I assume that the 64.5 is 94.5%
ISO 10156 Gases and gas mixtures - Determination of fire potential and oxidizing ability for the selection of cylinder valve outlets is the reference document for this under the Fire Code and GHS
While 4% in air is the LFL, in N2 it is 5.5%. Different dilution gases will change the LFL due to conductivity. In a release you will be well below this since it must mix with air
Chemically Speaking LLC
In light of the most recent incidents I would like to know if anyone considers a Hydrogen(0.1-5.5%)/Nitrogen(64.5-99%) mixture flammable? .
My name is Eugene Ngai, Chemically Speaking LLC. I have spent over 40 years in the compressed gas industry and currently consult for many universities, national labs and private companies. I have done projects for some of you.
I was retained to help the UC Center of Laboratory Safety investigate the U of Hawaii incident.
They have forwarded me a number of questions regarding hydrogen. I would like to comment as follows
In the US there has been considerable work done on H2 safety, currently NFPA 55 Compressed Gases and Cryogenic Fluids Code chapter 10 addresses gaseous systems and Chapter 11 liquid systems. We have been working closely with NFPA-2 which will be the new standard for H2 used as a fuel. Sandia National Labs is the lead research group on this. We are developing a test protocol to measure dispersion from a liquid vent stack so that we can accurately model it. Numerous meetings have been held to conduct a fault tree on siting a H2 fueling stations.
As to the Tsinghua incident, it is very difficult to get accurate information on an incident like this especially since it involves one of China's primer universities. I am told that the cylinder ruptured. Without seeing the cylinder it would be hard for me to speculate on what caused it. I don't know if he was filling the cylinder. As you will note in my article "Dangerous Gas Mixtures Avoiding Cylinder Accidents" which is hyperlinked in the webpage. People fill cylinders all the time with gases that could compromise the cylinder. There have been many incidents involving fuel cell research.
I had sent out an alert about this problem in 2011
A second possibility is hydrogen embrittlement. This typically occurs with high strength steels used for some cylinders. The Europeans suffered from hydrogen cylinder ruptures for many years. There is an ISO standard 11114-4 on how to test metals for H2 embrittlement. The typical US DOT 3AA and 3 A cylinders have a high enough alloy content to not become embrittled. Since it was H2 it was most likely to be filled in China. China has some very strict regulations under the GB standards on what cylinders can be used, but again not knowing the specifications or history of the cylinder I can only speculate.
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