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March 8, 2006
How Not to Do It: Liquid Nitrogen Tanks
A colleague of mine forwarded a copy of an accident report from Texas A&M. It seems that in mid-January they had a bit of a blowout there, thanks to a big liquid nitrogen tank. Now, liquid nitrogen cylinders are normally fairly benign, as long as you don't freeze your external organs off with the stuff or leave the liquid sitting around where it can condense oxygen out of the air. But idiocy will find a way - note the regular cylinder on the right and the new, improved model next to it.
These guys are usually equipped with pressure relief fittings, since nitrogen does tend to want to be a gas, and gases do tend to want to expand quite a bit. This tank, though, which seems to have been kicking around since 1980, had been retrofitted by a real buckaroo. Both the pressure relief and rupture disks had failed for some reason in the past, so they'd been removed and sealed off with metal plugs. You may commence shivering now.
Why it didn't blow long ago is a real stumper, but presumably people were taking nitrogen out of it quickly enough to keep things together. Not this time, though: at around 3 AM, things came to a head as the internal tank (these things are double-walled) expanded until it pressed against the outer one. That kept it from expanding anywhere else except on the ends, and as fate would have it, the bottom blew out first. The engineer's best guess is that this took place at around a 1200 psi load. It must have been quite a sight, although it's a damn good thing that no one was around to see it. I'll let the engineer's report take it from here:
The cylinder had been standing at one end of a ~20' x 40' laboratory on the second floor of the chemistry building. It was on a tile covered 4-6" thick concrete floor, directly over a reinforced concrete beam. The explosion blew all of the tile off of the floor for a 5' radius around the tank turning the tile into quarter sized pieces of shrapnel that embedded themselves in the walls and doors of the lab. The blast cracked the floor but due to the presence of the supporting beam, which shattered, the floor held. Since the floor held the force of the explosion was directed upward and propelled the cylinder, sans bottom, through the concrete ceiling of the lab into the mechanical room above. It struck two 3 inch water mains and drove them and the electrical wiring above them into the concrete roof of the building, cracking it. The cylinder came to rest on the third floor leaving a neat 20" diameter hole in its wake. The entrance door and wall of the lab were blown out into the hallway, all of the remaining walls of the lab were blown 4-8" off of their foundations. All of the windows, save one that was open, were blown out into the courtyard.
No one seems to have heard the celebrations, but someone noticed that the building's water pressure had gone a little wimpy and went to investigate, which I'll bet was a real eye-opener. I get the impression that they're still trying to track down the Mr. Fix-It who inadvertently rigged the tank for takeoff. The company engineer who came in to investigate noted that he's seen these kinds of "repair" jobs before, generally after they've powered through something.
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