O2 clean tank question

[Dr Wu]

I agree with all of this except for

"Also and most important: O2 will never "cause" anything to burn. "

While not necessarily incorrect, it is rather misleading.

O2 does not burn. That is a fact. It is a fact that is taught in even the most basic Nitrox course. It is a fact that is important in understanding what O2 clean means.


Facts are not missleading. People that speak of things they don't understand are misleading.

 

[Indian Valley Scuba]

Gotta love the academic confusion and twisting of standards, rules, facts and guidelines here. And no one has mentioned the only agency that has any real, supportable and legally enforceable documents regarding O2 content of a gas mixture - the Compressed Gas Association (CGA).

But it's still entertaining to see all the old myths (and some new ones) kept alive and recycled here in this this thread. Thank you!

 

[CaveMD]

O2 does not burn. That is a fact. It is a fact that is taught in even the most basic Nitrox course. It is a fact that is important in understanding what O2 clean means.


Facts are not missleading. People that speak of things they don't understand are misleading.

Facts are not misleading, however the method in which you state them can be.

 

[DA Aquamaster]

Gotta love the academic confusion and twisting of standards, rules, facts and guidelines here. And no one has mentioned the only agency that has any real, supportable and legally enforceable documents regarding O2 content of a gas mixture - the Compressed Gas Association (CGA).

But it's still entertaining to see all the old myths (and some new ones) kept alive and recycled here in this this thread. Thank you!

I wondered when someone was going to drag that out. The issue there is how relevent CGA standards really are to diving. According to the CGA, most diver shops and most divers would have blown up long ago.

 

[BigdogDown]

Im going to say this for the cheap seats OXYGEN IS NOT A FLAMABLE GAS! it IS explosive in the fact that it is an OXYGENATOR. Think of the fire triangle you learned in school, you need 1. heat 2. fuel and 3. oxygen. So when you fill a tank with pure oxygen, the worry isnt that the GAS will explode, rather that the heat of compression will cause some small contaminent to become heated to the point of ignition and then be rapidly oxidized by the oxygen, and become a larger combustion and in the confined area of a tank the expanding products of combustion wil find a way out and boom you have an explosion. It wasnt from the oxygen is was the oxygen rapidly feeding the combustion that happened.

 

[Texfrazer]

Ironically, the burst disc is installed in the tank valve in order to provide some sort of mitigation of what is going on inside the tank (of note is that the burst disc was not developed with a high O2 mixture in mind) in order to prevent a catastrophic failure of the tank. From some very crude calculations, the burst disc would have to be approximately the diameter of the tank in order to prevent a catastrophic failure on a fully pressurized tank of O2. Why are burst discs so important again (when internal combustion is the primary concern)?

Thngs that make you go hmmmm.....

 

[Indian Valley Scuba]

This is why some take the time to study engineering in college, to avoid the need to hum.

 

[Dr Wu]

Ironically, the burst disc is installed in the tank valve in order to provide some sort of mitigation of what is going on inside the tank (of note is that the burst disc was not developed with a high O2 mixture in mind) in order to prevent a catastrophic failure of the tank. From some very crude calculations, the burst disc would have to be approximately the diameter of the tank in order to prevent a catastrophic failure on a fully pressurized tank of O2. Why are burst discs so important again (when internal combustion is the primary concern)?

Thngs that make you go hmmmm.....

Of course one could also question that use of the term "Irony".

 

[Troutmaster]

Ironically, the burst disc is installed in the tank valve in order to provide some sort of mitigation of what is going on inside the tank (of note is that the burst disc was not developed with a high O2 mixture in mind) in order to prevent a catastrophic failure of the tank. From some very crude calculations, the burst disc would have to be approximately the diameter of the tank in order to prevent a catastrophic failure on a fully pressurized tank of O2. Why are burst discs so important again (when internal combustion is the primary concern)?

Thngs that make you go hmmmm.....

A burst disk is not going to prevent a failure of a tank due to an "internal combustion" and was never designed to. It vents the tank if the tank is grossly overpressurized to keep it from reaching failure pressure. Like if the tank is exposed to fire, it will rapidly increase in pressure, so the burst disk pops at 5000 psi and vents the tank so it dosen't pop like a balloon at 12,000 psi. If you really want to know why burst disks are important, ask a firefighter if he wants to go into your house to put it out if he knows there is a tank in there without one.

 

[DA Aquamaster]

Im going to say this for the cheap seats OXYGEN IS NOT A FLAMABLE GAS! it IS explosive in the fact that it is an OXYGENATOR. Think of the fire triangle you learned in school, you need 1. heat 2. fuel and 3. oxygen. So when you fill a tank with pure oxygen, the worry isnt that the GAS will explode, rather that the heat of compression will cause some small contaminent to become heated to the point of ignition and then be rapidly oxidized by the oxygen, and become a larger combustion and in the confined area of a tank the expanding products of combustion wil find a way out and boom you have an explosion. It wasnt from the oxygen is was the oxygen rapidly feeding the combustion that happened.

Nice theory but it totally ignores the PO2 issue.

For example, NASA used 20% O2 at about 3 psi for decades in space capsules as it allowed much lighter weight and weight was important as we were booster challenged compared to the Soviets until the Saturn 1B and Saturn V came along. It also provided for a great deal of fire safety in flight as with a PO2 of .20, fires propogated no worse than in air at 15psi and it was much easier, quicker and safer (no risk of DCI) to partially vent the spacecraft cabin to reduce the pressure further to extinguish any fire.

On the ground, it was a different story and the Apollo 1 fire was the result of a 100% O2 cabin environment at 15 psi during a ground test along with lots of cumbustable materials that had evolved into the interior of the spacecraft. NASA reacted to the problem of ground tests and launches by pressurizing with a mix of O2 and nitrogen, then venting the craft and replacing the cabin atmoshere with 100% O2 on ascent.

The point of this is that if I have a tank filled to 3600 psi with air, the PO2 is 51.64 compared to a PO2 of *only* 1.0 for 100% O2 at 14.7 psi. In other words, if I fill the tank with air, or any gas with significant amounts of O2 in it and then heat it enough to get a small piece of fuel in the tank to ignite, I have a potential problem just do to the PO2 even at low O2 percentages.

The really good news here is that it takes a great deal of heat to heat a tank to that point and even if it occurred, I need enough heat to not only ignite whatever contaminant is there, but I then need enough heat to reach the ignition temp of the tank material itself. If I don't, the contaminant goes "poof" in a flash fire and the fire goes out as there is just no fuel to continue combustion as O2 is an oxidizer, not a fuel.

So what that means is that I need to have a high ignition temp for the initial ignition of the contaminant(s) and I then need enough contaminants to burn to either a) increase the pressure in the tank enough to rupture the tank, or b) increase the temp enough to ignite the tank material itself.

Consequently, in terms of the "explosion" you describe, it makes very little practical difference whether I have air, nitrox or 100% O2 in the tank. The ignition temp of matrials such as steel, aluminum and brass will be lower in a pure O2 environment, but it will still take an ignition source and a lot of contaminants to produce enough heat inside a tank to get there, and a tank made from steel or aluminum tends to be a superb heat sink.

Tank explosions related to O2 tend to be related to careless handling, extreme ignorance or outright stupidity. The best example I can think of at the moment was a technician removing a valve from a medical O2 tank. The valve was in operable and could not be opened, so the tech's plan was to unscrew the valve enough to get it to leak slightly. Not the best plan in the world as it failed to consider the heat generated by the emormous amount of friction between the valve and tank threads under full service pressure.

Fill rates are important, and slower is definitely better to reduce the heat generated, especialyl with 100% O2. However, I have seen fire departments slam fill 3000 psi aluminum SCBA tanks in as little as 45 seconds, and the tank is still cool enough to touch. So even a tank that feels hot is still a long way from reaching the point of ignition of most o-ring mateirals and potential contaminants. Of greater concern is the potential for compression heating and hots spots in the flow path of the valve, and that is a much more imortant reason to use very slow fill rates with O2.