Needing more than a spark test?

[rwm]

I'm surprised with 10% flammable gas its actually flammable? Kind of like spirits that are less than 100 proof.

 

[WobblyHand]

Acetylene in air has the widest flammability of all gasses. From my faulty memory it is flammable from a few percent to over 70%. It's really easy to make a carbide cannon (calcium carbide releases acetylene has upon contact with water) because of the wide flammability limit of acetylene. It makes a heck of a bang!

But acetylene in argon wouldn't be inherently flammable, as long as there were no air leaks in or out of the mixture. Argon is inert and doesn't support combustion.

 

[Weldingrod1]

I thought hydrogen was the most bang-a-licious

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[WobblyHand]

I thought hydrogen was the most bang-a-licious

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Hydrogen is up there! Think acetylene is worse, but haven't checked flammability limits in a while.
Hydrogen leaks have their own quirks, as expanding hydrogen (leaks from high pressure to low) heats rather than cools, which ignites the hydrogen. Burning hydrogen has a lot of UV and a little bit of blue, the flame is nearly invisible to the eye. Unless of course you add a whole Zeppelin to the mix. We generally don't use hydrogen for balloons due to fire risk. The Hindenburg disaster in NJ put an end to that.

 

[rwm]

OK- What are the physics of hydrogen heating on expansion?

 

[WobblyHand]

Acetylene LEL 2.5%, UEL 100%
Hydrogen LEL 4%, UEL 70%
LEL = lower explosive limit
UEL = upper explosive limit

This means it's possible for pure acetylene to explode without air, or oxygen under some circumstances. Definitely a material to handle with care. I have a couple of tanks, and do take care, nothing beats an acetylene flame, save for an oxy-acetylene one!

 

[rwm]

Yes acetylene can decompose with heat or shock and it is exothermic so I can cause chain reaction and explosion. I believe that is why is it stored by dissolving in acetone in the tanks?
Joule-Thomson effect:
"Hydrogen has unique properties that lead to safety issues during compression and storage. While an ordinary gas cools down when expanding, hydrogen at any temperature above its inversion point (-80°C) heats up when expanding. The problem is that gas expands from compressor outlet to storage tank or in case of leakage. An ordinary gas would therefore be cooled and become less reactive, but hydrogen is becoming hotter and more reactive. In case of leak towards the ambient, a hot hydrogen/air mixture is created which can self-ignite if hydrogen concentration is above a few percents."

 

[WobblyHand]

OK- What are the physics of hydrogen heating on expansion?

Hydrogen and helium and maybe another gas, have Joule Thompson coefficients that are negative at our "normal" temperatures. So unlike oxygen or nitrogen or argon, expanding room temperature hydrogen will heat up instead of cooling. This presents problems if you want to make liquid helium or hydrogen. However, if you can cool the compressed gas below some magic temperature, then the JT coefficient becomes positive again. So the compressed hydrogen needs to be cooled below that magic temperature. If expanded at that temperature, it will then cool further.

 

[WobblyHand]

Yes acetylene can decompose with heat or shock and it is exothermic so I can cause chain reaction and explosion. I believe that is why is it stored by dissolving in acetone in the tanks?

That makes it safer, I believe. I had also heard that the inside of an acetylene tank contains balsawood, but I haven't ever opened one to find out. Think it's used to prevent the acetone from sloshing around.

 

[rwm]

Fascinating. I will read more...