When it comes to your safety
as a diver, you might not often
think about that little disk in the
neck of your tank valve -- a burst
disk -- that's supposed to "blow"
before pressure can build up to a
dangerous (explosive) level. It
could happen if a tank is left unattended
while being filled or if
there's a fire. And Undercurrent has
learned that it's more common
than one might care to believe
that tanks have the wrong disks,
making them potentially lethal.
Disks are designed to burst at
pressures well above the tank's
working pressure rating -- as
much as 40 percent above nominal.
For a 3,000 psi aluminum 80,
that would be 4,200 psi. To ensure
the proper margin of safety, each
burst disk must be matched to the
working pressure of the tank on
which it's installed.
In fact, one dive shop owner
told us that they received 2,400
psi LP steel cylinders with 3,000
psi DIN Valve upgrades. "When
we brought this issue to sellers'
attention they sold us the correct
burst disks; then we had to bring
it to their attention each time we
ordered more."
Many shop retailers order
tanks and valves separately --
often from different manufacturers
-- and assemble them in the
shop. That leaves lots of room for
human error, according to Jack
Kuhn, owner of Harbor Dive
Center in Sausalito, CA. The shop
could mis-order, the manufacturer
could ship the wrong valve, or
the shop technician could confuse,
say, a valve rated at 3,300 psi
and install it in a tank rated at
3,000 psi working pressure.
That unsafe combination
could allow a negligent fill technician
to overfill the tank dangerously.
While a tank might survive
such an overpressurization,
repeated overfilling will induce
cylinder wall fatigue that could
cause a catastrophic rupture.
Even if a rupture doesn't occur,
cylinders that have been frequently
overpressurized consistently fail
their first hydrostatic test and
therefore must be replaced.
Either way, the risk seems
hardly worth a few extra minutes
of breathing time. So it's a potential
problem you ought to
address.
On new valves, the burst disk
pressure rating should be noted
on a small sticker. So before you buy a valve, be sure it matches the
working pressure of the tank
you're planning to use it with. But
on older valves the disk pressure is
impossible to detect. Even worse,
burst disks can fatigue. Some
older disks were made with lead
slugs that could fire out of an
overpressurized tank like a .22
bullet.
Bill High, president of
Professional Scuba Inspectors,
Inc., reports that new Department
of Transportation regulations
require that during each five-year
hydrostatic test, burst disks must
be verified as certified to release at
the cylinder's test pressure or up
to 10% less. In reality, there's no
way for hydro testers to verify the
existing device is correct so they
replace each disk with one certified
to meet the DOT regulation.
Dive businesses following PSI's
recommended policy of replacing
the burst disk at time of hydro
should send cylinders to the
retester minus the store serviced
valve.
If you buy a used tank that's
between hydro tests, replace the
disk assembly for an extra margin
of safety and get that .22 bullet
out of your garage.