David A. Webb writes:

The theoretical maximum pressure would be less than 130psi.

Someone, please check my math.

Hard to predict precisely, but roughly speaking, you have on the order
of a doubling in molar volume (the molar ratio after combustion by your
analysis is 7:1, but the explosive limits are almost the inverse at 10
to 15 percent propane in air), and a 10-fold rise in absolute
temperature, so the ultimate pressure would be the product, which is to
say, upwards of 20 times the original pressure, or 20 atm, or upwards of
300 psi. Of course there is a sub-sonic shock wave as well.

The question is whether the rupture strength of a 20 lb propane tank can
contain an explosive rise from 1 atm to 10 or 20 atms of pressure. If
so, then one would be skeptical that these things *ever* explode.

Have these tanks actually exploded, say, in fires? It seems like the
reports that claim explosions were actually tanks that leaked into a
confined space that then mixed with air and exploded, not that the tank
itself ruptured or exploded.

There was a suicide case locally a few years ago where a guy tried to
deliberately explode several 20 lb tanks indoors by lighting fires
underneath them. He only succeeded in making an explosive mix in the
room, which was devastating, but the tanks themselves survived intact.