In article ,
Richard J Kinch wrote:

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.

anecdotal evidence time:

Back in September, I went out to feed the horses one morning, and in the
process, discovered that my landlord's house was on fire. Half an hour,
maybe 45 minutes later, when the FD got there (no fault or blame put to
them - even driving like an utterly insane bat out of hell, I can't make
it from the house to the nearest fire station in less than 35 minutes in
my car - they responded as fast as was physically possible, considering
the distance and terrain between them and the fire) the house was
essentially totally engulfed. It was a hot fire - the hottest I've ever
seen personally - It did a damn good job of trying to torch trees a
hundred feet away. When it was done, the fire left almost nothing but a
smoking crater where the basement used to be - Other than one small
section of bathroom wall, *EVERYTHING* burned. Glass melted and flowed
like water - I can go down to the site and pick up frozen "rivers" of
glass that would fool just about anybody into thinking they were actual
flowing water. Ceramic countertop tiles, and concrete tile roofing
slumped, cracked, and shattered. Concrete block walls in the basement
scaled and blistered and cracked. A coin collection was turned into a
blob of mixed metal, with only a very few recognizable coins in it.
Aluminum engine blocks on lawn equipment stored in the basement in
preparation for the soon-to-be-coming winter rain were reduced to
puddled lumps with barely recognizable partially melted steel chunks
sticking out of them. An entire woodworking shop - table saw, radial arm
saw, drill press, jointers, planers, routers, you name it, if it can be
used for woodworking, it was probably there - some still in the boxes
from Sears 'casue there hadn't been time or room to get them all
unpacked and set up yet, was destroyed. The cast iron table of the
jointer was twisted and warped like warm taffy. We still haven't found
(or at least, if we have, we don't recognize it) the table saw's table.
The motor was found - as a steel-shelled blob of melted copper. Several
rifles and shotguns got hot enough that the barrels slumped flat. A
fiberglass boat 30 feet away went up in smoke, and the aluminum heads on
the inboard engine melted into slag. The pickup truck 20 feet beyond the
boat had the paint blistered and plastic trim melted and distorted.
Another car near the truck had taillights and bumper melted, and
blistering to the paint job. *EVERYTHING* flammable in the house was
turned into either ash or charcoal chunks. As I say, it was a *VERY* hot
fire. Several of the firemen said it was possibly the hottest, worst
fire they'd ever attended. Usually, at least *SOMETHING* is left. Not in
this case.

Anyway, to the point of this post:

Four propane tanks, one full, one partial, two brand new (less than a
week old, and not yet been filled for the first time since being bought
to replace the recently outlawed pair without the overfill prevention
gizmo) were in the fire. All of them were the (I think) 5 gallon size -
They don't *QUITE* fit into a standard plastic 5 gallon hydraulic fluid
bucket. I guess that would probably make them 20 pounders.

The only visible damage to any of the tanks that could be seen is the
soot and charred paint, and the tracks of shiny metal where the aluminum
valve-wheels melted and dribbled down the sides. No sign of rupture or
expansion, no sign of warping, no sign of *ANY* sort of damage, other
than the char and melted valves. The partial one was on the front deck,
attached to the gas grille (which, other than the cast-iron grates, was
unrecognizable when we found it) and it provided one helluva show when
the overpresure valve let go - a 20 foot tounge of flame and soot that
roared like a ****ed off dragon and blasted out across the yard, lasting
for almost a full minute. I don't know for sure what the full one,
living down in the basement, did, but I'd bet it contributed
substantially to the heat.

Aside from the melted valve wheels, any of these four tanks could have
been repainted, and by looking at them, nobody would ever know they'd
been anywhere near a fire of any size, let alone in one that reduced all
but about a roughly 10 by 10 foot section of bathroom wall in a 50-ish
by 30-ish foot house to ash, charcoal, and broken dreams. (since the
fire, I have personally punched all four tanks to prevent exactly that
sort of thing from happening - As they sit now, they won't hold
atmosphere, let alone pressure. Leastwise, I figure a jagged 6 inch hole
smashed in the side of each one should be adequate to make it impossible
to pass them off as servicable.)

And before anybody asks if the cause of the fire is known, no, it isn't.
The hot water heater (electric) is the "most likely suspect", but
there's no conclusive evidence to pin the blame for certain. The arson
investigator said he could find nothing to suggest that it was
deliberate, and has closed the books on the investigation. Everybody
that was supposed to be in the house was gone at the time, and I managed
to get two of the three dogs that were inside safely out. (No sign of
the third - due to where she was last seen, it's almost certain she was
trapped on the other side of a wall of flames from where I made entry
and found the other two hiding - with the hallway ceiling falling in
flaming chunks, and a solid sheet of flames beyond that, I wasn't about
to try going down the hallway to hunt for a dog)

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