It all has to do with how quickly the pressure drops in the event the
container is compromised.
The pressure in a container of water, fed from [say] a 3/4" pipe, will
drop to half the supply pressure when the size of the leak/hole/crack
reaches a size similar to a 3/4" pipe. FIll the same tank with air,
and fully half the air has to escape before the pressure drops to
half. Meanwhile, the pressure is still available to further the
hole's size increase.
The difference is the compressibility of the contained material, which
controls the stored energy. Pull with a few pounds force on a rubber
band, let go suddenly, and you'll get a good snap on yourself. Apply
the same force to a piece of string with the same cross section as the
elastic band, and almost nothing happens when you release it. Pull
until the band breaks and you will be simulating the rupture of an air
vessel in a small way. The string breaking is quite boring. The
difference is in the ability of the material to store the energy
applied to it.
Containers for pressurised gases need to be built to hold that energy
safely. A water heater tank just isn't designed or built for that.
Donald.
> From: Conrad <conrad@conrad.uk.net>
> Date: Mon, 15 Apr 2002 15:29:21 +0100
>
>
> > (If you want to know why water is safe, let me know. I don't
> > want to bore everyone who doesn't want to know.)
>
> please continue... I am in the process of building a large petrol powered
> compressor for my sand blaster, and have everything except a tank...
>
> Conrad
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