made a big mistake

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freind needed his gas tank for his bike welded up,it was rusted through in a few spots.every time in the past that ive welded on gas tanks ive filled them with water and burned away no problem,this weekend same thing filled the tank with water leaned it at an angle so i was sure there was water on the back side and hit a small test arc..no problem ,second arc was a whole diffrent deal,woosh it knocked me into the wall the tank didnt actually explode but it is no longer useable,where did i go wrong??
Reply:Filling the tank with water was your first mistake...You need to remove the oxygen from in the tank and still be able to weld. You need to use an inert gas (like that big tank of argon!) The  tank must have a positive pressure at all times to prevent oxygen from migrating in and it needs a vent.  Now if no oxygen is in the tank it can not go BOOM!The tank can even have fuel in it! Also note that the gas exiting from the tank may burn, and if that is allowed to build up in your shop that will go boom.  The vent burning is not a problem unless the purge stops. Be Safe!Me!
Reply:ive heard running exast fumes from a car will keep it from blowing but im not sure ill ever try it actually i think im out of the gas tank repair bussiness,specially free-bees
Reply:Originally Posted by paintmanive heard running exast fumes from a car will keep it from blowing but im not sure ill ever try it actually i think im out of the gas tank repair bussiness,specially free-bees
Reply:true that!!!no more tanks
Reply:Originally Posted by paintmanive heard running exast fumes from a car will keep it from blowing but im not sure ill ever try it actually i think im out of the gas tank repair bussiness,specially free-bees
Reply:i somewhat agree with Me!... expanding gas (water vapor in this case) due to excessive heat build-up creates a higher pressure differential within the tank. as soon as you strike the arc and it turns the solid metal to a molten mass, it is a weak enough spot to create a hole and thus release the pressure. the main problem with this is that there would have to be a HUGE amount of pressure to create any real damage, especially in thin sheet metal. from what i have heard and understand, something like this situation would MAYBE (and that is a BIG maybe) would make a small "HISS" sound from the SMALL pressure differential popping thru the molten metal mass. this type of issue is mainly a problem when trying to completely seal an enclosure and the metal has a problem with that final, small pinhole.from the sounds of what has happened here, you had a small amount of gas fumes left inside the tank... how did you clean the tank before welding? when my uncle does his tanks, he always washes them with soap and water, twice, then lets them dry, upside-down, for a few hours in a warm shop before he ever welds on them. not only does this take out 99% of the contaminates inside the tank, but due to the fact that gas fumes are heavier than air, they will leave the tank via the filler-hole. it wouldn't take a very large amount of fumes to make a significant "boom" either... an ideal air:fuel ratio is about 14.7:1, so you only need a small amount of gas (maybe a few drops worth of fumes) inside a bike's gas tank to create a "perfect-burn" scenario.just my opinion.Later,Andy
Reply:My son's old dirt bike has a metal tank and it had about 4 pin holes along the bottom side,  I was too nervous to try welding it so I tried a product called "minute mend" made by CRC.  I was able to form it so that it blended into the shape of the tank, I never bothered painting it, but it's been holding up now for 2 years.  Might be an option for future tanks.
Reply:Originally Posted by aczelleri somewhat agree with Me!... expanding gas (water vapor in this case) due to excessive heat build-up creates a higher pressure differential within the tank. as soon as you strike the arc and it turns the solid metal to a molten mass, it is a weak enough spot to create a hole and thus release the pressure. the main problem with this is that there would have to be a HUGE amount of pressure to create any real damage, especially in thin sheet metal. from what i have heard and understand, something like this situation would MAYBE (and that is a BIG maybe) would make a small "HISS" sound from the SMALL pressure differential popping thru the molten metal mass. this type of issue is mainly a problem when trying to completely seal an enclosure and the metal has a problem with that final, small pinhole.from the sounds of what has happened here, you had a small amount of gas fumes left inside the tank... how did you clean the tank before welding? when my uncle does his tanks, he always washes them with soap and water, twice, then lets them dry, upside-down, for a few hours in a warm shop before he ever welds on them. not only does this take out 99% of the contaminates inside the tank, but due to the fact that gas fumes are heavier than air, they will leave the tank via the filler-hole. it wouldn't take a very large amount of fumes to make a significant "boom" either... an ideal air:fuel ratio is about 14.7:1, so you only need a small amount of gas (maybe a few drops worth of fumes) inside a bike's gas tank to create a "perfect-burn" scenario.just my opinion.Later,Andy
Reply:Dave is absolutely right.  I have seen the aftermath of folks that THOUGHT they knew what they were doing...gasoline has a lot of energy in a small package and it's amazing what a little bit can do when it's in a confined space.Smithboy...if it ain't broke, you ain't tryin'.
Reply:Not sure if I am doing it right but it seems to work for me so far. When ever I had a gas tank repair, all I do is remove the tank and make a exit hole. Now I stick a blower hose in the tank and force vent the tank for awhile. I have even used a hair dryer for this. I let the tank air for a day or so and heat it up again with the blower or hair dryer. After a couple days you can weld, throw matches in it or even put a torch on it. Even the gas smell is completely gone before I start welding.Is this one of the 249 ways and I am lucky or does this seam right Miller DVI2Lincoln Precision Tig 225Thermodynamics Cutmaster 38Everything else needed.
Reply:I use Jamlit's method pluss. I wash with dishwashing soap rins and dry with a cheap matress inflator and then I purge with argon or nitrogen due to cost on large fuel tanks. After I blew up my own tank years ago I learned the right way to do it now I feel safe doing it. I do nut use a hair dryer I dont like the glowing element. I have also used the blower hose off of a shop vac haped to  5/8"ish hose stuck in the tank. DO NOT CUT CORNERS OR GET IN A HURY! From start to finish when I do a tank it is about a week. After it is cleaned and dryed I let it sit in the shop open for 3 or 4 days. Like said before you should smell no gas and the purge is just a safty. It took 3 weeks for me to get over the bruising on the inside of my thighs after I welded on my tank full of water. (first and last with water) Steel Thunder Welding LLC. St. Thomas VI USAMM350PBOBCAT 3MAXSTAR 150 STH375 EXTREME PLASMA6.5 HP COMPRESORDEWALT 18, 24, AND 36 VOLTO/A TORCH SETAND SO MUCH MORE I DONT REMEMBERALL IN MY 2005 2500HD EXTENDED CAB L/B
Reply:Originally Posted by offrdfun73I use Jamlit's method pluss. I wash with dishwashing soap rins and dry with a cheap matress inflator and then I purge with argon or nitrogen due to cost on large fuel tanks. After I blew up my own tank years ago I learned the right way to do it now I feel safe doing it. I do nut use a hair dryer I dont like the glowing element. I have also used the blower hose off of a shop vac haped to  5/8"ish hose stuck in the tank. DO NOT CUT CORNERS OR GET IN A HURY! From start to finish when I do a tank it is about a week. After it is cleaned and dryed I let it sit in the shop open for 3 or 4 days. Like said before you should smell no gas and the purge is just a safty. It took 3 weeks for me to get over the bruising on the inside of my thighs after I welded on my tank full of water. (first and last with water)
Reply:I have done a few in the past.  One day I said "it aint worth it".  I haven't done one since.  I am a mechanic, I just tell the customer how much a new one is.I used to steam clean them.  Nothing less than that.  Hot steam for 20 minutes does clean em out.  I then purge with Co2.Some one brought a built tank in to a shop for modifications because it didn't fit.  They told him it had never had gas in it.  He struck an arc, it blew up and killed him.  I don't weld no stinkin gas tanks!In my business it seems if I try to save some one money or "help them out"  It bites me in the azz.  That kind of bite I don't need.David  Real world weldin.  When I grow up I want to be a tig weldor.
Reply:Originally Posted by paintmantrue that!!!no more tanks
Reply:well you all have convinced me that ill never EVER try to weld another
Reply:Your first mistake was to try it in the first place..Don't get me started..My cousin and alot of my friends were hurt BAD doing the "same thing" with a tank that was in a field for a year...Burned a whole factory down..No joke..You people that wanna play with used tanks..Have a good health insurance policy.....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:I patched up a aluminum diesel tank last week, removed excess fuel and purged with argon.  From what I know and what I was told, diesel is safer because of the lack of combustible vapors, but once you start welding the residue evaporates and becomes a flammable vapor.  Then, if someone moves the tank around while the weld is still really hot and the purge is gone a drop of liquid fuel can come in contact with the weld and ignite.  Kaboom!Well, im still alive but that doesnt mean I wasnt on pin and needles while welding it.Have we all gone mad?
Reply:Originally Posted by jamlitNot sure if I am doing it right but it seems to work for me so far. When ever I had a gas tank repair, all I do is remove the tank and make a exit hole. Now I stick a blower hose in the tank and force vent the tank for awhile. I have even used a hair dryer for this. I let the tank air for a day or so and heat it up again with the blower or hair dryer. After a couple days you can weld, throw matches in it or even put a torch on it. Even the gas smell is completely gone before I start welding.Is this one of the 249 ways and I am lucky or does this seam right