"...venting vapour. That's a whole lotta 'boom.'..."

Actually, the venting prevents the boom. It gives more of a whoosh. I once watched a motorhome burn to the ground. RV burning, every so often the propane tank would reach a certain temp/pressure and vent, creating a flame thrower effect out to the side for a few seconds. Temp/pressure went down, so did the flame thrower. Few minutes later, here came the flame thrower for a few seconds. On and off 'til the tank was empty.

I guess that in an enclosed space you could get a boom, which would be bad news...