Thread: hey guys an engineer would love some help to get an explanation, Thank you.

Problem:
Drilling in London and high pressure Nitrogen gas pockets are hit killing two workers due to lack of ventilation.



Background and proposed theory:

• Underneath London in the 1940s the water table was lowered underneath an impermeable layer of clay.
• This created a vacuum in certain areas
• Due to the vacuum the water cavitated
• Air came out of the water and formed above the water and below the clay.
• Iron present in the clay oxidised causing the removal of Oxygen in this air and resulted in Nitrogen rich air
• Water table increased (due to less demand officially) and resulted in the Nitrogen under high pressure.

Question:
Does this theory make sense to scientists?

Thanks for your help regardless I really appreciate it

Mahony

Just a few thoughts. I am not an expert.

If the water level was depleted by wells, then later rose when the demand from wells decreased, it would seem to me that the air would have gotten in from the surface. No need to postulate a vacuum. The trapped air would surely become stale after a while. I think oxidation of iron might be one way for that to happen, but maybe rotting of vegetation could play a part?

Thanks for your reply,

I believe that the clay surface is uneven. It is shaped like this ---^---- if u can imagine from my crap diagram. The high area is where the vacuum is been created and it is been pumped from the extreme left and right. The vegetation is a very good idea though too. I really appreciate your response

Two graphs in this report give a general view of the geology of the London basin and the changes in the aquifer. As a native Londoner I wasn't aware of this issue so I'm glad you brought it up.

Rising Groundwater in Central London

I'm curious if this is a real event, the deaths of two drillers by nitrogen suffocation. Do you have a link?

I agree with Harold that a vacuum is unlikely. The lowered water table occurred too slowly and air would have seeped in over the years, or so I would have thought. If a vacuum did occur then I think cavitation is the wrong word in this context. Cavitation refers to the rapid appearance and collapse of bubbles, not the slow outgassing from solution. However, your scenario does not need a vacuum, it just needs air to be present by some means.

How much unoxidized iron exists in London clay?

http://www.geolsoc.org.uk/webdav/site/GSL/shared/pdfs/specialist%20and%20regional%20groups/TVRG/London%20basin%20conference/Basement%20issues%20-%20Prof%20Cosgrove%20&%20Dr%20Ghail.pdf

Go to slide 27 of 28

It happened back in the 80s so I cant find much more relevant link just yet.

I'm currently working on it for a research project. I will provide better papers I think il have to upload them though.

Thanks for you opinion though.

Very little in the London Clay in general this just occurs in one layer known as the Upnor formation. But the London Clay provides the impermeable layer which is saturated with water and to get air through requires huge suction I would have thought.

Also

I was thinking that possible Henrys law would state that the if it was released by cavitation would surely go back into the water when high pressure was applied again by the water table rising?

If the water is saturated with dissolved air then the previously released nitrogen would not be able to redissolve. Perhaps the water has a path to atmosphere via the exposed sand and chalk at the edges of the clay that keeps it saturated.

Originally Posted by Bunbury

If the water is saturated with dissolved air then the previously released nitrogen would not be able to redissolve. Perhaps the water has a path to atmosphere via the exposed sand and chalk at the edges of the clay that keeps it saturated.

Much appreciated. thank you again

I agree this very logical that this is what could be happening.

With regards outgassing that you suggested before. Is their any chance that the Nitrogen comes out easier than the Oxygen? Any suggestions on where I could read up on this?

I think the answer lies in Henry's law which you have already mentioned. The Henry's law constant for nitrogen is much larger than the constant for oxygen, but the partial pressure of nitrogen is much higher than that of oxygen in atmospheric air so I think nitrogen would dissolve preferentially the system should come to equilibrium with the trapped air being somewhat oxygen rich. This just from looking at the henry's law equation and not doing any math - could be wrong and I don't feel like doing the math just now. You'd have to make some assumptions about what happens when the water level drops - does it pull a vacuum as you proposed, or does fresh air flow in, maybe through faults or old boreholes as well as through the permeable layers. Interesting though; I had no idea London was perforated like a colander.

Thanks Bunbury,

Hopefully this is my last question

If water is in a sealed unit and some of the water is pumped out until it reached a certain negative pressure say-100kPa. At this point the water cavitates and air comes out of the water. what will happen the pressure as the air comes out?




Thanks a million for any help

First you will evaporate water so there will be both air and water vapor present. Second I don't believe the air will suddenly erupt from solution; it will continuously evolve as you drop the pressure. Then if you lower the pressure to almost full vacuum, the system will be adjusting continuously so nothing special will happen at any particular pressure. Depending on the water temperature it might boil at some point, but the water temperature will drop as water evaporates due to removal of latent heat, so it could freeze instead. All in all it's quite complicated and probably involves both Henry's law and the Clausius Clapeyron equation.

Hello,

The maximum pressure that any system can reach is limited by outgassing. If the system was under vacuum then the pressure would drop and the water in the system would begin to boil at whatever temperature the vapor pressure is equal to the atmospheric pressure in the system. Clay adsorbs water vapor and water really really well, so this process would compete with the adsorption of air molecules.

So, first you would need to figure out the kinetics of adsorption between the air and clay vs. the water vapor and clay to determine if the system would adsorb water before adsorbing air and if the source of oxygen for the oxidation reaction would come from the adsorbed water or from the adsorbed air.

If oxidation of air molecules occurred by iron, you'd also need to know the molar equivalents of iron to air molecules adsorbed by the Clay in order to calculate how much nitrogen you'd have in the system. Air is a uniform gas, and so all the properties of air are from the average of the individual gasses in the system. Since this is a mixture and not a covalent bond the use of oxygen in air for oxidation of iron really only disrupts the ratio of the gasses in the uniform mixture and thus disrupts the properties.

The ratio in air is approximately 78% Nitrogen & 21% Oxygen, the rest is argon etc.

So, I seriously doubt that there would have been enough Iron present in the soil to release enough nitrogen to seriously disrupt the balance when there would have been a significant amount of water and water vapor present.

This is the chemical equation you need for oxidation of iron.. ([Fe2]^3+) + ([O3]^2-) ----> Fe2O3

So now you'd need to decide how big the system was and calculate how much air was present. Then calculate how much oxygen was present in that air, and how much water was present and how much oxygen was present in that water. Noting that you aren't going to get any release of nitrogen from water vapor so the only source of nitrogen would be from the air...

Also, how exactly did the vacuum come about? If it was due to hydraulics then you'd need to calculate the effect of the loss of vacuum on the system once the water level vaporized or postulate how the system might have become sealed and resulting in a closed system once all of the water creating the vacuum had boiled off or once the levels of water changed to reduce the pressure on the rest of the system, etc.

Now, personally - I think it is more likely the Nitrogen came from the soil. As the clay adsorbed water it could have easily released Nitrogen into the water resting above, and if there was a vacuum created and the water boiled the Nitrogen gas was released from the water.

Now since Nitrogen is less dense than air - each time this happens - more and more nitrogen from the clay that has been released into the water and possibly from oxidation reactions will be released into the system and will collect at the top of the system, wherever that is..

So now we have to ask the question of, what are the kinetics for adsorption of Nitrogen into the soil/clay at the top of the system? Was this in fact clay/soil or concrete etc?

So, if you could find out how much nitrogen is typically found in the clay around the area this happened, and from that calculate how much nitrogen could have been released into the water in your system through adsorption of water by the clay, you may be able to figure this out.

Henry's law will only tell you about the solubility of the gasses absent external parameters - so it isn't going to help you that much in this real world model - since equilibrium will be shifted each time water is vaporized via boiling due to the proposed vacuum or each time an oxidation reaction occurs.
You can use the Clausius-Clapeyron equation as well, but I do not think this will help you that much either - if you are suggesting that the nitrogen was under extremely high-pressure - and you require an extremely accurate analysis of the event the assumptions made in the CC equation are going to add up on your quickly.. So, you'd need to utilize the Antoine equation instead.

However, since nearly all of these values are going to be speculative at best, I'm guessing you'd first have to figure out what the minimum high-pressure the nitrogen would have needed to be at in order to cause the explosion, if there was an explosion. Because you mentioned the high pressure, but then you also mentioned ventilation, so are the people who reported this sure what happened? Was there an explosion or did the workers die from suffocation? If it was merely the latter than you don't need to do any of the math at all.

Nitrogen is less dense than air and will collect at the top of whatever system it finds itself in.. You can simply calculate how much nitrogen could have been present due to vaporization of water and/or oxidation of iron to determine how much nitrogen could have been present and if that would have in fact been enough nitrogen to suffocate those men... You'd also have to consider how fast all the gasses from entire system escaped from the holes that were drilled and how long it would take a human of the sex, age, size, and weight of the workers to suffocate, if they were in an open system when they released the nitrogen gas into it, and also if they were in a closed system (or mostly closed system due to calculation of exchange rates etc, if the size of that system would have allowed enough space for the nitrogen, which is less dense than air, to quickly rise to the top of that new system and eventually escape at whatever the exchange rate for their system was.

I'm sure there are likely other parameters that you'd need to consider as well. I think if you can you should try to get more information about what exactly happened to the workers..
Best of luck
Cheers