When will the Acetylene decompose explosively? I meant to say on what conditions Acetylene explosively decompose?

Quite a lot of conditions.

In particular if it's hot or under pressure (more than about half an atmosphere above ambient) or mixed with air or another oxidant.

You should probably have checked on this before it became urgent.

Acetylene is known to detonate by decomposition, including without oxygen. Pressure is a known factor, heat an other, many banal metals also - they act as a catalyst. You could see the MSDS.

 

Not only acetylene: all small acetylenic molecules behave this way. Including Propyne, which a rocket design hobbyist tries to advocate as a propellant.

 

It's the reason why acetylene for torches is kept dissolved in acetone, instead of just compressed in the bottle as is done with other gases like propane, oxygen...

 

A Russian team (at a very reknown manufacturer) presently tries to develop a rocket engine that burns an ammonia-acetylene mix. I hope this bizarre attempt did not result from any of my ideas. Both ammonia and acetylene are far too dangerous to store and use in 100 or 1000t amounts in a rocket.

http://www.sciencemadness.org/library/books/ignition.pdf

 

Rocket fuel has ALWAYS attracted those with nerves of steel or no common sense but the PDF of the above book is one I love. My favourite? An accident involving CF3 - you read it right; chlorine trifluoride.

 

I also love that German chemist who makes 'energetic materials. Thomas M. Klapötke - http://www.chemie.uni-muenchen.de/ac/klapoetke/content/klapoetke/research/energetic_materials.html How about this:

 

http://img139.imageshack.us/img139/4994/schemetou3.jpg[/img[

 

 

Try again!

Haha let's try scaling that one up.