Stainless Purging Pt.2

Frequently Asked Questions
Written By Sean Sanders

Hey this is Lori popping in. I am currently the editor of the current articles. This is Sean’s view of Purge Welding. You may notice there was another one written by Garrett last August. Here is the link to that if you would like to compare notes, Purge Welding. You are now free to continue reading. ENJOY!!

I’ve already written an article about sanitary welding, but let's talk a little more in depth about the purging process.

What’s weld purging and why do we purge? Weld purging is the process of removing all oxygen from the inside of the weld area to increase weld quality. When oxygen is not removed from the weld atmosphere it will cause defects in the weld, namely sugaring, which is the common name for oxidation on a stainless weld. Sugaring causes a very granular structure on the weld which weakens the weld and traps bacteria. This becomes an issue especially in process piping due to contamination from product being run through the pipe.

What do we use? The gas most people use to purge weld is argon. There’s already an ample supply around almost anytime you are tig welding and it works extremely well; however, there are better options for some situations.

100% Argon 

This is the most common purge gas. It's the most cost effective gas for purging sanitary piping and it is extremely predictable. It's very easy to know how it acts, especially if you already have some welding background. Argon is more dense than air and flows similarly to the way water would, which makes it easy to know how to use it most effectively -  just let it flow in from the lowest point possible and let air escape from the highest point.

100% Helium 

This is probably the next most useful purge gas because it’s lighter than air, which makes it perfect for jobs where you can't purge an entire piping system or a large tank. Helium has high thermal conductivity, which means, for a given amperage you can get more heat input. This is especially valuable if you're welding with a 120v welder or if you're welding on copper - but that's a lesson for a different time.

100% Nitrogen

This is commonly used for purging equipment in the medical field. It should not be used for sanitary welding as it will cause weld defects.

98/2% Argon / Hydrogen 

The value of an argon/hydrogen mix is that the hydrogen will react with any oxygen left after purging and turn into water. That could be the difference between a perfect weld in a place where it's not possible to remove sugaring, and having to re-work that piece because it's not acceptable.

Solar-flux

Solar-flux can be useful in specific scenarios, but typically I wouldn't recommend it. It's not 100% reliable and produces a very hard layer to be able remove surface finish. Although it’s not appropriate for sanitary tubing, it can be useful for situations where it's not feasible to purge with a gas and a sanitary weld isn't necessary, but you do need a good quality weld with no sugaring. I often used solar-flux working at a toilet paper factory that used only stainless pipe. It didn't need to be sanitary, but did need to be smooth inside so the pulp wouldn't clog anything.

Purge pressure

Typically you can flood gas at a fairly high rate for initial purge (ex.15-50 cfh) depending on the project and size of vessel to be purged. Once it’s been completely purged, you would drop that pressure to 5-10 cfh. It's worth saying that purge pressure and different gasses inside or the weldment will affect how it welds. With the right purge pressure, your weld will be smooth and free of defects. (Hopefully) If your pressure is too low, the weld will become concave and sink down or even “fall out” of the material. If purge pressure is too high, it will literally blow a hole through your weld from inside the material.

Purging equipment

All the different solutions to this problem could be its own article. The most common answer is using rubber plugs to limit the area that needs to be purged and a double flow regulator. You can also use a single regulator with a Y fitting, but it's not as accurate when setting a purge pressure. There are also water soluble dams that can be put into place, which are convenient when welding on a long run of piping that cannot be easily removed or closed. It’s commonplace to use any valves that may be in your piping run as a purge block, since they are watertight and already in place. The option I most prefer is using a fitting that matches the end of the piece you’re working on and having a tapped hole in a blind plate with an air tool exhaust as a gas diffuser.

Sean Sanders
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What makes a sanitary weld ?
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The Importance of a Clean Shop