Burst Disk Questions...

[DA Aquamaster]

Other than a failed cylinder wall....which is the whole purpose of using burst disks

Ok, I'll concede that point.

[DA Aquamaster]

This whole thread puzzles me. Why would anybody with a brain in their head disable the burst disk assembly? One reason would be that burst disk assemblies blow for no reason. But that's not the case. I have firsthand knowledge of FOUR times that burst disks blew.

1. A friend had a 2250 cylinder that was filled to 3000psi and left in a hot car.

2. A friend had 2400psi WP disks in a manifold and filled the doubles to 4000psi several times.

3. A co-worker walked away from the fill station while it was on "direct fill" on an AL80 and went to 4500psi.

4. Assuming the tank in his hand was rated at 3000psi, a moron at the shop filled a 2015 AL19 with a twenty-year-old burst disk assembly to 3200psi. (I won't say who this moron was, but he has an umlaut in his name.)

Twenty-four years of diving, fifteen of them working in at least one shop, and these four examples are the only time I experienced a disk going. EVERY single time, the disk *should have* blown. I don't see the problem.

Just put 3500psi WP disks in all your steel tanks and go with it.

Ok. I'll play. I agree with you and my observations are similar. The very small number of burst disc failures I have seen have all related to either really old burst discs that should have been replaced when the tank was hydro tested (a good interval to do it, every 3 years is better) or to a fill that was well over the intended service pressure for the burst disc assembly.

The one exception was a new 3500 psi service pressure burst disc I installed in a LP 95 that blew about 30 minutes after a fill to 3600 psi. I suspect the disc was defective and/or incorrectly marked.

[FW]

The bottom line is you have to plan your dives to survive, even if you have a burst disk failure, o-ring failure, etc.

Most of the burst disk failures I have seen were due to using disks designed for old steel 72s, in Nimrod, and early PST tanks. We used to replace the burst plugs with solid bolts. Once the air fill stations found out, they started inspecting the burst disk plugs. That was when the SS burst disks were invented. Modern burst disks come in a much wider range of design pressures, so there isn't a real need for "cheating". Like DA said, they should be replaced regularly. They flex each that the tank is used/filled, and wear out.

[chilldive]

The bottom line is you have to plan your dives to survive, even if you have a burst disk failure, o-ring failure, etc.

Most of the burst disk failures I have seen were due to using disks designed for old steel 72s, in Nimrod, and early PST tanks. We used to replace the burst plugs with solid bolts. Once the air fill stations found out, they started inspecting the burst disk plugs. That was when the SS burst disks were invented. Modern burst disks come in a much wider range of design pressures, so there isn't a real need for "cheating". Like DA said, they should be replaced regularly. They flex each that the tank is used/filled, and wear out.

No such thing as a SS burst disk... why use them if they would burst? Just messing with ya Forrest.

[chilldive]

Okay, it's been eight months. Will somebody PLEASE respond to my Post #43 and give me a reason you feel I'm wrong? I would like to hear someone's opinion as to why burst disks are a bad thing.

I wouldn't disagree with your reasoning. For me it's just eliminating one more "potential" point of failure. Call it peace of mind. Read all this discussion - with SS disks, it all becomes mute points.

Also, I have "heard" that with SS disks, the next thing to blow would be the valve neck o-ring, not the tank. Whether that is true or not, I would still use SS disks in all my tanks.

Cheers.

[DA Aquamaster]

I wouldn't disagree with your reasoning. For me it's just eliminating one more "potential" point of failure. Call it peace of mind. Read all this discussion - with SS disks, it all becomes mute points.

Also, I have "heard" that with SS disks, the next thing to blow would be the valve neck o-ring, not the tank. Whether that is true or not, I would still use SS disks in all my tanks.

Cheers.

It's a compromise. On the one hand if you install a stainless steel disc you do eliminate a potential source of failure. (Although I'd still replace it or at least inspect it for corrosion, etc, every time you hydro test the tank and at that time you should also rebuild the valve - and the burst disc replacement would be part of that rebuild process.)

On the other hand if you are using a burst disc intended for the actual fill pressures you are using and you are replacing it on a reasonable interval, then the risk of failure is very, very small to start with.

In exchange for the elimination of that very very small failure potential, you are adding the risk that the disc may not fail when needed, for example in case of a direct fill from a compressor or large high pressure bank or in the case of a fire where a tank with 3600 psi in it (at room temp) may fail catastrophically when the pressure increases to similar levels due to the heat of the fire.

Now...to play devils advocate here and in the interest of being both fair and transparent on the issue - it may not make much difference either way. Consider a burst disc intended for a special permit 3442 psi service pressure tanks. By law the (new) burst disc must rupture at no less than 90% of the test pressure and at no more than the test pressure. So in this case the disc must rupture between 4646 and 5163 psi. Even the low number may be more than you want to subject a 2400 psi low pressure steel tank to, especially when it is heating up in a fire given that the test pressure for a 2400 psi 3AA steel tank is only 4000 psi.

You can also get into a debate the issue of whether it is better to store tanks empty or full, both in terms of long term effect on the tank and in terms of what is better in the event of a fire, specifically in this case when the tank has a SS "burst"disc in it.

[chilldive]

You're makin' me dizzy!

[Walter Pickel]

Please don't double disk...

Double "disking" is akin to a solid plug...by doubling up a couple 3K disks (for example) will not blow at 6K, probably much much higher. The problem is, nobody really knows when it will go, at this point, it's unpredictable.

I have seen tank neck o-rings extrude before a burst disk blows. I guess this is why burst disks are not required in Europe.

[JLH]

OK I'll respond to post 43. Who was that moron again? I agree with the rest of your posting though.

Jack

[OutlawCaver]

I would like to hear from some of our European cave divers. I know many European valves do not even have burst disc ports.