Welding Tips and Tricks

Hello folks,

My shop just got a job to build many sections of tubing for a water treatment system. The sections consist of sanitary type fittings on each end with a variety of elbows and straight sections. The total length of each section varies from 1 ft to 5 ft long. The tubing and all fittings are 304 stailess steel, 2", 3" and 4 " dia, 0.083" wall thickness. the TIG welder on my team is an ace, produces excellent welds and a variety of setups and materials. He performs double duty for me, mostly a machinist and a part-time TIG welder. he has produced great looking welds on the practice pieces of tubing and fittings so we are ready to begin the the production work. I need to learn about the process for purging the inside of the tubing and fittings. We have a separate cylinder of argon with a regulator. We have a cap for one end of the tubing assembly which we will connect the argon line to and a cap with a 1/4" dia vent hole for the other end.

My initial plan is to tack the tubing and fittings together as an assembly then put the caps on each end. Purge the interior of the assembly for a few minutes with a flow of 20 to 30 CFH of argon then complete the welding of each section.

I need your expertise to learn how to do this the right way. What do you guys suggest?

Many Thanks!

Jim

Thanks

Jim

sounds to me like you got this nailed already.

throw some pics up of the process

My thoughts on this are - if you need to purge - then as argon is heavier than air try to arrange the feed in at the lowest point so as the tube fills up the air is pushed upward and out untill there is nearly pure argon inside - it is common to use numerous small holes on the exit & if possible use an improvised diffuser on the inlet otherwise there is a danger of creating a small flow which follows the most direct route to exit thus only shielding a small area of the component - it's much like using a gas lense for outside pipework to improve gas cover.

Do you need to purge at all - is the item your producing going to be X rayed or MPI'd or pressure tested that would show up if this had been done - if so there would normally be a spec or approved procedure for the part stated - in which case follow the procedure - if there is'nt one maybe enquire as the best looking weld in the world may not comply with the required procedure & it's a bit late once the job goes away & every part fails. - The reason I ask is that on 83" wall your not likely to burn through - if the joints are say butt welds with any root gap or V prep then purge gas is essential due to 100% penetration - but I've done a few similar sounding pipe welds where the end fittings sleeve over the tube - so the weld is effectively a lap joint - there is not much chance or need to put so much heat in that the inside surface is hot enough to really require extra shielding - but obviously this depends a bit on how critical the application & pressure rating is. I'm guessing this is not a part of a nuclear facility.

Anyhow - this is just my own experience - others may not agree - although I do a fair bit of stainless it's never tested stuff - my only pressure work is steam related pipework & boilers - these are regular steel so this issue has not come up.

All the best with your project.

I'm assuming for water treatment and food grade the purge will be essential, and I'd even do it for socket-weld on .083... It's too easy to get hung up in a awkward position and put a little too much heat in a spot. "Sugar" is a no-no on food-grade, you must maintain the parent metal properties throughout.

It's better to build a set of purge inlets, one for each size pipe with little tabs on them to grab the ID of the tube, and tape them on to seal them good. A sintered bronze "vent" filter in 1/8" pipe thread is a great, cheap inlet diffuser. For purge outlet, same story (minus filter, of course), but I'm with Brian on this. Eight or so 1/16" holes are better than one 1/4" hole. You want a GENTLE flow of purge gas, around 5 CFH. Brian's advice for "bottom-up" flow is dead on.

Tape every joint except the one you're welding (there's a special tape for this, if the welds are close enough for heat to affect the next weld in line, but I forget what it's called), and start at the weld nearest the purge inlet, working toward the outlet. A cheap alternative to the heat-safe tape is to wrap each tacked joint with a 1" wide strip of aluminum foil, then tape with 2" or 3" masking tape (keeps the tape glue away from the weld zone). For tubing 3" and above, you should even dam the part opposite where you start with foil to prevent atmosphere from mixing, and you should be in calm air.

When you're all set up, and turn on the purge at 5, go take a leak, smoke one, or have half a cup of coffee, and you're good to go.

If you find you need more detail than this, let me know. I still know a chap or two who do this stuff regularly, though we haven't talked in some time.

Steve S

Thanks guys, Excellent suggestions. I've started to put together the parts and pieces to make the purge setup. I'll have to adjust the internal purge flow and dwell time to find out what are the right for each length of pipe section. I like the idea of taping the joints so as to not lose the purge flow. Thank you very much!!! I am away on a business trip next week, my team will start the process. I'll keep you posted.

Jim

Several of us are here every day.

I don't have to do food grade, but many of my welds must meet "oxygen clean", and the specs are similar.

Steve S

Hey,

Is the purging gas left on full time or is it filled and then turned off?

Mick

Hey, Mick,

The purge is continuous flow. It's meant to displace any "atmosphere" that seeps in from the parts that aren't taped off, including the backside of the weld you're working. Thus the gentle slow flow... No turbulence.

The idea is that the back side of your weld will never see oxygen while hot.

Steve S

Hey,

Thanks,

Thats what i thought, but wasnt sure as ive never done it.

Mick

Hey there bud, I'm a union pipe welder here at the ancient COORS plant in Colorado and we do a lot of "beer tube". Food grade sch. 5 stainless. Everyone else is giving u great advice. We typically tape off both ends of both and poke a few holes in outlet side and also tape up the joint and weld a quarter at a time. This is all one pass according to spec and use .045 wire for filler. We also use an o2 monitor to ensure a atmosphere free environment. They also just purchased some purge dams to avoid having to purge an entire piece of fab but to b honest haven't used them a whole lot. We typically run between 20-30 Cfh depending on how large ur outlet holes. What Otto said about running a lower purge to avoid turbulence makes sense to though. We run up to 12" pipe though so at 5cfh that may take a while. I do know u have to b careful to over purge though. Had an issue with another contractor out here that had to high a purge and caused terrible suck back on a bunch of fab. Good luck.

uahoodpecker has a valuable point.

I based my 5cfh purge rate on the 1.5" and 2" sch. 5 and 10 I do for process. On larger tube and pipe, like the 12" he referred to, a higher flow is appropriate, and as long as it's accomodated for in the outlet (enough holes to let it all out), it won't create a positive pressure to push your root back.

When I weld larger than 2", it's vent-side piping or vacuum-jacketing, and I back-purge with nitrogen as oxygen-clean is not an issue on this side of the system.

Steve S