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Underwater accidental hydrogen / oxygen explosion

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Papyone:
Hello everyone,
First of all I want to inform you that I’ve not much knowledge with chemistry because my job was commercial diving. I’m now retired, but still very concerned about the security of my former colleagues and therefore I come to you with what some of you will maybe find a weird question, but which has some great and even vital importance to us.
One of the current things a commercial diver does is cutting steel underwater. Therefore we use a short thermal lance that works about the same way as a welding rod except that the lance is hollow to allow the passage of pure oxygen. Due to the DC current we use, as well as the heat of the flame (+/- 5500°c) we have electrolysis phenomena of the water that produce an explosive mixture merely composed of H2 and O2. For commercial divers, the formation of such gas mixture sometimes presents a lethal risk if it cannot escape freely to the surface and a lot of accidents did happen with deadly consequences.
Up to now, we were convinced that during the cutting process the percentage of hydrogen that was produced was situated above the H2 L.E.L which if I’m correct is situated around 3.9%
But, recently following another deadly accident it was decided to make a few cutting tests (6) to recover a few samples of residual cutting gasses for analysis and after seeing the results, I must say that I’m very astonished to see that the percentage of hydrogen present is much lower than this L.E.L of 3.9.
As for instance, the average values that the laboratory has measured are:
H2 0.0167% / O2 93.5% / N2 5.49% / CO 0.113% / CO2 0.855%. 
What is very confusing is that even with such a low hydrogen proportion we are confronted to explosions even if the trapped volume of gas is small (a few cubic centimetres).
So my question is: Why do explosions happen with such a low hydrogen %?
Does the combination of all these gases change the LEL of the mixture?
Sorry for that long description.
I also ad here a picture to help you to understand my question better.
Thanks in advance for your help.

Papyone:

--- Quote from: Papyone on December 23, 2018, 11:28:44 AM ---Hello everyone,
First of all I want to inform you that I’ve not much knowledge with chemistry because my job was commercial diving. I’m now retired, but still very concerned about the security of my former colleagues and therefore I come to you with what some of you will maybe find a weird question, but which has some great and even vital importance to us.
One of the current things a commercial diver does is cutting steel underwater. Therefore we use a short thermal lance that works about the same way as a welding rod except that the lance is hollow to allow the passage of pure oxygen. Due to the DC current we use, as well as the heat of the flame (+/- 5500°c) we have electrolysis phenomena of the water that produce an explosive mixture merely composed of H2 and O2. For commercial divers, the formation of such gas mixture sometimes presents a lethal risk if it cannot escape freely to the surface and a lot of accidents did happen with deadly consequences.
Up to now, we were convinced that during the cutting process the percentage of hydrogen that was produced was situated above the H2 L.E.L which if I’m correct is situated around 3.9%
But, recently following another deadly accident it was decided to make a few cutting tests (6) to recover a few samples of residual cutting gasses for analysis and after seeing the results, I must say that I’m very astonished to see that the percentage of hydrogen present is much lower than this L.E.L of 3.9.
As for instance, the average values that the laboratory has measured are:
H2 0.0167% / O2 93.5% / N2 5.49% / CO 0.113% / CO2 0.855%. 
What is very confusing is that even with such a low hydrogen proportion we are confronted to explosions even if the trapped volume of gas is small (a few cubic centimetres).
So my question is: Why do explosions happen with such a low hydrogen %?
Does the combination of all these gases change the LEL of the mixture?
Sorry for that long description.
I also ad here a picture to help you to understand my question better.
Thanks in advance for your help.

--- End quote ---

Enthalpy:
Welcome, Papyone!

Of the measured gases, only H2 and CO can burn, and both concentrations are very far below the explosion limit. No interaction known to me, nor do I expect any.

Explosions must result from other conditions than the measured composition. The dangerous conditions may not happen every time.

I don't see how an electric arc would produce significant hydrogen by electrolysis. The evolved hydrogen must burn immediately under these conditions, before any amount accumulates. That's even more the case if (?) you use AC current. But surprises happen.

A different scenario? From the item being cut or welded, or from the tools, metal still hot reacts with water when or where no oxygen is present, and produces metal oxide and hydrogen. If hydrogen accumulates in something like a sunken hull, it can detonate.

Still an other hypothesis: methane produced by bacterial decomposition of organic material already accumulated in a place where the diver adds oxygen. The mix can deflagrate.

In case (?) these explosions occur where hydrogen or oxygen accumulates, away from the tools, a solution would be to cap the operation site with a funnel and a pipe to evacuate the gas to the surface.

Papyone:
Already thank you for your reply. I will comment more on it in two days. ;)

Papyone:
Hello enthalpy,
Here I’m again with a few more information concerning our cutting process.
To do what you see on the photo, we use a rod that needs about 150 Amps to create the electric arc which ignite the rod and once ignited such a rod burns during about 50 seconds and consumes about 250 litres (at atmospheric pressure) of pure oxygen per minute.
To limit the electrocution risks we always use DC current.
Concerning the formation of hydrogen by electrolysis we can see that it happens in salt water, because when we start a new job we have to make a polarity test to verify if the cutting torch is correctly connected to the negative pole of the welding group.
Therefore we need to use a bucked of salt water into which we approach the extremity of the electrode at close proximity of the earth clamp. Once the current is on, we can see that a lot of bubbles travel in the water from the clamp towards the electrode (test illustration on the photo).
In most commercial diving schools and manuals they tell the divers that this phenomenon (electrolysis) continuous during the cutting process, but apparently from what I have found during my research on the net this is not really the case.
If I’m correct, thanks to a Faraday law it is possible to calculate the theoretical value of the electrolysis flow generated during the consumption of the rod.
For instance by using one of his formulas I’ve learned that whatever the voltage, 1 ampere will produce 0.209 l of oxygen and 0.418 l of hydrogen, or a total of 0.627 l of gas per hour.
Knowing that when we are cutting, the thermal rod will burn for about 50 seconds with an intensity of 150 amperes, I’ve calculated that this amount of gas mixture would be produced.
(0.627 / 3600) x 150 x 50 = 1.3 l of O2 / H2 mixture
This amount of gas will actually be produced as long as the tip of the electrode does not touch the workpiece but and at contrary to what is often taught to the divers as soon as we start to cut, a large part of the current will pass directly through the electric arc and in this case only the leakage currents located at the periphery of the arc (and therefore still in contact with the water) will be able to create the electrolysis of the water.
I guess therefore that the production of our explosive mixture is not due electrolysis.
Going through the web, I’ve also learned that if water is brought into contact with a very high heat source greater than 2200 ° C, it will start to split into their atomic components hydrogen and oxygen.
At the same temperature of 2200 ° C about 3% of the water surrounding the heat source is decomposed into hydrogen and oxygen, and that this percentage of dissolution increases sharply when the temperature increase. As the combustion temperature at the extremity of our rod is close to 5534 ° C (10000° F) I ‘ve tried to find out  how much water is then decomposed, but unfortunately up to now nobody has given me an answer. 
Do you by chance know how to calculate this amount?
Anyway, I suppose that most of the hydrogen produced in our cutting comes from the heat and not from the electricity.
Concerning the production of methane by bacterial decomposition of organic material you are quite correct. In the latest accident that happened in Japan this was the cause (methane accumulation in a sealed sheet pile tube).
The use of your funnel would be difficult to install in most situation, but anyway thanks for the suggestion. Just to let you know that one of the first things a diver learns when he starts to cut is to make sure the residual gas can escape freely to the surface.
If not, they are taught to make vent holes above their cut or if not possible the must then vent the cut with air.
But unfortunately this is not always done correctly and accidents continuous to happen (35 in the latest forty years).

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