IIRC, existence of one or more pressure relief valves meets the spec. The valves allow a controlled release of stored material to stabilize containment pressure.
Once released, that material may ignite, but that is secondary to keeping the tank integrity.
The danger is when fire outside the containment vessel is up against the vessel wall. In such a case, the fire outside the tank will simultaneously heat both the tank wall and the material inside the tank. As long as the level inside the tank remains ABOVE the level of this heated zone the material itself will absorb the heat, and tend to keep the temperature of the vessel wall below a temperature at which it might fail. If the fire goes on long enough to boil off the contained material until the level gets down to where the heated zone is, the tank wall temperature will rise rapidly to a point where the strength of the steel falls below the set point of the pressure relief valve(s). Under those conditions the tank wall will burst, and the vaporized material inside the vessel will be forcibly expelled directly into the fire until internal tank pressure drops below external atmospheric pressure.
If that material is flammable, it will convert a burning fire into a massive blowtorch until the tank pressure falls below atmospheric. The rate of release will cause material to exit the ruptured tank so quickly that it will not just suddenly stop when internal pressure equals external pressure, but will overshoot, so there will be a moment of pressure reversal that will cause the tank to suck back and draw burning material back into the tank.
THAT is when the BIG KABOOOOM will happen.
What vaporized material remains inside the tank — IF its molecular structure includes oxygen enough to support self-combustion, as is the case with peroxides — will ignite instantly causing a pressure spike that will act upon the area of the existing tank rupture with such force as to completely tear the rest of the tank apart.
This is called a BLEVE (ble-vee) and is the most dangerous situation possible with flammable materials under compromised containment. Pieces of the shattered containment vessel can be hurled through the air to astounding distances.
This subject-specific video is instructive:
During my career I went to quite a few tanks that were on fire. The pucker factor was always quite high. My first experience with a BLEVE was while I was still on probation. My lieutenant and I were just about to enter a burning house when an explosion blew nearly all the windows and doors out. A barbecue tank in the basement had BLEVE’d and caused a pretty good amount of additional damage since it was in an enclosed space.
The last one I was on came in as a semi-truck on fire at a motel about 5 in the morning. It was still dark and we could see from a freeway overpass half a mile away that we had an impressive looking blaze. So I called for back-up. When we arrived the cab was fully involved, there were large flames coming out of the top of the tank, and the truck driver was going crazy running around the motel screaming that his giant tank full of propane was going to blow and flatten the whole large multi-story motel and the huge casino across the street... He had caused a serious panic so we had half dressed people running and screaming and tearing out of the parking lot. Looking back it was sort of a comical scene of pandemonium.
Fortunately, semi-truck fires are generally not very hard to put out and even though the impressive looking flames from the cab and the tires were impinging on it, and even though the pressure relief valve was releasing gas that had also caught on fire, the tank still had a low probability of causing a BLEVE. We put it out in a few minutes, the pressure relief valve then closed on its own and we had the units after us try to calm everyone before someone was run over or fell down some stairs. A lot of them were drunk to start with.
The truck and possibly the tank were both probably total losses, but no one got hurt during the panic. We managed to get back to the station within an hour or two with an amusing story to tell our coworkers arriving at shift change. A good day.
As a long time member of our HAZMAT team my largest concern at this point with this chemical plant is the release of large amounts of noxious chemicals into the water and air. Although the flood waters may have some benefit... One of our sayings on HAZMAT was “the solution to pollution is dilution.” In low enough concentrations most chemicals used even in industrial applications become mostly harmless. It looks like there is a lot of water running through there so whatever leaks out may be largely washed away with no long term problems able to be identified.