tried to use multiple hoses with low flow. This will increase the volume of gas entering the cavity while at the same time reducing purge time.
Parent - By ssbn727 (*****) Date 12-14-2010 14:47
Excellent point Jeff! ;) That's how it's supposed to be done IMHO.

Parent - - By rangerod (**) Date 12-14-2010 17:02

I was wondering if something in my techique might be an issue. I was concerned about turbulence but never considered my purge rate might be a problem. I will add multiple hoses in future, very good information. Thanks to all for your assistance.

Parent - - By ZCat (***) Date 12-14-2010 21:14
I guess I'm a little confussled on what your setup is. You have a hole in the bottom of the pipe for the argon to come in or are you purging thru the gap? Why is the purge outlet only 3/32?
If you had a bigger outlet hole you could turn up the purge and not have to worry about the root blowing out, and fill the pipe up with argon a lot faster. That's the way I do it. Most places would laugh at you for taking 3 hours or using multiple hoses to purge a piece of 4" pipe.
Parent - - By rangerod (**) Date 12-15-2010 03:25

This one was a backing pass on the inner diameter of the tube with inlet purge in the bottom of the pipe. I guess I got confusing because I reference previous vessels where there was an open root where I could not even add a purge port, I had to use only the open 2" long seam being welded at a time for the vent. You can laugh if you want but I'm here trying to inform myself as best I can, some of us do not have the luxury of mentors to guides us in our ways. Thanks for you help it is appreciated.

Parent - By Tommyjoking (****) Date 12-17-2010 01:37

I hope this is starting to work out for you at this point.  Your initial posts and other comments on here can give a clue to such long purge times for such a small area.  I did say push hard initially and taper down....pushing hard would be about 30-40cfm.  At very high flow rates with everything sealed off the mixing issue will come into play.   You should be able to fully purge out this pipe 100% in 20/30 minutes and weld it out with no color at all.  I agree with others here that some blue, without any code stating otherwise is perfectly acceptable

Good luck to you
Up Topic Welding Industry / General Welding Discussion / SS Purge Issue

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