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June 2003 Vol. 18, No. 6   RSS Feed for Undercurrent Issues
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What if the Burst Disk Doesn’t Burst?

from the June, 2003 issue of Undercurrent   Subscribe Now

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.

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