xXCARL1992Xx wrote: ↑28 Jun 2022 19:43there is no vacuum, these trailers are all emptied via pressure
My bad here, I was remembering from old; maybe it was used time ago instead of pressured aas you explained is used in those.
xXCARL1992Xx wrote: ↑28 Jun 2022 19:43compressed air can get hot as hell
Literally up to double the air temperature (in kelvin) at intake; as the max pressure is 2bar as you said. But again, those parameters aren't taken randomly; if they allow 2 bars is because they know that temperature will NOT cause ignition on the flour with a good margin of safety. That's engineering, not guessing; those vehicles had to conform to lots of regulations to be certified and allowed to operate.
And that returns again what I said: bad maintenance (that caused malfunctioning/bad readings on the pressure being pumped, for example) or bad procedure (pumping into the tank air that was way hotter than max allowed at intake). Would that make flour to be considered dangerous, then everything hauled on a truck is because there's always big dangers when a trucker is reckless with procedures or the mechanic doesn't does a good maintenance. ADR cargo is ever dangerous and specially if leaked on accidents; while flour at most would be a pain in the back to clean it up.
And, BTW, bad design of that trailer too. It has been centuries already that it's know in engineering that an explosion on a confined space will break through the weakest point. Everywhere prone to suffer an explosion is tried to be engineered with a controlled weak spot to at least redirect most of the damage to the less dangerous place. You can find examples of that in lots of military equipment, but also in chemical facilities, fireworks factories... Should that had been considered when designing that trailer, no way the weakest point should had been facing the place where most of the time (with the trailer loaded) you can find almost sure a person (the driver). Some cover on the top should had broken way before that welding on the front collapsed. It could even be engineered that this cover would open "safely" (remaining attached to the trailer instead of being blown away to avoid landing over somebody's head). At that range of pressures isn't that difficult.
Regards
PS:
@abasstreppas agree with the static spark, with all the friction of the materials and air through the whole system during charge/discharge, probably a lot of static could build up. But then the grounding of the trailer should had to care about this if it was properly set. And you are right, sugar has the same problem and due the same carbs nature that share with flour. The smaller and more refined the particle, the more prone to burn.