View Full Version : NItro purge weld, but pipe full of soot

01-11-2016, 08:41 AM
Hi fellas,
Got a bit of an issue here.
I've just finished welding bends, etc into my pipe run for a 16kwr ducted mhi unit.
The copper is 1/2 and 7/8.
I religiously purge with dry nitrogen when I weld anything, but noticed this time round there was a whole heap of flaky black sooty **** blow out of the 7/8 when I cranked up the nitro.
Looks as though no nitro was used almost.

What's gone wrong? I'll try and post up some pics of the welds I cut out.

I'm very Hesitant to fire the machine up as is. I think at least I'll add a suction drier and hopefully that'll save my rmd from being clogged up with rubbish inside a month.

Thanks in advance guys

The Viking
01-11-2016, 12:04 PM
Sometimes we get it wrong and there is no flow at the point we are brazing other times the gas supplier get it wrong when they fill the bottles.
Did you use Oxygen Free Nitrogen [OFN]? There are other grades of nitrogen available that contain oxygen as well as nitrogen and these are no good for us when we braze.

Moving forward, personally I would put a liquid line drier/filter in just before the indoor unit and a suction line drier/filter just before the outdoor unit, that way both the EEV and the compressor would get the best possible protection.


Glenn Moore
01-11-2016, 11:11 PM
Was the pipe sealed at the purging point .Often I see the nitrogen line just layed in the end of the pipe being brazed. The flow of nitrogen then draws more oxygen into the tube like a chimney causing more oxides to be deposited inside the tube

02-11-2016, 09:38 AM
Hi fellas,
The nitro is from our local fridgie gear wholesaler, so I can only assume it's correct. I'll check this and see what the msds says about purity.

I started welding from the condensor end, the copper was flared there and hooked up to the machine. NItro was fed in via the service port on the line being welded.

11-11-2016, 09:22 AM
Hi guys, so the issue was traced back to nitro being sold by a wholesaler as "dry nitrogen " actually containing oxygen, and not really dry either.

I can only assume there are hundreds of ticking time bomb installs that people have welded in good faith that are just waiting to have a clogged up orifice of some sort.

I sourced high purity nitro from boc gas ( Australia ) which is less than 10ppm oxygen and water.
Welds up a treat, only issue is I need to basically re pipe my job, as what's there is trashed.

Rob White
11-11-2016, 09:35 AM

Interesting situation and one a lot of us take for granted.
OFN purity as proven by you is a real issue and therefore
something to keep an eye on. I know I very rarely think
about the purity, I just use the stuff supplied. I have always
used gasses from the big reputable companies like BOC, A-Gas
and such but it does make you wonder.

Thanks for the update.