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Up Topic Welding Industry / General Welding Discussion / GTAW Purge
- - By emccorma Date 08-17-2007 22:30
Hey guys, I might be asking a question that some think is simple ( and it probably is ) but here goes. I am going to be welding 316 ss sc 40 pipe 4-10 in. butt joint and some socket weld. I understand the internal argon purge but the tape that I've heard you use to cover the root gap is what is confusing me. What type of tape, how much of the circumfrence should be covered at any given time. I am not new to GTAW but my shop has not done any stainless as of yet.   Thanks guys, Ed
Parent - - By ZCat (***) Date 08-17-2007 23:13
masking tape for the outside. Pull back however much you feel like welding, just make sure the purge is set high enough to compensate. You want to be able to feel it coming out on your fingers at the gap and at the bleed point, if set too low you will draw air in the pipe. also, if you do any grinding, say like on a tack, you need to wait for it to purge up again, the grinder shoots a lot of air in the pipe.
Parent - - By chris2698 (****) Date 08-18-2007 02:22
another thing to remmeber is when you are purging when you are just about to fully weld the root you need to back off on the purge some and even punch about 10 holes with the filler rod in at where you have the end of the pipe taped off so when you fully weld the root it won't blow back into your face. I work at a shop that all we weld is stainless very little carbon. Any masking tape will work but I have seen some special tape that is like a yellowish color that is made I guess for tig welding which would be kinda cool to have but probally not worth paying for.
Parent - By ZCat (***) Date 08-18-2007 02:45
that blue painter's tape works real good, too...not as sticky as regular masking tape
Parent - - By Shane Feder (****) Date 08-18-2007 02:50
Emccorma,
You can buy a type of aluminium tape that is brilliant as it can handle the heat much better.
It is more expensive than masking tape but you can weld right up to it without it catching fire like masking tape.
Just jump on good ol google and you should be able to find it,
Regards,
Shane
Parent - - By Kix (****) Date 08-18-2007 04:43
For weld neck flanges you can put a clear welding lens up against the flange and tape it on so you can check your root out and make sure your getting a good purge.  I tape off the end of the pipe somewhere and punch a few holes in for a vent and completely tape off the joint.  After a bit i will peel back a bit of the tape on the joint and hold a zippo lighter over the root opening and see if it puts out the flame.  If it does then your ready to weld.  I like to start off on one of the bottom quarters first because the argon will want to come out there first.  Tape some steel wool or a gas lens to the end of your purge hose and stick it in the pipe also.  This will cut down on the turbulence inside the pipe and get you a better purge faster.
Parent - - By Tommyjoking (****) Date 08-18-2007 09:26
Good advice Ray!!!

I disagree with the aluminum tape for the simple reason of cost + the adhesive is much more resilant....so at the surface it will cause probs.
Chris I have never seen a special tape for this type of work....if you come across some info please pass it along.

Uncover what is gonna make since....say 4-6" dependant on size pipe and gap...what your gonna run in one pass...basically if you do the KIX flame test at the top of where you uncover and it goes out ....you got gas coverage....if not crank it up a bit.  Chris is right on providing holes for the gas to escape especially towards the end of the joint but I will put vents on the pipe end if its open.  I personally will weld it with no land (to speak of) no gap beveled accordingly and a lot of purge if I am allowed to do so.

Best of luck\
Tommy
Parent - - By emccorma Date 08-18-2007 11:16
Guys, thanks for all the info it was a big help , gonna give it a try Mon. If anyone is interested ,I came accross an internal backing tape that looks like it would work also ( providing you have access to the inside of the joint )www.huntingdonfusion.com . I would like peoples opinions.

                                                                                                                 Thanks again Ed
Parent - - By Kix (****) Date 08-19-2007 18:31
There is tape that you can put on inside that will desolve in water that holds the purge paper  or rice paper up inside the pipe to make a purge dam if thats what your talking about.  Thats mostly for tying into lines that don't have a valve close by or an open end close by to keep ya from using a whole bottle of argon to purge one pipe.  Your basically using the stuff you put over the toilet seats of public restrooms.;-)  That stuff would be good for some small bore pipe.lol
Parent - - By chris2698 (****) Date 08-19-2007 20:05
here is a really good website for all tig welding stuff and they sell that aluminum tape but the guy I seen that had this clear tape I emailed him and when he emails me back I'll get back with yall. but here is a good website to check out for all your tig welding stuff http://www.arc-zone.com they sell purging kits but god there a fortune looks like a good place to buy tungsten and stuff

Chris
Parent - - By Shane Feder (****) Date 08-19-2007 20:37
Tommyjoking,
The aluminium tape has been used in New Zealand for years by quality companies who specialise in welding stainless steels for the dairy industry. We had to wear disposable overshoes and change our overalls when entering the workshops so if there was even the slightest chance of contamination it would not have been used.
As I said it is more expensive than masking tape but as I have mentioned in previous postings regarding welder quals / WPSs etc if all the companies in an industry use it, all the companies factor it into there tender price then it is not an issue.
Regards,
Shane
Parent - By Tommyjoking (****) Date 08-19-2007 21:50
Shane

I am sure the Al tape works great...it certainly wont get hot and let go like masking tape will...or get inadvertant holes in it and allow in oxygen in.  Just not sure how easy the glue would be to brush away or the tape to remove after heat exposure around a joint because the stuff sticks so well.  I have been wrong a time or two believe it or not  LOL!  ..Might give it a try myself ..

Best Regards
Tommy
Parent - - By 803056 (*****) Date 08-19-2007 20:39
I typically allow the purge gas to flow until I've displaced the volume of the system six to seven times. That is, if the volume to be purged is one cubic foot, than the purge has to be sufficient to allow five cubic feet of gas to flow through the system. At 15 cfh, it would take about 30 minutes. That sounds like a long time, but it is necessary to assure the oxygen level is reduced to a sufficiently low percentage. Look at it this way. The system starts with 100% air (20% O2 and 80% Nitrogen and assorted other gases), displace the volume once, the percentage of air is reduced to 50%, displace the volume again, and the air is down to 25%, one more time and the air is at 13%, once again, 7%, again and the air is at 3.5%, again and the air is at 1.25% (oxygen is about 0.3%) and one last time the air is down to .6% of which O2 is 20% or about 0.12% by volume.  The math may not be to the first of second decimal point, but so what, you get the idea.

The oxygen level influences the quality of the weld in the system. Too much oxygen and you'll have discoloration. You want a maximum of 0.5% oxygen in your system when you start to weld. As mentioned by some of the other knowledgeable folks, tape the open joints to stem the flow of shielding through the open roots. It is for that reason I prefer to use U-grooves with a 0 root opening and a 1/64 to /32 inch root face. It reduces the chance of aspirating oxygen into the system and it reduces the flow needed to maintain proper purge.

I encounter many jobs where insufficient purge time is allowed to properly displace the oxygen in the system before welding and then the contractor fails to properly maintain the purge while welding by taping the open root joints.

As for maintaining the proper pressure, a magnehelix can be used to monitor the internal gas pressure to prevent blowing the root pass out of the joint. You want any bleed holes to be located at the highest points in your system because the argon is heavier than the air. The water soluble dams and removable bladders do a good job of reducing the volume to be purged and reduce the amount of gas required and the time it takes to obtain a proper purge.

Good luck - Al
 
Parent - - By Shane Feder (****) Date 08-20-2007 00:39
Tommyjoking,
The Al tape does leave residue but all the work we did was food grade stainless so every weld was pickled/passivated.
One hour with the paste on and then a quick rub over with water and a plastic scourer and everything was removed from the surface.
Regards,
Shane
Parent - By chris2698 (****) Date 08-20-2007 00:53
I emailed the guy and he said it is a 3m tape  he wasn't sure of the name just that the stuff was 60 dollars a roll so I did a search at the 3m website and came across this tape 5490 film tape http://solutions.3m.com/wps/portal/3M/en_US/3M-SpecialtyTapes/SpcltyTape/ProdInfo/Catalog/Slick-Surface/ I did a google search and yeah this stuff is around 60 dollars a roll not worth it to me but maybe to some of you.

Chris
Parent - - By jonesy70 (*) Date 08-20-2007 00:57
how about something simple like aluminum foil from your local grocer...its cheap and works great...it holds up agenst the heat pretty well and does not leave the sticky residue from the tape when it gets hot...if you are welding fanges or open fittings and want to look inside with out loosing your purge..tape a clear protective lens over the end...that way you can see  inside the pipe...regular rubbing alcahol will remove the tape sticky residue...you just need to use a little elbow grease!!
Parent - - By chris2698 (****) Date 08-20-2007 01:02
regular masking tape works fine for me it's free at work I pay nothing for it and the sticky stuff well I never have a problem with it. It gets burned off. I just thought the clear stuff was kinda cool.

Chris
Parent - By swsweld (****) Date 08-20-2007 02:11
Usually a lighter works OK to see if the oxygen is displaced with argon or helium. An oxygen anilyzer is better. Using too high of volume to purge can blow out a lighter or match if its mixed with oxygen and argon.

I use a lighter if that's all I have. If I have an anilyzer I try to get the oxygen level to 1% or lower if possible.

We were doing R & D for a project years ago to repair a reactor head. It was a tough design, alloy, etc.
Some very good welders working on it.
The company welding manager (a very scientific type guy) bought a very expensive anilyzer that measured parts per million. We thought it was a waste of time and money. After nothing worked to get the root pass in successfully we finally tried it his way. We got the oxygen levels down below 100 PPM and it made a huge difference. I think it was 100 PPM; many moons ago. That is a minute amount.

Of course all alloys do not require this but that one did.
Parent - - By OBEWAN (***) Date 08-20-2007 16:53
Caution is required regarding any burnoff of masking tape or residue.  Our welder here burned his masking tape and got so much co2 in the shielding gas that he ruined the tungsten, and severely oxidized the weld.  I would say for critical applications to go with the aluminum rather than risk the contamination if the welder is careless.
Parent - - By Shane Feder (****) Date 08-20-2007 22:45
Hello guys,
Just to add a bit more regarding ali tape versus masking tape. OBEWAN is correct, it has to be critical work to justify the expense.
When you start your weld on the bottom of the pipe it is near impossible to stop the masking tape from catching fire, what happens then is your argon starts leaking out the lowest point which is at the bottom of the pipe and this can affect the purge in the uppermost part of your pipe. You can strike an arc almost touching the ali tape and you do not have to worry about it catching fire,
Regards,
Shane
Parent - By CarmenElectrode Date 08-22-2007 16:50
Hey guys,  thanks for the kudos!

We do have a tape that is adhesive free along the center, called EZ Zone tape:

http://arc-zone.com/catalog/web_store.cgi?page=purget.html
(scroll down a bit to see it)

It's made by Aquasol...  here's the link:
http://www.aquasolwelding.com/index.php?option=com_content&task=view&id=37&Itemid=
Up Topic Welding Industry / General Welding Discussion / GTAW Purge

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