O2 clean or not O2 clean...

RebreatherNut

Hi all,
Well, I was wondering about the whole O2 clean tank issue. I know about O2 cleaning, read the Oxyhacker book, know about the hydrocarbons etc etc.

But then I hear from various sources, well when they do the annual visual inspection and the dive shop sticks its light into the tank, there goes your O2 cleanliness? True or false?

Other stuff I have read is about air travel, some people argue that, if you check EANx or O2 tanks, and you don't know what happens behind the TSA scene, your tank is no longer O2 clean.

In other words, how much does it really take to "mess up" a O2 clean tank so it becomes dangerous? I sure don't want to try it out . I know the saying, better safe than sorry.

Thoughts, comments??

Thanks,
RBN

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"One minute to go, Luck-meister! Then I'll be down on you like a buzzard on a gut wagon." - ALF

Mixaddict

If your dive shop is going to do a VIP, they should O2 clean your tank at the same time and sticker it so. In our shop, we won't even do O2 fills unless the O2 cleaning has been done in the last year. Additionally, unless one of our techs did the cleaning, we won't fill them, unless we know the person personally that did the O2 cleaning. Additionally, we want to know where you have gotten the tank filled during the last year. If we are at all suspicious about cleanliness issue, we will require a new cleaning prior to filling. As far as tanks go, the most dangerous part of the O2 equation is to the person doing the filling, so we don't take any chances. I had a close friend killed in an O2 explotion about 3 years ago at a filling station.

I know there are a lot of divers out there, that think the whole O2 cleanliness issue is completely overblown, but it only takes one accident to ruin your whole day! My advice is to make sure that your O2 tanks and valves (as well as deco bailout tanks) are O2 cleaned at least every year, and also that you are very careful about procedures to keep them clean in between. If you have any concerns about your filling station, find another place or get them to clean it up. If their standards are not up to your requirements, you will spend a lot of money and energy constantly cleaning your tanks, valves, regs, etc.

By the way, if the shop doing your VIP doesn't know enough to clean a light off before they stick it into your O2 clean tank, time to find another shop!

As fas as TSA goes, who knows what they may do, but more than likely, they will just shine a flash light into the opening and see that there is nothing inside. If I am traveling with O2 tanks, I usually clean them when I get home anyway, as I don't completely trust most remote fill stations. (Just my paranoia!)

It's your responsibility to keep your equipment clean and to make sure that you are not endangering the person filling your tanks. Also, it doesn't take a very much hydrocarbons in your tank to create gas that is dangerous to you as the diver at depth.

Just my $.02
Randy

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Randy Thornton (MixAddict)
Inspiration, Evolution,Hammerhead & Sentinel
CCR Instructor

rdmmdr

randy
well said. an extra hour or two to avoid the burn unit. no question. you only have to experiance an o2 fire once to understand that you never want to have another one. have some green.

rick

nigelh

There is a lot of silly thinking on this matter.
There is a world of difference between having O2 service components (a must) and being scrubbed up clean and shiny.

Firstly it is not surgically sterile. I've heard some fool in a dive shop (staff) utter the 'gasp - you touched it now it's not clean anymore' line.

Back on uk.rec.scuba the physics cabal did some sums on a 12L single and deduced that 10cc of hexane (roughly petrol) ignited explosively in a tank would take it from its working pressure to it's test pressure. We rated that as nasty but contained. However 10cc inhaled was over the lethal dose so good flammable pollution, the stuff we want not to have in our tanks, would kill the diver before it damaged the tanks and hence the outside world.

I'm far more concerned with contaminants in the valve and more so the regs than the tank. I forget the origin of the numbers but allegedly most oxygen fires are down to filling procedures and then they seem to be down to first stages.

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nigelh

marekm

Actually hexane is nowhere near as toxic as portrayed by the 'physics cabal'. The LC50 for a rat is 48,000ppm over 4 hours. The example you give above works out to only 800ppm+-10% for an expsoure of only one hour, well below the LC50. Given that hexane is not very narcotic (unlike say toluene, which is a known scuba cylinder contaminant) the diver won't even pass out from it at recreational depths. Lesson here is you should not send physicists to do a toxicologists job.

My own simplified take on O2 cleaning is that everything is O2 filthy, only some things are less O2 filthy than others. There is simply no way you can clean out every single last molecule of flammable hydrocarbon/particle from a tank/valve/reg. There will always be some left, hence you cannot call it 'O2 clean' in an absolute sense. Furthermore, how O2 filthy something is, directly relates to the probability of a significant O2 fire occurring. By periodically 'O2 cleaning' you simply lower the likelihood of an O2 fire occurring to a level more to your personal liking.

-Marek

nigelh

ROTFL!

The main figure was 10ccs. That's not just surface contamination.
The main problem for oxygen fire risk is volatiles and volatiles don't stay in the tank they end up in the diver. Compressor oil may be nasty but only the stuff in the valve appears a risk to me. The body of the tank is actually a rather benign environment that I would be happy to only clean when it gets corrosion tested/inspected.

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nigelh

RebreatherNut

I have to agree that probably the valve is the "more concerning" part than the tank. Also, it is in the valve where the gas must make the 90 degrees bend in order to get into the tank.

__________________
"One minute to go, Luck-meister! Then I'll be down on you like a buzzard on a gut wagon." - ALF

Mixaddict

All the more reason to make sure that the tank as well as the valve are clean and kept clean. If you put crap in the tank, it must go through the valve, and will certainly come back out the valve. Thus, even though the valve may be the source of the "adiabatic compression", it's what goes through the valve that make's it a source of potential disaster. "Garbage in, garbage out", but in this case, it can be worse, "garbage in, BANG out!"

__________________
Randy Thornton (MixAddict)
Inspiration, Evolution,Hammerhead & Sentinel
CCR Instructor

RebreatherNut

You are right Randy, the O2 Big Bang Theorie .
A friend of mine had a very bad experience while filling O2. There was a 90 Degrees curve before even entering the valve, and some particle struck that 90 degree angle......Kaaa-Boom!!!! He sustained 3rd degree burns on face and elbow, the wall had a big black mark and the filling yoke and hose were trashed.

__________________
"One minute to go, Luck-meister! Then I'll be down on you like a buzzard on a gut wagon." - ALF

Genesis

I've seen the consequences of an O2 fire and do not want to experience one first-hand. Generally speaking IF you have an O2 fire you can forget about being able to put it out, irrespective of what you have for fire-fighting equipment, until and unless you can cut the O2 flow off. If the fire is between you and the valve, preventing access to it, you're hosed - about all you can do is run.

With that said, you have three generalized risks here:

1. Scuba valves have a number of nasty sharp bends and edges in them. These are bad news around O2; quick changes in direction in a gas flow produces heat, and heat is required to initiate a fire. Those same quick changes also give rise to the possibility of impingement (particles in the gas flow striking something and sparking) ignition events.
2. Once a fire is initiated in a pure O2 atmosphere (anything over 50% FO2 may as well be pure O2 for fire purposes according to a study NASA did on this matter) absolutely anything that can burn will do so. This includes both aluminum and steel and in the valve it includes the seat. Among scuba valves I've NEVER seen a seat made of an O2-rated material such as KEL-F, although there might be someone making them. Just about the only safe metals in this regard are brass (will not burn under 3,000 psi of pure O2) and monel (for all intents and purposes won't burn) Therefore the brass valve body will not burn, but the plating and valve seat will. So will the tank under the right circumstances!
3. Aluminum reaches its critical temperature at roughly 300F. This is very bad, because once that temperature is reached the strength of the metal is reduced by about 80%. If the tank has any material amount of pressure in it (certainly over 1,000 psi or thereabouts) and reaches this temperature, it will rupture. Now you have an explosive event to add to the O2 fire.

Not long ago here in Florida an aluminum O2 deco tank was dropped. It was connected to a fill whip in preparation for filling but the fill valve was closed; the fill had not yet started. Something internally ignited on impact; the tank reached critical temperature internally and exploded. An employee of the shop was killed in the blast. Parts of the tank were recovered and clearly showed that the failure was not a simple mechanical fracture (e.g. overstress from being dropped on the valve) but that combustion occurred internally in the tank. The valve, which of course was the suspected ignition location, was never found.

I take O2 cleanliness very seriously because I fill my own tanks at my home and both PP mix as well as fill my supply (and OC deco) tanks full of pure O2. So long as you are the only one doing it and are only transfilling (no boosting!) in theory once clean it should remain that way. In practice it doesn't work like that which is why any tank exposed to 40% FO2 gets cleaned annually with Blue Gold and the valve disassembled and run through the ultrasonic. Ditto with the O2 regulator. If you run a booster then you have an additional place where contamination can concentrate and be transferred into the tank and valve.

The single biggest thing you can do to control risk is to control fill rates and open valves SLOWLY. If transfilling, make sure you have a true needle valve for fine control of the fill rate. If boosting, take your time. Keep the fill rate to 60psi/minute and you greatly reduce the risk. Brass is a preferrable material for fittings and such as it does not spark and will not burn. Teflon-core SS braided hose WILL burn (both the stainless AND teflon!) if it gets lit, so make sure it doesn't. Anything made out of steel (stainless or not) can potentially spark if particles impinge on it at high speed, which is all you need for an ignition event. Fitting selection can be a problem as it is difficult to find brass fittings rated for 3,000 psi and nearly impossible to find ones rated for more, which drives people to use SS instead.

Most regulators have their bodies made of brass (good) but some internal parts are stainless (e.g. valve lifters in the Apeks DS4, etc) - not so good. The seats are also typically some form of nylon - not so good - and there is almost always at least one 90 degree and sometimes 180 degree turns in the gas path (not good at all.) Very few regs are truly rated for pure O2 use, but with care and cleaning it is reasonably safe.

Welders use a lot of O2 and rarely have incidents, but they do happen. Then again T and K cylinders here in the US have valves specifically designed for O2 service and are steel, and welding regulators are designed specifically for O2 service.

We kinda break the rules in the diving world with oxygen, in that basically none of our gear is really designed for that use, and nearly all of the time we get away with it....... I try to stack the deck as much as possible.....

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