Saturday, August 23, 2008

SCUBA Cylinder Explosions

So, we got a new compressor at the aquarium to fill our SCUBA cylinders with between dives. Unlike the old one, this compressor has some better safety features. For one, it only fills the cylinders when the door is closed and the cylinders are encased in a thick steel compartment; the idea being that, if a rupture were to occur, the extra metal would minimze the damage to the surrounding dive locker, where there are typically lots of people walking around. The process of loading a tank into the compressor and heaving the door shut before the compressor can start filling the tank reminded me of how the Ghostbusters would load their ghost trap into their "containment system" in the movie.

Anyway, I got curious about just how often SCUBA cylinder ruptures occur, and found some information on the website of Luxfer, on of the biggest manufacturers of SCUBA cylinders in the world.

Here is one of their charts, listing 13 SCUBA cylinder ruptures/
explosions since 1988, the most recent in 2007.

It is unclear to me whether this is a list of only SLC (sustained-load-
cracking) ruptures, or all SCUBA cylinder ruptures worldwide. Either
way, it appears that many of them are due to a specific alloy of
aluminum (6351) that is more susceptible to SLC than other alloys of
aluminum or steel. Luxfer stopped making these tanks in 1988 (in the
U.S.) and in 1990 (in Australia), afterwhich they switched to 6061,
which is apparently not susceptible to SLC.

Still, statistically speaking the risk is low. . . but if you google
around for any of the events on the list, you're likely to find
personal accounts and photos of the aftermath of the explosions. On
the couple I looked at, the scuba-shop people filling the tanks
noticed no defects on the tanks whatsoever. In one case, the cylinder
exploded after being placed in a water-tank to be filled, but BEFORE
the staff member even opened the compressor valve to fill the tank
(IE: it exploded when it was not being filled, at least, according to
the account at this web site, near the bottom of the page).

Note that the energy contained in the compressed air of a SCUBA cylinder, relative to atmospheric pressure around it, is equivalent to the energy of about 2 hand grenades, according to the link above.

Not sure how reliable that account was, but, anyway, with the number
of cylinders we have being using in the dive locker, and the fact that
the fill station is right next to where divers and tour groups walk
all day long, I'm happy to have the new ghost-buster containment system.

Of course, a person is MUCH MUCH more likely to be injured or killed by driving a car, getting hit by lightning, having a coconut fall on his head, or getting crushed by a vending machine than being injured or killed by an exploding scuba tank. . . but I see no problem in mitigating risk, especially since the most likely time a tank would rupture is during the filling process.

Finally, for fun, I'd like you to use your current state of mind
thinking about exploding 3000 psi scuba cylinder to reflect on the
idea of a new "air car" that will be powered by an on-board compressed
air tank holding air at 4500 psi. . . situated right next to a fuel
tank. . . in a car that can drive up to 90 mph. All in the name of
saving energy. (Because I'm sure air compressors that can fill tanks
up to 4500 psi are highly energy efficient, right? ;) )