I asked this question on Reddit a while back but nobody seemed to know the answer, so here we go.

In the welding/soldering/heating world, it is well known that at pressures above 15 PSI, acetylene gas becomes unstable and can violently and exothermically decompose into hydrogen gas and carbon, causing an explosion even without the presence of air/oxygen.

Acetylene cylinders have a pressure of around 250 PSI, but this is a-ok because the acetylene is dissolved in acetone or DMF in a sponge of calcium silicate. But not all of the cylinder is occupied by acetone and calcium silicate, specifically what I am talking about is the valve assembly.

In the valve (and high pressure side of the regulator if present), acetylene gas exists at around 250 PSI. Now I know this is safe because it hasn't changed in decades, but how is it safe? Wouldn't this be a large pocket of high pressure acetylene that can explode at any moment?

I am clearly not understanding something, does anybody know?

As far as I understood, it still needs a trigger (a temperature surge,...) to start the reaction.
As a gas (so not dissolved) there is only limited amount of the acetylene.
And unless the whole bottle is heated up, even when that small volume triggers the reaction, it does not release that much of energy and the products, so both the products (mainly the hydrogen), as well as the heat can be absorbed by the other materials within the bottle (the sponge holding the acetone, the metal mesh holding the sponge,...).
So the bottle construction is able to contain and quench that reaction.
Of course everything changes if the bottle gets heated up (e.g. in a fire,...), there the ability of the acetylene to dissolve into the carrier materials droops, so the pressure goes up, increasing the amount of the acetylene in the gaseous form, so the amount of the released hydrogen and heat increases, the preheated material has no margin to absorb any more heat, so it really runs away and kaboom...

Something similar goes with the oxygene: At that high pressure and purity, practically all materials usable for pressure bottle construction burn readily in the oxygene. So the only thing missing is the heat for an ignition. Typical remedy is to have all the internals clean, so any combustible material is in a solid chunk, so needing a lot of heat to warm up, but not that large surface to generate that much heat from combustion. So unless disturbed (heat of surrounding fire, or someone shooting at the cylinder), it is quite stable, so it could be used in that form.

@Medved
Thank you so much! That definitely makes sense now, and now that I looked up pictures of acetylene tank valves they do seem pretty cramped inside, with all of the acetylene gas being pretty close to the thermally conductive brass.

Very fortunately, for glassblowing, you can readily use natural gas/propane + low pressure oxygen from a concentrator, which removes most of the risks.
Soft glass is workable with safer gas + air. Acetylene is mainly reserved for metalworking, and even there are safe toys like MIG/TIG welding.

@RRK
Yes of course, I know I was just talking about glassworking before, but my hyperfixation has suddenly shifted as it often does . In my opinion acetylene is kind of obsolete for most purposes, but that is apparently a controversial one.