was interesting but not actually very conclusive. The original poster's use of chemical equation notation gave me flashbacks to interactions with one of my subordinates who was a chemical engineer. His dad who was one of my best friends on the department and was also a chemical engineer, but his son was someone who could be very difficult to get along with and typically did not take orders well on scene.
Both methane and hydrogen sulfide are commonly found in well water so it is not necessary to come up with the formulas explaining how explosive gasses could be formed within the tank. They are both lighter than air so a slowly accumulating explosive mixture of air and gas would be found somewhere near the top of the tank.
Hydrogen sulfide has a wider flammability range from 4.5% to 46% vs methane which is 4.4% to 16.4% but it has a very strong odor and it also dissolves in water forming a weak acid and bad tasting drinking water. If the water company was having enough trouble with hydrogen sulfide that they were accumulating volumes in their tanks large enough to create an explosive hazard... they would know about it and have to be treating their water.
Methane is colorless and odorless and commonly found in well water but again it seems a little unlikely that the water company would not know about it and be treating their water if they had enough that it would be forming explosive mixtures near the tops of their tanks. But stranger things have happened.
I did find the posts mentioning hydrogen to be very interesting. Hydrogen has a broad range of flammability from 4% to 74%. It is formed when an acidy solution comes into contact with magnesium, aluminum, zinc and iron. And also when a basic solution from sodium hydroxide and other chemicals used to treat water comes into contact with aluminum or zinc. But if it were hydrogen the ignition would have to have been at the top of the tank and that is not what it sounds like happened here. The video linked to in the thread here makes no mention of what they were welding on but the Fresno Bee says the ignition happened when the four workers “went to start the tie-in”.
https://www.fresnobee.com/news/local/article252270353.html
That type of operation would take place near the bottom of the tank. I appreciated your link to the EPA paper about the reason older tanks become weaker at the bottom than the top through corrosion. I am at a loss to explain the mechanism of how this mishap occurred with the information provided.
It occurred while welding or cutting was taking place so that most likely was the triggering event, but how could that operation ignite flammable gasses at the top of the tank? It does not make much sense. There is not a flammable mixture the bottom of a tank full of gasoline.
It does not seem possible that atmospheric pressure could hurl all that steel 70 feet in the air. I could understand the tank structure going up a few feet from a catastrophic failure of the bottom of the tank but the video clearly shows that much more pressure was involved.
My best guess is that somehow air pressure was feeding back into the tank and something that was done to “tie in the tanks” either welding or cutting caused a weak spot that suddenly pricked the balloon at a weak area at the bottom of the tank. Electrical connections arcing as the structure broke loose might explain the light show that some people think that they see in the video.
Good insights. Thanks. What puzzles me is that atmospheric pressure water tanks are open to the atmosphere at top (right?). The openings are covered with a screen to keep out debris and animals. So one would think that would prevent an explosive regime from being reached, wouldn’t it? Especially because most of the resulting hydrocarbon gases are less dense than air.
It’s puzzling why a side seam didn’t fail first, but all the bolts or welds at the connection between the sidewalls and floor let go simultaneously. That is what suggested to me corrosion problems at the floor/sidewall junction.