Liquid Nitrogen

[Welder99]

Need some advise,im going to look at a weld job on a alum,Tank that has held liguid nitrogen.seems it had a leak around a ball valve in the main structure.The customer said the tank is in the range of 4000 gal.only a little product is left in the tank.theres a fill opening in the top that can be unbolted to vent before welding.i plan on making a patch to weld over the bad area,then retap for a new ball valve.Before i attempt the job im trying to get any info on the liguid nitrogen.The tank is outside in the open.The repair is on the side close to the bottom of the tank.i was told the the tank can be tilted some to keep what product thats still in the tank away from the weld area.(may only be around 50 gal or so left that cant be pumped.)any information would be helpful....

Welder99..

[Andy249]

Welder99

I'll assume by using the product in the tank is liquid nitrogen? Is it not possible to just let nitrogen in the tank boil off completely before attempting the job? I have a feeling that trying to run a weld on material that could have a temperature that is approaching 0 kelvin (-263 degrees Celcius I think) is asking for trouble. Aluminium is prone to cracking enough as it is without adding the complication of a such a huge temperature difference on such a thermally conductive material!! Also if I knew I was going to have 4000 gallons of liquid nitrogen above my weld I would leave nothing to chance in terms of weld preparation. Try and find out what grade of aluminium it is and use a filler wire that is compatible with it.

Have fun and good luck!

Andy249

[timw]

Before I try to answer your questions please note, you are entering a very dangerous area of welding skill. I can't stress enough how much you need to know EXACTLY WHAT YOU ARE DOING. I'm not there so don't take anything I say as fact,if you don't cover all the bases you can DIE!
I worked at FRUEHAUF Trailers in the low 70's. We repaired gasoline tankers among other product carriers. We had a strict safety procedure that included Checking and signature of supperviser every hour for flameable and other hazards. We had a major ventilation system that kept fresh air in the tank. Also a mechanic could only enter a tank with a safety man on top, outside of hole. If the safety man needed a drink of water or what ever, the man came out of the tank until he got back.
It's my understanding that Liquid Nitrogen boils (converts to gas) at room temp/atmosphere. Do not come into contact with the liquid it will freeze/burn through clothing. I have been getting treated for minor skin cancer for over 10 years, the doctor puts Liq Nit on them to freeze (frost bite burn) them off my skin.
Another concern, even if the tank is "empty" if you entered it without ventilation YOU COULD SUFFOCATE from lack of O2, the Nitrogen would displace the air in the tank.
I hope I have scared you enough by now to know your skill level and proceed or walk away, you may get only one chance! Sorry for the long post but I feel it is important.

[klsm54]

I agree with Andy249. I would get rid of ALL of the liquid nitrogen before attempting this repair. The temperature difference between the liquid effected area and the welded area is just to great, and the aluminum, being such a great thermal conductor, can be effected quite a way from both the weld area and the liquid nitrogen.

Good advice on matching the filler metal to the base material also. On this type of repair, you want to leave nothing to chance. Good prep, proper procedures and materials.

I would also make sure that the tank was not filled completely on the first fill after the repair. I would want just enough liquid nitrogen put back into the tank to thouroughly cool the repaired area for some time before continuing the fill. The last thing you want to happen is for the weld to crack with 4000 gallons of liquid nitrogen behind it.

It should be no problem getting rid of the residual product in the tank before making the repair. After you open the top of the tank the product should boil off pretty quickly.

And of course, dittoes to evrything timw said about the safety during this project.

[GTA/SPEC]

Can you describe the tanks application. Every cryogenic container has an inner container, insulated with a vacuum jacket. I assume that the repair is to the outer jacket? You should dump all of the liquid and purge the vacuum prior to repair. After the outer jacket is repaired you should purge the vacuumed area with helium and use a helium sniffer to check for leaks in the repair. You should have a professional cryogenic repair specialist do this kind of work.

[HAWK]

Welder99,

I MUST 2ND GTA/SPEC'S ADVICE. HE IS GIVING THE STRAIGHT AND NARROW ON BASIC CRYOGENIC TANK CONSTRUCTION. UNLESS YOU HAVE SPECIFIC KNOWLEDGE, EXPERIENCE, AND EQUIPMENT FOR THIS TYPE OF CRYOGENIC REPAIR WALK WHILE YOU ARE ABLE. I consider myself to be a well qualified and skilled in the process of GTAW. However, I do not specialize in cryogenics and have more than once walked from such jobs including liquid nitrogen, oxygen, and hydrogen tank repairs.

Here's a scenario to consider. You boil of the tank contents. Repair the outer jacket and collect your money. The tank accepts the new fill and all seems well. A week later something goes wrong and somebody gets killed because the tank leaked or ruptured. Now what? Maybe it was your fault. Maybe not. Guess who will be blamed.

[H80N]

welder99
GTA/SPEC and HAWK are right on the money... what you are dealing with is a "dewar" or in plainspeak a huge thermos bottle .. when repaired they must be recertified..
here is a link to one outfit that works on them with an overview of the recert...

http://www.ustankcryo.com/process.cfm

for my money.. the liability and danger far outweighs any profit... and have some grave suspicions about a customer that insists that the tank be left with product in it...
my guess is that any reputable cryo repair outfit would tell him that he is crazy and walk away from the job...
is your decision... but if it were me... I would decline ....
hope this helps
Heiti

BTW Liquid Nitrogen has a temperature of -321 Farenheit ...(same as the surface temp on the planet Pluto) if you were to splash some on your hand it would probably freeze instantly.. crack and fall off... where was that valve again?????

[ASKANDY]

I would go along with that as well. Too scary for me.

A-

[Welder99]

Sorry for the delay in responding.....But THANK YOU to all that posted a reply to my question.After reading all the replies and deeper investagation ive backed away from the tank repair.I feel confindent in making the repair as far as the welding goes,but was uncertain with the contents that were inside.i have welded in oil refinerys before,but never have been in contact with the nitrogen.I found out it was not just liguid nitrogen with what they call 28.Its used in the ag area.The MSDS stated that if it dries it becomes unstable.And uncertain to explosion if heated or open flame.Thanks again ANDY for a great site with a wealth of information......Welder99.

[H80N]

Welder99
if I understand correctly it looks like you may have URAN (Urea + Ammonium Nitrate) in that tank..... and if so... YEP EXPLOSIVE as the devil....under the right circumstances... ammonium nitrate and diesel fuel makes a very powerful secondary explosive called ANFO.... the same stuff that was used in Oklahoma city... pretty stable as explosives go ... usually requires a fast primary explosive to kick it off.. but can be triggered otherwise....
you are too old to die this young...
take care
Heiti

ps... reading the MSDS was a very smart thing to do...