I've built/used lots of different purge and trailing shields and find that something simple is generally just as good as something super complicated. The biggest thing I've seen impact quality is that you'll want a good seal between your purge system and the work (glass cloth heat tape is what we use) and fairly even coverage over the length of the joint to be shielded, and a vent so there only slight positive pressure on the purge volume (it takes fractions of a PSI to move a weld pool). For something like what you're doing, I'd make up small modular sections (maybe 2' long) of SS tubing (we use small diameter stuff, like 3/16"-1/4") with saw cuts in it every two inches, supply them with two gas feeds at 6" from each end, and tape them to the backside of the weld groove so they're maybe 1/2" to an inch off the work. If you have some drops from the job, you can give this a run and see if it'll work for what you're doing before you try it on the job.
Topic American Welding Society Services / AWS Learning & Education / Stainless Steel Butt Welds With Backing
