Pure O2 Handling?

[Jaap]

Why do so many people indicate the maximum safe filling rate in psi? That doesn't make any sense to me as it isn't a measure of the filling rate!

Lets say you fill with maximum 100psi per minute. That is only a small trickle of gas if your filling a 13cu ft bottle but the same 'recomendation' applied to a 120-twin set would result in a considerably greater flow!

Why not talk in terms of flow instead of that arbitrary 'psi per minute'-talk?

By the way in europe the industy standard oxygen bottles are usually 200bar (approx 3000psi) and that works just fine.

/Anders

 

[Stone]

I can't measure flow directly. I *can* set my valve so that the pressure gauge needle is moving at 100 psi per minute.

 

[blacknet]

Hello,

welding supply shops do carry flow meters. Also doesn't the o2 hackers manual state that filling smaller bottles becomes more artful due to it's small size? Think it also states you have to use a reduced flow rate depending on the bottle size.

Ed

 

[Jaap]

Well using the manometer to provide information on the filling rate is not what I'm questioning.

What I find strange is the fact that a lot of people seem to over simplify things when they think that 'x psi per minute' is a measure of the filling rate regardless of the cylinder size.

 

[runvus4]

Expanding on the heat "Fire Triangle", a fire requires three components: Fuel, Oxygen and Heat.

Oxygen itself does not burn, but the greater amount of Oxygen present, the lower the "flash point" of a subtance, or the temperature required for an item to ignite. For O2 cannisters or high O2 mixes, lets just assume that you can't remove O2 from the fire triangle for SCUBA tank purposes.

Fuel is just that, anything that may burn, which while highly unlikely, can include metal should the flash point be reached, which is why TI is usually not cleared for high O2 concentrations, the flash point is lower than steel/brass/aluminum. Also, CGA grade E gas has a hydrocarbon limit of 25ppm and hyperfiltered air has a limit of .1 ppm, which explains why Nitrox tanks should only be topped off or filled with Hyperfiltered or "Oxygen compatible" air.

Ignition or heat for fill purposes usually means the temperature by which any fuel sources can ignite. This is the only side of the fire triangle that you can really manipulate. Heat caused by compressing gas is also known as Adiabatic Heat. This effect should be readily observed from the "hot fill" that. what you can't see is that the gas inside is significantly higher than the tank itself, especially at the point of entry to the tank from the valve. In my gas blending manual, it notes that a rapid pressuraization of a tank to 2000psi from zero can generate gas temperatures as high as 1600 degrees. If you are doing this and have not had a problem, then your equipment is properly spec'ed out in that you are using O2 compatable components and you are lucky enough not to have had any contaminants. I would highly reccomend that you stop that practice and not be anywhere near your LDS fill station when they are filling if they continue to do fill pure Oxygen that fast. You may get away with it 10000 times, but you only need one time to get caught behind the eight ball to ruin what would have otherwise have been the rest of your life. Since it is the pressurization of gas that generates the heat (change in PSI, not the CuFt/min pumped), you control the heat by controling the PSI per minute. The recomended max fill rate for High O2 gasses is 70 to 100 psi per minute, though 60 psi per minute is often used as slower is better as far as heat generation is concerned, and 60psi/min is far easier to eyeball than 70 or 100psi/min. As always, there may be local regulations concerning gas handling that may apply to your specific situation so you may want to check with whomever supplies your gasses.

 

[Jaap]

Just trying to clearify a few things. The term 'Adiabatic' refers to a process that is thermally insulated from the surrounding so that no heat can escape. The filling of tanks is not strictly adiabatic since the outside of the bottles lose heat during the process.

I agree with the fact that the temperature inside a bottle being filled is highly dependent of the pressure rise and not directly dependant of the actual flow. The actual bottle temperature can become dangerously high during rapid high oxygen fills. I have heard that this is a specific problem for oxgen and nitroxtanks made out of composite materials without metallic liners since the actual bottel can ignite if the temperatur is allowed to high. A friend that used to work for a company making bottles told me a rapid fill giving a bottel an outside temperature of around 50-60 Celsius (around 120 F) could give an internal bottle temperatur as high as 259 C (480 F) wich is damaging to some composites.

But I don't think that that is always the most important issue dealing with oxygenfills.

Most high pressure oxygen related accidents seem to be caused by adiabatic compession related problems originating in the filling line and not in the tank.

Near adiabatic compession occurs is when a gas in a confined space such as a filling line is suddenly subjected to a high pressure that rapidly compress the gas to a fraction of its original volume in such a short time that the heat that is evolved cannot escape. For example when a decanting hose filled with oxygen at ambient pressure is rapidly subjected to high pressur oxygen.

As said in the last posting the temperaute can sertainly sometimes reach around 1600F (approx 850C) or even higher during these events. But this maximum temperatur occurs in the filling line and only for a very brief amount of time not in the tank. If the decanting hose is contaminated or if the polymer in the hose has a flamepoint that is to low ignition could occur if enought energy is transfered to the contaminant/polymer.

So high temperature is not enough in order to achive ignition, the heat transfer to the material that is to ignite must also be taken into consideration. Just like if you would try to ignite a pice of paper or some oil with a spark, just a spark is not always enough just because the energytransfer is to limited.

So in my way of thinking several things are important in order to stay safe. Slow opening of valves reduce the risk of sudden adiabatic comperssion in the fill line. The increase in pressure and the actual flow rate are also important. If the flow in the filling line is to high friction and vibrations could tranfer enough energy to casue an ignition of the polymer or possible contaminants in the valves and fillinghose.