Green flame in HVOF hydrogen - oxygen combustion

[JeroenB]

Dear, 

We have a HVOF installation fueled by hydrogen and oxygen. Normally our flame is almost invisible but sometimes it turns green. We have narrowed down the origin from the hydrogen gas, when changing a hydrogen pack the flame can suddenly turn green. In the past we always rejected these shipments of hydrogen but it has become more frequent since we switched from hydrogen supplier. Our accuraspray camera measures no change in particle speed or temperature with a green or invisible flame. 

My current suspects are:
1. Impurities in the hydrogen weaken the copper somehow and it is burned, turning the flame green
2. Impurities in the hydrogen gas itself turn green when burned

Do others have similar issues? Do you have other ideas what it might be? I'm not sure if I should be worried about the green color.

Best regards,
Jeroen

[Vadim Verlotski]

Dear, 

We have a HVOF installation fueled by hydrogen and oxygen. Normally our flame is almost invisible but sometimes it turns green. We have narrowed down the origin from the hydrogen gas, when changing a hydrogen pack the flame can suddenly turn green. In the past we always rejected these shipments of hydrogen but it has become more frequent since we switched from hydrogen supplier. Our accuraspray camera measures no change in particle speed or temperature with a green or invisible flame. 

My current suspects are:
1. Impurities in the hydrogen weaken the copper somehow and it is burned, turning the flame green
2. Impurities in the hydrogen gas itself turn green when burned

Do others have similar issues? Do you have other ideas what it might be? I'm not sure if I should be worried about the green color.

Best regards,
Jeroen

Dear Jeroen,

A green flame is unambiguous evidence of copper oxidation in the combustion chamber or nozzle. There can be no more explanation for this phenomenon. The main question is, where can excess oxygen come from in the combustion chamber? Since you see a green flame when changing a hydrogen cylinder, a possible explanation could be the presence of air in the pipeline. When changing the cylinder, air enters the pipeline and it must be removed by purging with hydrogen from the new cylinder. If such a purge is not done, then the first portion of hydrogen from the new cylinder will contain a lot of air and the flame will become oxidizing for a short time (copper will oxidize in the combustion chamber and the flame will turn green). In other words, the appearance of a green flame after changing the hydrogen cylinder may be the result of an incorrectly performed cylinder replacement.

[loriolo]

Hi Jeroen
I had this problem with APS (NiCrAlY yellow colored), it was a very little gas leaking while the system was inactive, but after the system was working and the gas was flowing on tubes, the outside air ((air+H2O) was aspired for venturi effect, this was clear after check of gas quality after gas pack (Ok) and the gas from gun (with water), I think that for HVOF that pressure and flow are higher the phenomena’s could be worse

[JeroenB]

Dear Vadim,
Thank you for the reply. Throughout the entire hydrogen rack (16 cylinders) we have the green flame. Therefore I have excluded remaining air, dirt/solids in the cylinder, dirt/solids in the tubes,... Whatever it is I'm confident that it is dissolved in the hydrogen. I have requested to do a full lab analysis of the hydrogen cylinders causing a green flame and the one causing a normal flame. I'm still worries that some organic compounds (oils durning manufacturing?) or acids (for cleaning?) are present in the hydrogen which affect the copper parts of our gun. I will keep you posted when I have the results.

Dear Loriolo,
Interesting experience! I don't think this applies here as we don't have the problem with certain racks of hydrogen and it is really changing the cylinders that turns the flame green (so I expect no issue with piping or leaking in our system). Also we run a leak test every week. But I will certainly keep your messge in mind for (potential) future problems!

Best regards,
Jeroen