Rupture Discs, FYI:

[Norm]

Folks often ask how long a tank will last if a rupture disc blows?
It's NOT always the advocated, "It'll discharge at a controlled rate so you exit the water safely", cliche.....They often DUMP, as mine did this morning. I had 30 cubic foot pony to empty in 30 seconds. .:boom:

 

[JamesK]

I have never seen a rupture disc on a scuba tank burst, but I have had it happen on my 10lb nitrous oxide bottle. It emptied in a matter of seconds. It don't take long.


What happened that your disc burst?

 

[FredT]

Given that a well maintained burst disk will generally go with the tank full, the time to exit should be in double digit seconds. Deco liability will not have had time to build up. A steel 72 that dumps a disk during an inattentive fill will scream for 30 seconds or more. The diver who can't get to the surface within 30 seconds from almost any depth he is likely to reach before failure with one of these screaming behind his ear is already dead. The adrenalin rush involved with any HP leak is absolute!

Happy Lundi Gras

FT

 

[JamesK]

Good point Fred. I guess if you think about it for a second, the disc blows because of to much pressure build up. If you are breathing off of the tank, you are relieving pressure anyway. You would have to just be starting your dive for it to blow, otherwise, you would have already reduced the pressure just from breathing off of it.

However, can the discs get gradually weaker? Let's say a disc is rated to blow at 3750psi. You tank is filled to 3500psi. In the heat, the pressure builds to 3750, but it does not blow. Could the seal get weaker, and then just give way at anytime? I don't see how it could happen, just wondering.

 

[metridium]

Originally posted by FredT
Given that a well maintained burst disk will generally go with the tank full

"Generally"? Could you tell me more about the exceptions, i.e. why, at what level?

 

[luvdiving]

I've had a burst disk "go" at the dock while I was gearing up. I think the tank was empty in about a long minute. It sounded like my valve o-ring broke. The first thing that most people do is to try and shut the tank valve. I think that a ruptured disk noise alone at depth would scare the hell out of me.:boom:

 

[Lost Yooper]

I saw a disk in a rental 72 go about two months ago. It'll get your attention for sure. That tank had just gotten filled and was lying on the pool deck. It is my understanding that the copper disk will weaken over time. If you're going to have disks, they should be replaced once in a while.

Mike

 

[The Chairman]

is best during the yearly tank inspection but no longer than when you hydro the tank. Corrosion, heat and overfilling play havoc on a burst disc. Do NOT let your tank technician put in a burst disc that is not rated for your tank. Too weak, and your disc will cut loose during a normal fill, or when the sun hits your tank. Too strong, and it may cause your tank to fracture instead, which would be catastrophic indeed! They are thermal/pressure fuses that are intended to keep us safe.

 

[Rick Murchison]

Norm,
Where was your 30 when it blew?
I've seen 'em blow on fill, in a trunk, in the sun, on the dock... I've never seen one go in the water... was yours?
Rick

 

[FredT]

Engineering wise burst disks are a lot more complex than the first look at them indicates. Failure mode can be single shear, bending or, inthe example of the H2 disks used onthe Space Shuttle main tank, a combination of bending, shear and cutting. Disks are rated at the max failure point, not the minimum, but generally that is within a 10 or 20% of full scale range. Tis is also why a faied disk has the requirement of eplacing both the disk and the plug. In the process of failing the disk "rollls the edge" of the plug changing failure mode form shear to bending, and raising the failure point by a significant amount,

Corrosion and re-using a disk after removal for inspection are two of the common causes of "low pressure" failure. (if the disk is removed and replaced the shear points will NOT line up, creating a secondary failure line.) Overtorquing a burst disk during installation is another way to cause premature failure. MOst specs only call for tightening the disk plug to 4 to 8 INCH-pounds to prohibit early failure of the disk.

Age can also cause some downward creep to the failure point. I have seen unmaintained disks fail due to simply leaving the tank out in the sun, and having a tank let go next to you while taking a nap during a SI will "start your heart" in nothing flat!

OTOH If the valve is rebuilt every time a hydro is performed, including replacing the disk, most divers will never have a disk fail, and _I've_ never seen one fail underwater. While it may be _possible_ to fail one there, I have never seen one, or even heard of one, fail underwater except in the urban legend category.

Keep in mind that the disk is there to protect firefighters, not as a safety function for the "normal operation" of the tank. This is why Aluminum tanks should be stored full or "empty" (50-200PSI). If stored between 1500 and 2500 psi it's possible for the tank wall to fail from thermal damage before the burst disk goes.

FT