Wrapping your head around purging can get a little crazy if you don’t have a habit of pipe welding corrosion-resistant materials. In reality, it's quite a simple concept. Imagine an older woman who’s house is packed full of cats. So many cats. And their causing problems (listening to loud music, not putting down the toilet seat, etc.). You need to get rid of these cats. But when you push the cats out, they just come back. So you decide to “purge” the house by bringing in a whole group of dogs. The dogs chase the cats out of the house. As long as the dogs stay there, the cats can’t come back.
Oxygen is the cats. When you’re purging, you’re trying to get rid of all the oxygen inside the middle of a pair of tubes. However, the moment you suck oxygen out of the tube, more oxygen just fills the empty space. So you bring in the dogs, which is your purging gas (Argon, etc.). As long as the purging gas fills the space, the oxygen can’t weasel its way back in.
Why don’t we like oxygen? When working with corrosion-resistant material, welds exposed to oxygen have oxidation occur. This causes the oxidized workpiece surface to no longer be corrosion-resistant. Because the oxidation occurs on the inside of your tubes, grinding isn’t an option (unless you have a highly-specialized grinder). Oxidation can also cause structural problems with your welds including reduced weld strength and premature cracking. So, it’s bad. Real bad. Especially if these pipes are used for something like pasteurizing milk. Or a slippery slide for gerbils.
The actual process of purging your workpiece will vary depending on your welding process and the equipment you’re using, but here are some pro tips that everyone should know.
Use Halogen-Free, Heat Resistant Aluminum Tape
You’ll want to seal off any gaps with Halogen-Free, heat resistant aluminum tape. A beginner mistake is to use whatever tape you have on hand (scotch tape anyone?). Most normal types of tape contain halogens, specifically chlorine. At high temperatures, chlorine absorbs into reactive metals, causing embrittlement in the weld surface.
Use An Oxygen Indicator
The general rule of purging is “The less oxygen, the better.” Oxygen content is often measured in ppm (part per million). Most precision purging welds are done well below 70 PPM. You can hope your purging gas has pushed out all the oxygen and that you’ve properly sealed your workpiece. But without an oxygen indicator, it’s really just guesswork.
Maintain The Right Flow Rate
Setting your purging gas to the proper flow rate gets difficult, mainly because there’s not many accurate ways to measure it. The goal is to achieve enough flow to gently force oxygen out of your tube. This keeps the pressure inside the tube slightly higher than the pressure out, meaning oxygen can’t flow back in. Some welders might be tempted to just open the valves up and let that purge gas flow like the wind. While this might seem like an easy solution to a lack of precision, there’s some definite downsides. First of all, you’ll burn through your purging gas a lot quicker. Second, increased turbulence inside your tube can cause instability with your welding arc.
So, to purge or not to purge? That is no longer the question. Purging is one of the best ways to maintain pipes’ corrosion-resistant surfaces while achieving a high weld strength. For you visual learners, here's one of our favorite purging videos: