"Gunner" wrote in message
...
On Sun, 16 Jul 2006 04:54:01 GMT, CJT wrote:

steamer wrote:
--FWIW the fact that many of these things seem to blow *up* and
land a fair distance from where they started out makes me think the
rupture
is beginning at the bottom of the pressure vessel. This might mean the
thing's filled up with crud and has never been drained properly, thus
there
is a weak band around the bottom, yes?

The pressure is highest at the bottom.

I thought pressure was lighter than air and always rose upwards. Which
is why airplanes fly and blimps float.


At last! A topic I can contribute to on this NG. First, the pressure is
higher in the bottom when the contents is all liquid. And yes, it is
possible that the crud has corroded a region around or near the bottom. But
the real interesting thing is that with a bigger tank you get a bigger
explosion. Here's why: The state of a gas is determined by pressure,
temperature, quantity of the gas, and the contained volume (remember high
school?). For most gasses, if you increase the pressure enough it will
liquify. Water, or more precisely water vapor, condenses to a liquid under
normal conditions. So the exploding water tank reverses the process. The
water is heated throughout so all the water is heated way above the boiling
point and most importantly, the pressure is not relieved (it builds up, as a
mater of fact). When the tank cracks, even a little, the pressure is
suddenly and dramatically reduced within the tank. ALL of the water flashes
to steam. Worse, now that the water is a gas it continues to expand and
release energy causing more damage (i.e.an explosion). This is why high
pressure systems (3000 psi) are tested with liquid and not air.
Hope this helps.