I can't comprehend a "pressure relief valve" in the head of a 100# propane cylinder. First because a functioning valve that goes open/closed would be a nightmare keeping sealed day and night for months, years. Also it would require a spring that would play hell during fluctuating temps particularly heat of fires.
Finally having worked in the fire equipment industry where everything from Halon, CO2, Scott Packs, to SCUBA tanks were hydro tested. One of our service guys took a piece of a burst disc in his arm from an overfill. It had shot through the "holed" bolt that covers the entire disc.
I believe the large bulk gas storage tanks at propane filling stations and tanker vessels are the ones with valves to ON/OFF gas release.
When they are under "fire" they can bleed-off the pressure and do go ON/OFF.
A rupture
disk, also known as a pressure safety
disc, burst
disc, bursting
disc, or burst diaphragm, is a non-reclosing pressure relief safety device that, in most uses, protects a pressure vessel, equipment or system from overpressurization or potentially damaging vacuum conditions.. A rupture
disk is a type of sacrificial part because it has a one-time-use membrane that fails at a ...
Because there is a fire present it cooks off the tank. the pressure relief valve pops on, and off. Just before the explosion you see the jet stop at about 1.45. The amount of propane in the tank actually is the cooling agent, until it is vented out, then the tank explodes. Big tanks can do huge damage.
https://en.wikipedia.org/wiki/Boiling_liquid_expanding_vapor_explosion
