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jdunmyer
06-22-2015, 06:45 PM
I have an old 80-gallon air tank, I assume ASME, as it has a (rusty, unreadable) metal tag that I am turning into a lawn roller, planning on filling with water. It has a bunch of tapped holes, including a 2" one in each end, a 2" and 3 1.5" ports in the sides. To avoid protruding plugs, I cut out some blanks with a hole saw and welded them into all these ports.

All was going as expected, with a couple or 3 slight leaks from my amateur welding, when my buddy stepped in to clean up the job. (He used to be a nuclear certified welder and is still pretty good.) We had been pressurizing the tank to find the leaks, not high pressure, probably 10 PSI or so. Anyway, after one of the runs, he did some welding, and I opened the fill valve that I had forgotten to open before he began. The air exiting the fitting was hot and smokey, and the tank was literally too hot to keep your hand on.

The tank was given to me and had been laying around for at least several years. Rolling it around resulted in the sounds of a pile of rust or other debris, not a lot, but "some".

What was burning, and did we escape a dangerous situation? IE: could it have exploded?

ironmonger
06-22-2015, 07:17 PM
Old tank = years of oil from the compressor. Donít think it was likely to explode, but vaporized oil is still a hydrocarbon, it could have... it was sure burning.

So don't do that again...

paul

Willy
06-22-2015, 07:18 PM
It's all speculation and conjecture at this point, but I'd say you probably dodged a bullet. No proof of course but years of use as an air tank I'd say it's very likely that it picked up some oil vapor. Cold water won't get rid of that of course, as I'm sure you are aware. Without knowing the exact composition of the gas and the air/fuel ratio inside the tank at that instant we can only guess.

I'm thinking though by the concerned tone of your post that you won't be doing that again. Good lessons usually don't come easy but I doubt you'll forget this one for a while. Glad all went well.

Mark Rand
06-22-2015, 07:42 PM
Probably set some residual oil on fire. I did a calculation about 30 years ago concerning a duct under my kitchen floor that contains both water, gas and electrical supplies. my conclusion was that, starting from atmospheric pressure, it could reach about 600psi from a worst case gas explosion. An air compressor receiver with some lube oil on the inside and a welder on the outside is probably much less than that because of the slower rate of combustion leading to less temperature rise before all the oxygen is used up, but it doesn't take many psi to blow a molten weld out into your mask...


PS. There was some amusement at work a couple of decades ago, when some old 6" steam pipes were cut out of the boiler house. Turns out that they were the fuel oil lines to the boilers:)

Jon Heron
06-22-2015, 08:20 PM
I'd say you dodged the bullet!
http://www.cp24.com/18-year-old-student-dies-following-school-explosion-1.649126
It doesn't take much residue to vaporize into a gas and then boom...
Also it is a very bad idea to use compressed air to check for leaks in tanks, there is an incredible amount of energy stored and even at 10PSI if a patch or something blew off the tank it could kill or maim you.
Much safer to fill the tank with water then pressurize that.
Play safe!
Cheers,
Jon

EddyCurr
06-22-2015, 08:36 PM
It is important for folks like us to be familiar with
the following:

From Engineer's Toolbox: Gases - Explosive and Flammability Concentration Limits (http://www.engineeringtoolbox.com/explosive-concentration-limits-d_423.html)


Flammable Range (Explosive Range)

The Flammable Range (Explosive Range) is the range of a concentration
of a gas or vapor that will burn (or explode) if an ignition source is introduced.

Below the explosive or flammable range the mixture is too lean to burn and
above the upper explosive or flammable limit the mixture is too rich to burn.
The limits are commonly called the "Lower Explosive or Flammable Limit"
(LEL/LFL) and the "Upper Explosive or Flammable Limit" (UEL/UFL).

Take some time to examine the gases discussed and
notice how some have a very narrow band between
their lower/upper % explosive range concentrations,
while others have a very wide band (like acetylene:
2.5% - 81%).

Purging a tank with an inert gas can improve safety.
However, carbon monoxide from auto exhaust
is NOT a suitable purging agent - it has a UEL/UFL
of 12% to 75%.

.

J Tiers
06-22-2015, 10:40 PM
The other half of that is oxygen.... if you fill the tank with nitrogen, or CO2 before you work on it, you are "safer". Far less able to get an explosive mixture.

But, it's a lot better to get the tank purged of all fuel first, and THEN add inert gas.

Or just don't use old fuel tanks....

Boostinjdm
06-22-2015, 11:35 PM
Do yourself a favor. Skip the water. Cut a 12" x 12" window in the side and fill with pea gravel. Then weld the window back in. I'm pretty picky about my stuff, but still forgot to drain the water 2 years in a row. Switched to pea gravel and never looked back. I think a 60 gallon tank is roughly 1300 lbs full of pea gravel. You may want to consider that depending on what you are towing it with.

A.K. Boomer
06-22-2015, 11:45 PM
This thread gives me the Willies *(no offense Willy - different kind)

where there's smoke there's potential for fire - vapors are what get ya,
I hope when and if the time comes I remember all this stuff from 45 gal. drums killing people to anything enclosed being potential trouble... I say this because it's an easy one to forget and skip...

ikdor
06-23-2015, 02:28 AM
Purging a tank with an inert gas can improve safety.
However, carbon monoxide from auto exhaust is NOT a suitable purging agent - it has a UEL/UFL of 12% to 75%.

I don't think modern cars make any appreciable amount of CO nowadays. One would be hard pressed to make an explosion with something like 0.5% CO and the rest of the mix CO2 and H20.
Don't try it though, some vehicles have an exhaust ventilator to fire up the catalyst extra quick after startup. If the cat has been removed you're blowing combustible mixture out of the exhaust....

Igor

_Paul_
06-23-2015, 06:50 AM
How cold are your winters? if full of water and it freezes will it split?

michigan doug
06-23-2015, 08:42 AM
Yup, sand or gravel. Denser, won't leak, can't freeze...

jdunmyer
06-23-2015, 08:50 AM
J Tiers,
This is an AIR tank, not fuel tank; I'm well aware of the risks with the latter.

Some of the problem could have been due to my forgetting to open the fill/drain valve after a quick leak check. More air, under pressure could have made the thing more flammable. After all, we had done a bunch of welding on the tank prior to its catching fire. Although the pressure wasn't excessive when I opened the valve, the gases were definitely smokey and hot.

As far as purging, I've worked on fuel tanks and used the gas from my MIG outfit, Argon/CO2 mix. Didn't even think of it for this job. Of course, we didn't use the MIG because the gas regulator was leaking since last use, probably a year or 2 ago.

Thanks for the commiseration, it's been a lesson to me, and hopefully others.

A.K. Boomer
06-23-2015, 09:27 AM
A quick not on this,,, I do not know about steel - but with aluminum tanks or enclosures that were once used for any kind of flammable I believe no matter how much detergent you run through it or whatever the danger can still be present - aluminum is porous and as soon as you apply heat the oils will come creeping out of it...

I can attest to this fact after trying to tig up a motorcycle engine case - brand new cleaning fluid in the parts tank --- get the thing cleaned and bone dry --- start to pre-heat and then all the sudden it starts smoking on me with a little halo of oil around the far parameters of the weld...

vpt
06-23-2015, 10:06 AM
A quick not on this,,, I do not know about steel - but with aluminum tanks or enclosures that were once used for any kind of flammable I believe no matter how much detergent you run through it or whatever the danger can still be present - aluminum is porous and as soon as you apply heat the oils will come creeping out of it...

I can attest to this fact after trying to tig up a motorcycle engine case - brand new cleaning fluid in the parts tank --- get the thing cleaned and bone dry --- start to pre-heat and then all the sudden it starts smoking on me with a little halo of oil around the far parameters of the weld...


Was this cast aluminum? I find cast holds more "stuff" than anything else. What I do is go over the area to be welded with the tig torch and no filler sometimes a few times brushing or grinding out the contaminants that come to the surface between each pass. Helps get a clean weld down eventually.


As for welding the air tank, pretty scary stuff. Pretty much any flammable fluid when atomized can be explosive or "flash burn". Put this in a sealed tank and it is a pipe bomb waiting to go off.

A.K. Boomer
06-23-2015, 10:25 AM
Andy you are correct - always seems to have been cast - but that's not to say I would trust the other, although I guess if it's got a polish too it its pretty resistant to being porous ? still would not want to find out the hard way...

J Tiers
06-23-2015, 10:34 AM
J Tiers,
This is an AIR tank, that now has fuel in it.



There, I fixed it for you

There is now FUEL IN IT. Otherwise it would have a hard time burning.

So really, it's NOT an air tank, it's now a fuel tank, and should be treated as one.

What you have is really one of those "I didn't know it was loaded" accidents, fortunately not a bad one.

jdunmyer
06-23-2015, 11:18 AM
JT,
Yes, I now know that this was actually a fuel tank and was dangerous.

Consider this: There is a 2" NPT port off-center in one end, that's to be my fill/drain port. It was open until the end, when it came time to find the leaks, and there was NO fire or smoke from within. It was only when the tank was left slightly pressurized that the fire occurred. I suspect that the pressurization had something to do with the fire, as much more heat was applied during welding of the patches than by touching up the pinholes in my welding, and there was no fire then.

Before reading this story, I'd bet that at least half the members here would have blithely welded on this tank, same as me. Remember, it has been laying around, outside, for many years (at least 10, maybe more) and is quite rusty, inside and out. Most of us wouldn't weld on an actual FUEL tank, even if it was old & rusty, but an AIR tank?

I now know better, and so do a bunch of others. Thanks for your commiseration.

J Tiers
06-23-2015, 01:10 PM
Pressure can change the air vs fuel ratio.

I might have welded, but not with it closed up. Not for that, but just due to heating and pressure blowing out the weld.

So long as you don't do it again, you don't get "honorary bubba of the day". Agree not expected with an air tank.

Any chance what you smelled etc was just from the welding? It's possible that there was oil in, but not much, and the heat of welding boiled it off of the heated area, charring and burning some just from the local heat. Some of the tank would be pretty hot from welding, and the rest might get that way after a bit, by conduction, or just due to hot air inside. Especially if there were several places to weld.

(edited for clarity and to add thought)

Royldean
06-23-2015, 02:37 PM
Pressure can change the air vs fuel ratio.

In a closed system?

jdunmyer
06-23-2015, 02:58 PM
JT said:


I might have welded, but not with it closed up. Not for that, but just due to heating and pressure blowing out the weld.


Welding the plugs/blanks was done with the 2" fill/drain port open. We installed a plug in order to test it, and put a valve and air fitting in a 1/2" port. We then ran a bit of air in, closed the ball valve, and unplugged the air line. Listened/felt for leaks, bled the air down, and touched up the pinholes. The last time, I forgot to open the ball valve, leaving a bit of pressure in the tank. When I opened it, hot, smokey air escaped, and the tank was hot. We did NOT intentionally weld on a completely closed tank.

You said that the pressure can change the air/fuel ratio..it seems to me that that is correct, it would make the mixture more oxygen-rich, would it not? That would make it easier to light off.

J Tiers
06-23-2015, 03:15 PM
You said that the pressure can change the air/fuel ratio..it seems to me that that is correct, it would make the mixture more oxygen-rich, would it not? That would make it easier to light off.

It puts more oxygen in, and can change the ratio by affecting the vapor pressure equilibrium, as I understand it.

Royldean
06-23-2015, 03:25 PM
Ok, let's be specific. Adding more AIR can increase the air/fuel ratio.

Adding PRESSURE doesn't affect the air/fuel ratio at all. For example, if you seal the tank and weld on it, and the air inside gets hotter, the pressure will go up. The air/fuel ratio does not change.
Likewise, if you were to seal the tank and then squeeze it in a vise, the pressure inside will increase, but the air/fuel ratio will not change.

Now, taking a tank with a fixed amount of fuel in it, and adding air (in this case, pumping it up with compressed air), WILL increase the air fuel ratio. But it's not the pressure that increases the ratio, it's the increase in air... which raises both the pressure and the A/F ratio....

PStechPaul
06-23-2015, 04:17 PM
If the fuel is oil which is held in the porous surfaces of the tank, the air/fuel mixture might change with temperature and pressure. High temperature and low pressure might cause the oil to vaporize and increase the percentage of fuel. The opposite might drive some of it back, but I doubt it.

J Tiers
06-23-2015, 04:54 PM
Ok, let's be specific. Adding more AIR can increase the air/fuel ratio.

Adding PRESSURE doesn't affect the air/fuel ratio at all. ...

It may affect it by forcing some vapor to drop out as liquid..... if the dew point is reached. That would decrease the mass fuel/air ratio.

Water drops out in a tank, despite the fact that all the water comes in as vapor in the ambient air, at ambient temp. Heating in compression is followed by cooling again, but it's a closed system, material mass is conserved.

Only if cooling with air use causes water dropout, which cannot be taken up by the next load, can the water accumulate. There is no source of water except the air coming in. That supposes incoming air to be saturated, OR that conditions inside the tank change the water capacity of the air, or both.

In this case, the heat of welding may have caused added evaporation of whatever oil was in there, INcreasing the fuel content of the air.

Black_Moons
06-23-2015, 08:04 PM
Purging a tank with an inert gas can improve safety.
However, carbon monoxide from auto exhaust
is NOT a suitable purging agent - it has a UEL/UFL
of 12% to 75%.

.

Find me a car that puts out 12% CO and i'll show you a car that has had the catalytic converter cut out and someone tuned to be so rich it must be blowing out black smoke, assuming it could even run... There is only what, 14% oxygen in the atmosphere so good luck putting out 12% CO after combustion occurs at any level. If you do manage it however I am sure that some chemical manufactures would love to know your secret of highly efficient CO production!


http://aircarecolorado.com/about-the-test/1981-and-older/i-m-240-exceptions/ Says since 1971 cars have had to have lower then 3PPM to pass air care.. or 0.0003%. The hydrocarbon limit was a much higher 1000 but even that is still only 0.1%.

Not that I am saying go use a cars exhaust as a purge gas (Or if you absolutely must, wait till the car warms up and the catalytic converter is operating), just that CO levels are NOT the problem. If anything the hydrocarbon levels (Completely unburnt gasoline) is a much bigger problem, but even those have a narrow combustion range. a cylinder that is not firing and cold/damaged catalytic converter however could output a lot of hydrocarbons.

That said, if your a welder, you have a very large bottle of inert 'purging' gas sitting by. Or if you just go to the paintball shop and buy a $20 CO2 tank (CO2 is a LOT cheaper then welding gas!) that should be able to displace enough oxygen to bring the combustion risk down to next to 0.

Also fun fact about pressure: If you close a cylinder, then heat it, you *increase* the *partial pressure* of oxygen and can make things more explosive that way. In reactivity its not always the % that matters but the partial pressure can matter too. Its just given in % because people expect you to keep your fuel/air mixtures at sea level pressure if you are sane. Higher pressure makes things more reactive as all the little atoms are smashing into each other more often, even if that pressure was generated from heating, the pressure is a side effect of colliding with each other more often.

PStechPaul
06-23-2015, 08:36 PM
I have never heard of using CO to purge a tank, but I did find some references to it regarding propane tanks.

http://www.millerwelds.com/resources/communities/mboard/showthread.php?7059-Cutting-Open-Old-Propane-Tanks/page2
http://www.texasbbqforum.com/viewtopic.php?f=4&t=12962
http://www.polytechforum.com/metalworking/purge-a-oil-tank-for-safty-144101-.htm

Usually it seems that an "inert gas" is used, but I would not consider CO to be inert.

http://www.meridianeng.com/inertgas.html
https://en.wikipedia.org/wiki/Inert_gas

It may be that the CO is supposed to "scrub" oxygen from the tank, and if it is to be used for propane, then the fact that it is flammable is not an issue. The more reasonable assumption is that car exhaust has very low O2 and very high CO2 and H2O, both of which can be considered inert and safe. I think it is an old "mechanics wives" tale that it is the CO in exhaust that is used for purging, with the incorrect assumption that even an old ICE running poorly blasts a huge amount of CO out the tailpipe. It's just that it doesn't take much to kill you.

Royldean
06-24-2015, 07:49 AM
In this case, the heat of welding may have caused added evaporation of whatever oil was in there, INcreasing the fuel content of the air.

Oh good lord. Some people just have to be right.... Have it your way.

J Tiers
06-24-2015, 11:23 AM
Oh good lord. Some people just have to be right.... Have it your way.

Not at all.... Just trying to figure out what actually happened.... If there was burning, and there wasn't before with it open, then something must have changed to allow it when it was inadvertently closed up. So what WAS it that changed?

Seems either the air/fuel ratio changed to have MORE, or to have LESS fuel, whatever got it to where it WOULD burn when it wasn't in a situation where it could before.

OR, as mentioned, and which you apparently decided was perverted, it isn't about that at all, and all that happened was that the heat of welding boiled off some oil and it just charred where it was. Frankly, that's the most likely, and the heat was just from all the other welding done on the various plugs he covered up.

In that case, he was never in any particular danger, and no bullets were dodged.

I don't know, and YOU don't know, but it's interesting to try to figure out what happened.

As for being right, I DO think the pressure has to affect the ratio, but I am willing to let you be as right as you can be... I really don't care what you do, it doesn't affect me! How's that for just plain COLD?