I've never heard of purging with exhaust, but if you think of it as Carbon Monoxide, it does'nt sound doable, but who knows? I know some fools who weld on fuel tanks, that are full of fuel, which keeps them from exploding, as long as they don't burn through or have a leak. I prefer to fill the tank with water.
Pat,
you're talking of trucks' (or cars') fuel tanks. I don't know whether there's some recommended practice issued by SAE (the Society of Automotive Engineers). You can check and, who knows, find something interesting for you. Their site is www.sae.org
What I know is a recommended practice issued by API (the American Petroleum Institute) for welding new petroleum products tanks during construction AND OLD TANKS FOR MAINTENANCE. Granted, the standard applies to the storage tanks you see in oil refineries and also service stations whereas you're talking about the tanks found in trucks, but I'm sure you can get more than a couple of useful ideas from the standard, especially when safety is involved. The standard number is API RP 582. The API site is www.api.org
Regarding what you call the "backyard procedure" for purging tanks, of course it's not based on any written, prestigious, internationally known standard. I'd say, however, that it's based on the "popular science" that is often valid to help us in a hurry or in a difficult situation, when we havn't got the required tools to make a first class job.
Tim is right. The exahust from an car motor doesn't contain any oxigen (or contains very little), just combustion gases, mainly CO2, CO and water vapor, so it actually purges the tank from air, and consequently, oxigen. Of course, the purging should be done outdoors, in an open place, so the gases will dissipate in the atmosphere.
Giovanni S. Crisi
Sao Paulo - Brazil
From the backyard to the graveyard!! Pat, the procedure that you describe should only be attempted by those intent upon committing suicide. What you are doing is introducing hot gasses that may contain sparks or flame into a potentially explosive atmosphere.
For more information on safe practices you should see ANSI/AWS F4.1-1999 Recommended Safe Practices for the Preparation for Welding and Cutting of Containers That Have Held Hazardous Substances. It's available at this web site. Got to the AWS home page and click on e-store then search for F4.1.
Dave
Sometimes in remote parts of the bush, absolute safety gives way to necessity. I have repaired a 500 gal. stove oil tank with oxy-acetylene torch using the exhaust purge method. If no alternative exists, it can be done, if you are willing to accept the risk of not having a procedure to follow from a book. Good luck.
As is the case with most of the other respondents, I would not be close to the tank when you are welding, but just a couple of thoughts:
1)The exhaust gases from petrol engines contain large amounts of CO. This is itself a flammable gas, so I would be weary of using this gas.
2)The exhaust gas from Diesel engines contains mostly CO2, which would be a much better purge gas.
3)You mention "venting" the tank. Make sure you vent it after you finished welding and not before, or else you will just put the oxygen back into it.
4)Theoretically, if you purge the tank long enough, any fuel residues should vaporize and leave a "non-explosive" atmosphere, even if you have oxygen present. But what if the vehicle you are using to purge the tank is pushing a lot of unburned oils through, which then again condense on the inside of the tank?
Before performing this, please let me know so I can give you my details for inclusion in your will.
Niekie
I have heard of, and know some people who have purged a gas tank with engine exhaust to remove oxygen from the tank. From what I have seen, and from Niekie's 4th comment, I wonder if what was really being accomplished was to remove the fuel and vapors. Or at least reduced the levels to avoid explosion. At any rate, those people are still alive to tell about it and also say that their nerves were on edge the whole time.
Patches I have made were made with epoxy compounds or with solder using an electric soldering gun. I removed the tank from the vehicle, drained it, and filled it with water before working on it. The non-welded patches withstood pressure testing. ( I have since learned that air-pressures tests are not supposed to make a somewhat rectangular tank look like round ball. I didn't realize that I was exposing myself to a different explosion hazard.)
Anyway, that was in my teenage years when my earnings were out weighed by my desire to drive an old Impala. The last time I had a gas tank leak, I discovered I could get a new tank relatively cheaply. And my point in saying that is that if a tank is leaking from corrosion, it may be a good idea to look at replacement costs. Especially in view of soil and water decontamination costs. I am assuming a small fuel tank here. If the object of this discussion is a large tank, then I believe
G S Crisi mentioned API standards that can give guidance.
CHGuilford
I have a friend that had a bad experience while trying to weld some new legs on an old air compressor holding tank. He never opened up the tank, in any way. He noticed the tank was quite hot, 3 feet from where the welding was being done. This was just before the hemispherical end blew off the tank, destroying his bathroom and part of his roof! He was doing the work in his cellar.
I think this is a good example of "close enough" and "oh, that'll be alright".
Thank God no one was killed.
Vent. Vent. Vent!
Dale Simonds
Dale,
in my opinion, there are two possible reasons why your friend's compressed air tank busted:
First. As the tank was hermetically closed, as you say, when welding was under way, the air inside the tank warmed up and, not being able to expand, put pressure inside. This pressure eventually made the tank end to blow off.
Second. The inside of the tank was contaminated with the lube oil used to lubricate the compressor. When welding was under way, the lube oil vapors caught on fire and the tank exploded. This seems to be the most likely reason: if the tank was never opened it was indeed dirty with oil.
Giovanni S. Crisi
Sao Paulo - Brazil
Agreed, the oil was his problem, and not doing some sort of venting. A potentially deadly situation.
Dale Simonds
Hi, I've tackled quite a number of diesel, gas, & oilfield tank repairs. My unofficial advice is to have the tank steamed out.We also test them with a cumbustible gas detector before welding or cutting. Another option is to fill the tank with an inert gas & purge continuosly.As for filling with water, you must have the tank completely full to be safe & it is harder to weld with water running out. The welding instructors I had pointed out that using exhaust fumes can be dangerous due to the fact that the engine may not be burning the fuel cleanly. The exhaust could be a combustible mixture. The old boys used to blow large volumes of air through the tanks with a shop vac or air compessor. In my opinion this greatly reduces the chances of explosion but does not entirely garrantee safety.Remember it only takes once. We get our tanks steamed at a trucking company nearby.They use the steamer to wash trucks & trailers. Hope this helps.
Having worked for a shop (back in my years) which used to specialize in repairing fuel tanks of all kinds, my advice would be replace the exhaust with CO2. An inlet where purge gas is introduced and an outlet at a point furthest away from the inlet always worked, every time. A period to allow dispacement of the fumes is also required. Also, watch that flow meter to ensure it does not freeze up. Rags soaked in warm water were constantly being placed on the flow meter to avoid freezing.
I was extremely reluctant to perform this work until someone showed me how it was done. Never had an accident in the 3 years or so performing this type of repair. I have seen the results of this activity when performed incorrectly, it's not pretty. This included a motorcycle gas tank that was filled with water (but not all the way to the top where the welding was being performed-oops!) And don't let anyone tell you diesel won't burn, it burns at a higher temperature than gas is all, it burns!
The insurance for this type of work eventually put an end to this practice. I would not recommend repairing of any container without specific knowledge of what was being contained and how to displace or completely remove that product, how much is your life worth to you?
I know a welder that uses exhaust fumes to purge the tank but, he has more balls that I do, one backfire and your project turned into a trip to the hospital. I weld gas tanks on occassion, I play it safe, pull the tank, flush the tank with water position it with the area your going to weld facing up, fill the tank with water to where there is very little "air space"
weld it. Check the tank by pressurizing it with air (the water can still be in there) use soapy water spray bottle to check for leaks. If it checks out, drain water, & dry with compressed air. "That's my junk yard dog approch." You can also purge with shielding gas but I personally am a little leary of this cause it's hard to tell when the tank is purged, and if
it's staying that way.
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