I work around these fertilizer facilities quite a bit for my job as an environmental scientist. Reading through all the info out there about this particular site, I feel like I should attempt to clarify what likely happened. Follow below for my first diary.

One of two things likely occurred at this site. Unfortunately, I haven't been able to confirm all the facts of the fire and their particular manufacturing practices. Here is my take on all of this:

1) They manufactured/stored ammonium nitrate, which was used in the OKC bombing. That stuff is dangerous and obviously very explosive.

2) The other option, and the one I'm leaning toward is that the anhydrous ammonia tank/s (usually between 12,000 and 30,000 gallons) exploded. Let me explain. Anhydrous isn't relatively explosive, or flammable (although certain conditions allow it to be), but it is stored under pressure. This seems to me to be the likely culprit.

The anhydrous is stored as a liquid in the tanks, and when a fire burns uncontrolled under or around these tanks the anhydrous starts to boil (it's boiling point is below standard room temperature). As a liquid boils it turns into a gas, increasing the pressure inside the tank. The tanks are equipped with a pressure relief valve that will release excess pressure once it hits a certain psi. At this point there is enough liquid to cool the skin of the tank, and keep it from rupturing. As the liquid continues to boil, more gas is created and the pressure once again builds triggering the relief valve. This cycle continues until the level of the liquid is below the flames. The exposure of the tank skin to these flames, coupled with no cooling system (the liquid) eventually causes the tank to fail...catastrophically. You may be wondering why the relief valve didn't release the pressure. The explanation is that at some point the pressure required to trigger the valve is lower than what the tank skin can contain.  The result is what we witnessed last night.

Typically these explosions are enormous and a minimum of a 1-mile radius is required to be safe. Obviously, this radius was not kept by many last night and unfortunately some paid the price. What is not clear to me, is if the firemen knew exactly what they were facing. In my estimation, they never should have tried to fight the fire once it was out of control, they should have focused on getting everyone evacuated.  I am in no way criticizing anyone at the scene, as I know how difficult those decisions are.

As I stated above, I don't have all the facts, and don't know what caused the initial fire, but from the eyewitness and videos, this explanation seems plausible to me.

See a visual explanation of the BLEVE search youtube.

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