Vanity burst discs

[Luis H]

According to my reading of the bulk of responses on this board, engineers that design high pressure cylinders know nothing of their craft and create arbitrary safety standards just to annoy us, we actually are much better educated and understand the issues much better than they do.

That is why I like to call this ScaryBoard.
Many of the comments here are pretty scary.

 

[Luis H]

It is a very common misconception that neck o-ring extrusion will protect a tank from rupture. I believe that this idea has been promulgated by some of those of the DIR persuasion to justify their contention that burst disc's should be plugged. While, I will not argue the relative merits of plugs vs. higher rated discs, I have seen what happens to tanks in a fire. In both cases that I have seen, aluminum tanks have ruptured and LP steel tanks, blown their over-rated burst discs without the tanks, themselves failing and without the o-rings extruding.

I don’t know who came up with that nonsense (IMHO), but a properly seated (O-ring style) tank valve should have a zero clearance between the tank neck and the valve body. As long as you have a zero clearance, it is impossible to extrude an O-ring.

 

[anzac65]

According to my reading of the bulk of responses on this board, engineers that design high pressure cylinders know nothing of their craft and create arbitrary safety standards just to annoy us, we actually are much better educated and understand the issues much better than they do.

Agreed !!!!!!!!!! I've heard some of them only spend 4 or five years at college qualifying. Darn, my lights have blown again. That's it. I'm replacing the fuse with a 4 inch nail. Stupid engineer didn't think to do that, did he.....

 

[Gilldiver]

The problem in the US, and other places is that standards live forever. The buerocrats don't want to stick their necks out, so they CYA forever.

When tanks had pipe threads a burst disk was required to protect from bursting of the tanks.

But when they went to O-rings it all changed. That is when the Europeans eliminated the burst disks. Under normal conditions an o-ring will not extrude, but under non-normal conditions it will fail. Those are higher temps that soften the material, coke it so that it cracks, etc.

The teflon valve seat may also fail prior to an aluminum tank going and will fail prior to a steel tank going.

 

[Luis H]

The problem in the US, and other places is that standards live forever. The buerocrats don't want to stick their necks out, so they CYA forever.

When tanks had pipe threads a burst disk was required to protect from bursting of the tanks.

But when they went to O-rings it all changed. That is when the Europeans eliminated the burst disks. Under normal conditions an o-ring will not extrude, but under non-normal conditions it will fail. Those are higher temps that soften the material, coke it so that it cracks, etc.

The Teflon valve seat may also fail prior to an aluminum tank going and will fail prior to a steel tank going.

Your first sentence has a little bit of truth to it, but everything else is totally incorrect!

For starters I have two sets of European double tanks sets with tapered pipe thread an no burst disc. The Europeans have never seemed that as concern about fire danger and SCUBA tanks.

The aluminum alloy strength is compromised at a very low temperature (I am fairly certain below that of Teflon). Steel is a bit better, but not great.

Even a compromised O-ring seal will not relieve the pressure as you are hoping since it will not be extruded.

I don't have the time to get more into it, but your statements are IMO dangerous.
IMHO, it is OK to take a risk, but do it knowingly. Don't kid yourself with a false sense of security.

 

[Mauifish]

I do believe an o-ring will melt as will teflon causing the extremely high pressure gas to LEAK OUT therefore the tank should not explode, I work with high pressure tanks on aircraft

 

[Gilldiver]

Your first sentence has a little bit of truth to it, but everything else is totally incorrect!

For starters I have two sets of European double tanks sets with tapered pipe thread an no burst disc. The Europeans have never seemed that as concern about fire danger and SCUBA tanks.

The aluminum alloy strength is compromised at a very low temperature (I am fairly certain below that of Teflon). Steel is a bit better, but not great.

Even a compromised O-ring seal will not relieve the pressure as you are hoping since it will not be extruded.

I don't have the time to get more into it, but your statements are IMO dangerous.
IMHO, it is OK to take a risk, but do it knowingly. Don't kid yourself with a false sense of security.

The failure would not be by extrusion but by degradation at temperature which can lead to a completely different failure mode.

Neoprene and nitrile O-rings have a service temp of 160-180F, Buna-N is 250F, Silicone and Viton is 400F. From past experience I know that at about 100F, or less, above the service temp these rubbers will either soften/melt, harden and crack, or coke. Any of the three will lessen, if not eliminate, the sealing properties

As the worst case is a full tank in a fire we will only talk about instantaneous heating of the metal. This eliminates the alloy age effects on aluminum alloys which is quite low at 200-250F for a period of time. Usually 1.5 to 2.5 hours for usage such as in tanks.

So, will the o-ring degrade prior to the alloy losing its ability to withstand the internal pressures? For aluminum that creep temp is between 600F and 1,000F. For steel it will be between 1000F and 1400F+ depending on the alloy.

The problem will be the race between the increase in internal pressure, creep strength of the alloy, and degradation of the o-ring. I think the o-ring will lose. As you say "The Europeans have never seemed that as concern about fire danger and SCUBA tanks" which would seem to support this opinion.


PS, no tank should ever be used after being in a fire.

 

[oxyhacker]

Tanks are tested to failure, usually at 2.5x working pressure though exemption tanks may be at 2.25X. They are also examined for the mode of failure - it should be "LBB", leak before break meaning the tank should leak and not burst.

Sometimes other tests to failure are specified -some of the exemption tanks must have saw cuts made in the side wall and then are tested to failure at 1.5X working pressure.

I believe part of the process of a manufacture getting DOT approval on a tank design is that they be tested to determine the actual burst pressure. I seem to recall seeing about 8,000 to 10,000 psi as the burst pressure on a LP steel tank. Luis do you have any info on this.

 

[rjack321]

Thanks

The copy of CFR49 section180.205 I have reads a bit different, basically the same information. I think you have a newer revision.

I don't have it on paper (too bulky). The latest and greatest is always available here:
http://www.gpoaccess.gov/cfr/index.html