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Old 15th August 2011, 05:11   #1 (permalink)
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Underwater Oxygen Fire

I was diving open circuit in the weekend and one of my two buddies made the switch from 50% to 100% oxygen at the 6m point. Immediately, he spat his reg out again and it was apparently free flowing. He jumped back on his 50% mix and we extended that stage of the deco to allow him to decompress properly.

On the surface he told us that he had had an oxygen fire and it tasted like burnt o-ring or plastic and it was difficult for him to recover his breathing after he switched back to his 50% bottle.

Once back on the boat, we removed the first stage regulator and the 1st stage o-ring (Scubapro Polyurethane o-ring) was dislodged and there was evidence of it being burnt both visually and by smell, although it was not too badly burnt.

We suspected that the tank valve seat was burnt quite badly as the tank valve would not seal the last few bar of gas when it was closed fully and it felt like it was seating metal to metal.

When he got home, he pulled the tank valve apart and found that the seat was completely burned up and just a black hole was remaining.

We can only surmise that when he charged his bottle, the velocity of the gas must have hit the valve seat - which must have had some silicone lube or dirt on it and created the ignition.

Valve type: Thermo
Tank type: Luxfer 40cuft
Tank and valve age: New with 3 dives on it - not oxygen cleaned but only ever had oxygen in it
Reg type: Scubapro MK17 2 years old - oxygen cleanliness unknown
Hose types: Standard Rubber
SPG type: Termo small 1.5"

To his credit, while this was occurring, he didn't panic but handled the situation well. Between the team, we managed our gasses and swapped deco tanks around within the team to ensure the diver had the best available deco gas with the most amount of pressure and was not further task loaded while dealing with a scary situation.

Things learned that we wish to share with the community:

Always pre charge your oxygen tanks on the surface.
Always ensure everything is oxygen cleaned regularly.
Always open oxygen valves slowly.
Always purge a reg prior to breathing it.
Always ensure you have sufficient redundant deco gas if there is a total gas loss of a single deco gas.

Given we were in a three person team and the diving tasks had been equally shared between the divers, this situation was able to be handled in a reasonably smooth manner and further issues did not arise.

I am extremely pleased that the outcome was that all team members exited the water having completed sufficient deco and without any further issue or major stresses. Heck, he even kept his trim throughout the whole affair.

Interested to hear if anyone else has ever heard of this happening underwater.
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Old 15th August 2011, 12:23   #2 (permalink)
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Re: Underwater Oxygen Fire

I actually know of 2 other incidences of the same nature - one on the surface while test-breathing an O2 bottle and one underwater during a gas switch. Both divers survived but had to take some time off to let their lungs heal and recover.

In your post you mentioned that the reg was a SP Mk17 - they are not designed or approved by SP for 100% O2 use. I would not used them for that purpose anymore ...

Also if you (insert buddy here) are using 100%, you may want to ensure that the cylinders, valves and regs are designed, cleaned and approved for such use - in addition to all the parts and lubs to be compliant with 100% service.

Apart from the obvious issues of debris/contaminants in the bottle or valve, excessive fill speed seems to degrade and deteriorate the valve seats which then leads to a flash fire inside the tank. At least in both cases I am familiar with locally, that was thought to be the case.

Thermo valves are generally good valves - I use them on all my stage and deco bottles without any problems. A tell-tale sign of seat wear is the fact that it may only take 1/4 turn to "fully" open the valve. If any of my valves start acting like that I rebuild right way. Hope this helps.

Andy
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Old 15th August 2011, 13:58   #3 (permalink)
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Re: Underwater Oxygen Fire

I don't bother cleaning brand new cylinders, but I will O2 clean all valves that I plan on introducing 100% O2 into. Being that there was evidence of burning in the valve, I would not blame the 1st stage. There's no way the burning could have gotten into the valve if it started in the 1st stage. 240 bar of pressure pushing the O2 out would prevent that.

It also appears your buddy didn't pre-charge his reg before getting in the water. That is not a good practice. Not only does it not allow him to ensure the o-ring is seated properly, but it will also allow salt water intrusion into the 1st stage, requiring it to be rebuilt. Although, in this situation it probably saved him in that being submerged may have kept it from becoming a worse incident.
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Old 15th August 2011, 23:14   #4 (permalink)
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Re: Underwater Oxygen Fire

Thanks guys,

I've shown him the post and he's told me to correct a couple of things and to add some more observations.

The regulator was a MK11 not a MK17. I personally only use MK25's for everything and oxygen clean all of my regs/replace o-rings etc.

He sent me some pictures last night and the valve seat was just a black hole and the DIN o-ring was just coloured a little black but otherwise is fully intact he says. All of the rest of the regulator 1st stage is fine, but interestingly enough, the christolube that he had in there got blasted into the diaphragm and 'cooked' as such. So it appears that his 1st stage was cleaned for oxygen service.

We are now certain that the fire occurred within the valve seat and can only assume that the seat had some contaminant on it as it was only 3 dives old as such.

He may have pre charged his tank, but the reg could have purged slightly as we were working our way through the wreck or while he was switching bottles during deco. We are fairly certain that the gas velocity was a major contributing factor in this. We wondered about the SPG initially, but this is ruled out as the valve seat has fully burned to nothing and the 1st stage is essentially fine.

Last edited by jasace; 15th August 2011 at 23:17..
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Old 16th August 2011, 03:59   #5 (permalink)
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Re: Underwater Oxygen Fire

I can now be certain on the material type of DIN o-ring it is, I've just spoken to Scubapro and it is Polyurethane. There is a little more info on poly o-rings here:

Oxygen Compatible O-Rings, Lubricant and Tools - Dive Gear Express

It seems that the weak point is the valve seat material and we agree that 40% clean or 'Nitrox clean' is not oxygen clean.


Further research seems to suggest that the valve seats are made from Polyamide 6/6, which has an oxygen index number of 22. This is considered slow burning. Anything greater than 28 is considered self extinguishing and anything at or below 20.95 will readily burn in air. So, we introduce pure oxygen into this equation and we have a material that will burn fairly easily. Makes you wonder what a shop does when they certify a tank as being oxygen clean and if they replace the valve seat at all. For those who prefer to work with UL standards, Polyamide 6/6 has a UL94 rating of HB, which is the least flame retardant of the UL94 rating. I hope I am reading and understanding this stuff correctly.

I spoke to one professional oxygen cleaning company who does Scuba tanks and equipment and they told me that they go with what the manufacturers suggest and it was a bit of a grey area as no manufacturer it seems is willing to sign a tank off as oxygen compatible. The valve seat is not replaced for an oxygen clean unless it has signs of excessive wear or discoloration it seems.

I may now look into having some custom valve seats manufactured with a better choice of materials for the seat. To date, I've had no issues and this one, while the first one I've heard of is not the only one to happen in the world it seems. Thanks Andy your input was quite valuable on this.
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Old 16th August 2011, 06:58   #6 (permalink)
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Re: Underwater Oxygen Fire

Quote: (Originally Posted by jasace) View Original Post
Further research seems to suggest that the valve seats are made from Polyamide 6/6,
Valve seats are IMO the real issue.

The problem is virtually all materials that have the necessary compliance to work as a bubble tight valve seat have a low index, and the materials that have a higher index (PTFE) will cold flow.

Rulon does look promising.

I'm sorry I can't cite the source but I do recall some correlation between worn soft seats and increased failure rate. IIRC the seats can develop small filament like "hairs" from use. Keep 'em nice and fresh!

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Old 16th August 2011, 09:05   #7 (permalink)
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Re: Underwater Oxygen Fire

Using a reg not knowing if it is O2 clean or not and asking what might have caused the fire ....
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Old 16th August 2011, 10:21   #8 (permalink)
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Re: Underwater Oxygen Fire

I have seen several O2 degraded seats over the years, most of them have been the OMS modular valves. I don't know who makes their valves.

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Old 16th August 2011, 15:54   #9 (permalink)
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Re: Underwater Oxygen Fire

Quote: (Originally Posted by bletso) View Original Post
I have seen several O2 degraded seats over the years, most of them have been the OMS modular valves. I don't know who makes their valves.

Dale
Sanosub makes OMS Valves

Sanosub Diving catalogue
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Old 16th August 2011, 16:01   #10 (permalink)
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Re: Underwater Oxygen Fire

Quote: (Originally Posted by jasace) View Original Post
All of the rest of the regulator 1st stage is fine, but interestingly enough, the christolube that he had in there got blasted into the diaphragm and 'cooked' as such. So it appears that his 1st stage was cleaned for oxygen service.
No surprise on the Christolube.. it is not stricly O2 compatible at least not for high pressures used in divcing and certainly not from its heat resistance..
There is better stuff available, but more expensive also of course..
Never understood why Christolube was so wide spread on the other sides of the oceans..

Quote: (Originally Posted by jasace) View Original Post
To his credit, while this was occurring, he didn't panic but handled the situation well. Between the team, we managed our gasses and swapped deco tanks around within the team to ensure the diver had the best available deco gas with the most amount of pressure and was not further task loaded while dealing with a scary situation.

Things learned that we wish to share with the community:

Always pre charge your oxygen tanks on the surface.
Always ensure everything is oxygen cleaned regularly.
Always open oxygen valves slowly.
Always purge a reg prior to breathing it.
Always ensure you have sufficient redundant deco gas if there is a total gas loss of a single deco gas.
Reading you description you should really focus on the last mentioned lessons learned.. No reason that missing one deco gas should turn into a scary situation.. Also I don't quite get why you had to swap tanks around in the team.. Seems that the dive planning was not quite by the book then..
As far as comes to the oxygen issue.. your 3rd point is the most important in my opinion.. with opnening slowly even under the given circumstances nothing should have happened..
Personally I am not overconcerned on the clealiness hype which is widely spread in the diving community.. I always have the welders in my image and how they "handle things"..

But where I always get very anal is at slooooooowly applying pressure on any rich mixes or pure oxygen whether at opening valves or transfilling as such..
No ignition source/energy..No burn/fire... simple..
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