Nitrox Only

[Hickdive]

When partial-pressure blending nitrox the air being used needs to be hydrocarbon "free". From most compressors this cannot be absolutely guaranteed so instead a second filter stack is included between the primary filter and outlet so that any oil contamination that does break through the primary is caught by the secondary. Unfortunately this can only go so far - some hydrocarbon contamination is possible even with double filtration but at such low doses that any single fill does not cause a problem. However, over a period of time and repeated fills any contamination might build up until it is potentially a problem. This is why oxygen-clean certification is time limited.

If you have an oxygen-clean cylinder for nitrox you can still use regular air in it without invalidating the oxygen-clean status provided the air used is "oxygen compatible".

Hypergolic ignition is extremely unlikely as contaminants that ignite in the presence of oxygen are rare - the primary source of ignition in oxygen fires is heat caused by friction or impingement. You can significantly mitigate the risk of heating by filling slowly; 4 bar per minute or less. However, once ignition occurs materials that are normally fairly inert can burn with extreme ferocity in high-pressure oxygen.

 

[Bob DBF]

No source of ignition is necessary. These substances can self-ignite. This may be even more likely under pressure.
https://www.google.com/search?as_q=&as_epq=Petroleum+based+products%2C+oils%2C+grease+and+lotions+can+self-ignite+in+the+presence+of+&as_oq=&as_eq=&as_nlo=&as_nhi=&lr=&cr=&as_qdr=all&as_sitesearch=&as_occt=any&safe=images&tbs=&as_filetype=&as_rights=#as_qdr=all&lr=&q=can+petroleum+and+pure+oxygen+self-ignite+

The references given, that I just read, do not give one instance of that happening, even though they give examples of other problems, most of which are smoking in a raised O2 environment.

I might add, that just because one does not see the ignition method, does not mean the source of ignition is not present. If said petroleum product had a flashpoint of 100 degrees F, you might not think that the "spontaneous combustion" of the petroleum had an ignition source. Also, should the O2 combine with compounds in the petroleum product, and have an exothermic reaction, you may miss the ignition source.

Oxygen, fuel and ignition source are necessary, or we might as well go back to the days when fire was magic.



Bob
-------------------------------------
I may have been born at night, but it wasn't last night.

 

[101recon]

http://www.usfa.fema.gov/citizens/home_fire_prev/hazmat.shtm

Quote from the above link: "Keep oil, grease, and similar petroleum-based products away from oxygen valves. They can cause a spontaneous explosion."



http://www.wakehealth.edu/Health-Central/NMR/Home-Medical-Oxygen-Safety-Tips/


Quote from the above link:
"Oils, grease and petroleum products can spontaneously ignite when exposed to high levels of oxygen. Also avoid using oil-based lotions, lip balm or aerosol sprays."

 

[Tigerman]

NOTHING truely "spontanously combust" - some things however can APPEAR to do so because they have a flashpoint that can be reached i.e. because the tank temprature increase as it get filled or the room with those thinners have a big window straight into the sun, but there IS a source of ignition, namely the increased temprature.
If the temprature is reached due to increased pressure or by a match is irrelevant - its still a source of ignition..

 

[eponym]

NOTHING truly "spontanously combusts" - some things however can APPEAR to do so because they have a flashpoint that can be reached e.g. because the tank temprature increases as it get filled or the room with those thinners have a big window straight into the sun, but there IS a source of ignition, namely the increased temprature. If the temprature is reached due to increased pressure or by a match is irrelevant - its still a source of ignition.

Discontinuities in flow during cylinder filling (for example, at the edges on the end of the dip tube) can result in extrememly small bursts of high temperature which dissipate quickly but which can be enough to ignite nearby hydrocarbons. At any rate, this is what my advanced deco instructor told us. He said momentary spikes to one thousand F were possible.

Bryan

PS. No, I have no citation or Rubicon link to back this up. Anyone?

 

[FM1520]

Membrane systems aside, here's a question I'd like to ask:

Assume that the compressor air is grade E, which goes into a blending stick fed by a dedicated pure oxygen tank. The appropriate electronic controls are in place to create and bank 40% nitrox in the bank of nitrox tanks.

I believe that when my cylinder is filled partially with this premix and then topped off with grade E air to produce my desired mix (let's say 34%), this procedure invalidates the O2-clean rating of my cylinder. In other words, I would not later want to do in-tank blending using that cylinder. Comments? -Bryan

Yes, at least thats my understanding.

 

[Tigerman]

Discontinuities in flow during cylinder filling (for example, at the edges on the end of the dip tube) can result in extrememly small bursts of high temperature which dissipate quickly but which can be enough to ignite nearby hydrocarbons. At any rate, this is what my advanced deco instructor told us. He said momentary spikes to one thousand F were possible.

Bryan

PS. No, I have no citation or Rubicon link to back this up. Anyone?

Would not surprise me and that would easilly serve as ignition..

 

[kwinter]

Adiabatic flow creates heat, and that can and often does serve as the source of ignition. The tiniest speck or nick in the way of the flow can and will cause temp to rise. But the key is to add the O2 very SLOWLY. I swear I could coat my tank, valve and whip with vasoline and still not have a problem if I filled it slowly enough. But that doesn't make it smart.

 

[Tigerman]

I think theres a reason why its not recommended.. Probably because divers aint patient enough to fill that slow

 

[boulderjohn]

. I swear I could coat my tank, valve and whip with vasoline and still not have a problem if I filled it slowly enough. But that doesn't make it smart.

The Oxygen Hackers Companion agrees, saying that using good technique while filling can allow you to fill an unclean tank. As you suggest, that does not make it a good idea to fill a tank you know to be unclean--the results will not be good for you if you slip up on your technique. It is instead an argument for using good technique at all times, because you can never be sure that a tank is as clean as it is supposed to be.