Double Your Burst Disk - Arguments For & Against, Please...

[Al Mialkovsky]

I've seen a few disks fail. It happens. Scare the tar out of me every time.

 

[Dive-aholic]

So, let's say you put a 5000psi disk into a LP tank with a plus rating to 2700. Now there's a fire in the house all around the tank. Are we saying that the tank will explode before the 5000psi disk blows??

What about the o-ring in the tank neck?

Most tanks are given a rating based on liability, not actual capability. The tank may be able to withstand 2-3x its rated pressure, but the manufacturer will give it a lower rating anyway.

Or use them because;
a. it's the law...

Not in the US unless you're transporting them commercially. And obviously not in Europe and the UK.

 

[Quarrior]

Not commenting on the practice in particular, but DOT only governs cylinders "offered for transportation". I believe this refers to commercial operations conducting interstate transports. I don't think there is a legal issue for privately owned cylinders for personal use.

Actually, before I got my top on my truck, my LDS cautioned me not to carry my tanks standing up so they can't be seen by the Law folks.

He said if they can see the tanks, they can pull you over and do a DOT inspection, not only of the tanks, but of the manner in which you have them mounted for transport. They can ticket you if you are in violation.

 

[Mark Vlahos]

I don't really care for the practice. On all of my cylinders the burst disk is installed. I do make sure that the disk on the valve is the correct one for the cylinder in question. I carry spares for each type of cylinder with me in my save a dive kit. When the valve gets serviced (by a shop) I make a point of letting them know that I would be happy to have the disk replaced, and if that request means that I will need to pay a slight additional charge that I am willing to pay the cost.

I have seen one disk fail in person, it was in a shop while the cylinder was getting its first fill after an inspection. The cylinder was not mine, just coincidence that I was in the shop when the disk failed. Yes, they are really loud and it did scare the crap out of us.

I do not lay awake at night worrying if the disks on my cylinders will fail. I do not spend my entire dive worrying if the disk will fail. I am concerned for firefighters and other possible bystanders in the unexpected event of a fire. Firefighters have enough to worry about without my adding the risk of exploding diving cylinders to the mix. I also make a point of storing cylinders either full or nearly empty.

Mark Vlahos

 

[lakewinni]

First of all for all of those using the law argument who cares about the law. Its illegal to overfill but that hasn't stopped that all the cave divers and everyone else who does it.
Secondly how often do tanks that are in Vis and hydro spontaneously explode when getting filled to a pressure that is something close to what it is rated to? Pretty much never.
Thirdly I HAVE seen burst disks hit in an overhead environment and go that way, but not just spontaneously.
Lastly if Europe isn't worried about thier firemen dieing from exploding tanks then I don't think we should be. I think the risk to you is much greater if you hit the disc on the back of your manifold against a cave ceiling then start losing all your gas and have to isolate and then exit a cave system than it is to a firefighter in this hypothetical fire.

 

[wedivebc]

Thirdly I HAVE seen burst disks hit in an overhead environment and go that way, but not just spontaneously.

This is the first case I have heard of a burst disk failing underwater. Can you share more details?

 

[do it easy]

For the record- I'm not a fan of double-disking. If it happens to blow at the surface, you just lose the gas and maybe a dive. If it blows underwater, it's inconvenient, but that's what back up plans and training are for. Either way, it shouldn't be the end of your world.

 

[PvilleStang]

Not commenting on the practice in particular, but DOT only governs cylinders "offered for transportation". I believe this refers to commercial operations conducting interstate transports. I don't think there is a legal issue for privately owned cylinders for personal use.

So do you walk your tanks to the dive site? I agree, and have, double disked tanks, but ponder this:

Faber, the manufacturer, warranties their tanks for 10,000 fills to 4,000 PSI. In no place where I get my tanks filled are they capable of filling beyond 3800 PSI. Now with that in mind, if you were to have a house fire, a fireman would really only be able to enter the house with temperatures reaching 150-200 degrees, at which point a rescue would be pointless. At 200 degrees, pressure would reach 1.34 times the pressure previously in the tank. I personally won't leave the tank in my home with more than 2640 in it, and that would reach 3432 psi. Now the average house fire reaches around 500 degrees at floor level (where most tanks would be stored). At this level, a tank stored with 2640 would be holding 5781 PSI, which most tanks would be tested to between 4000 and 5000. It would be high, but the probability of failure at that point is still not great.

Now say you blow a burst disk in a deco dive, and are forced to go to a back gas. This situation poses greater problems... You could blow your PPO2, you could be lacking the quantity of gas to make it to the surface... there's a lot of issues with that situation. I personally would rather the safety factor underwater.

And in case you need to know, I got my facts from a firefighter about the temperatures of a fire. He, as a matter of fact, had a buddy involved in a scuba tank incedent during a house fire. The tank fell through the ceiling and almost killed him in the fire. Tank hadn't busted open or anything.

 

[PvilleStang]

Oh, and I can't forget, the metals used in burst disks are thin and fairly maleable (look at a burst disk when you replace them, its like a little bowl). When metals heat up, they loose some of their strength and integrity. I would bet money the burst disks loosen up and blow in a house fire situation rather than the tank blow, since there is more metal surrounding the tank than there is in two burst disks! Maybe there's a myth busters episode idea in all of this...

 

[Mark Vlahos]

... I think the risk to you is much greater if you hit the disc on the back of your manifold against a cave ceiling then start losing all your gas and have to isolate and then exit a cave system...

I am concerned about hitting the overhead with the valve. To that end, I chose Thermo brand valves for all of my cylinders, including my doubles. The Thermo valves have a collar around the bolt head offering an increased level of protection against impact. And the collar was the consideration that had me choose Thermo rather than Halcyon.

Mark Vlahos