[TheForge] Metal porosity and purging tanks (was: cutting propane cylinders- fuel-air mixture)

[Steve Smith]

Excellent Bruce, thanks.

Steve

Bruce Freeman wrote:

> Bob,
> 
> This is an ongoing debate on this forum, though usually not so explicitly.
> 
> My answer to your question is another question:  What do you mean by "pores"?  
> 
> Pick up a book with photomicrographs of metal surfaces and you'll see that they are not clean and pretty, but rather more like lunar landscapes (close-up, no craters, generally speaking:^).  Add a bit of rust or scale, and the surface can be quite "porous".  
> 
> Even polished metal has surface scratches from the polishing operation, the size of which depend upon the degree of polishing.  I believe surface poricity can be largely eliminated by appropriate planishing, but I don't have evidence to prove this.
> 
> It's my take, however, that the bulk metal typically is not porous at all.  One notable exception is castings.  Castings can be quite porous, typically due to outgassing during cooling. (This CAN be the sign of a poor casting, but I understand that controlled outgassing is sometimes taken advantage of by the foundryman.  I don't remember why.  I would have to look it up.  To control shrinkage, maybe?)  
> 
> I believe that forged and/or rolled metals lose much if not all of this porosity.  (Most metals start out as casting - ingots or billets - after all.)
> 
> So, if the question is whether metal is porous like wood, then the answer is "no, definately not."
> 
> If the question is whether there are "pores," or gas bubbles (like in swiss cheese * but a LOT smaller:^), in metal, the answer is "very likely, but not certainly," if the metal is cast; "no," if the metal is forged or rolled; or "maybe," if it is minimally forged or rolled from the original cast billet.
> 
> If the question is whether metal has surface "porosity", then the answer is "usually yes, more or less."
> 
> If the question is whether metal is ever free of porosity, the the answer is "effectively yes, unless you consider surface scratches to be porosity."
> 
> But now let's consider the usual source of this question:  Cutting fuel tanks.  The issue here is not really porosity, but whether residual combustables may be present in quantity sufficient to be hazardous.  The answer is clearly "yes."  
> 
> Why is this?  Well, I can't possibly account for all situations, but I can discuss a few.  Propane and other gases are treated with odorants and are pumped with oil-containing pumps.  In addition, water can never be completely excluded.  Hence, a propane tank is likely to contain a small amount of liquid sludge, which may be in the form of a hydrate.  Liquid fuels are less likely to stratify, but certainly will coat ever internal surface of the tank, even when the tank is almost empty (by evaporation and recondensation).  In either case, polymerization could possibly occur, especially when unsaturated hydrocarbons are present along with levels of oxygen and either signifiant heat (sun-heated tanks) or metallic catylists (which might include iron, rust, or other metallics).  Hence, the interior of the tank is likely to be quite messy on the small scale.  (Think about the bottom of an old iron frying pan, and you've been given the job to scrub it "clean".)
> 
> To cut to the quick, the interior of any fuel tank is likely to contain significant quantities of combustables, and simple cleaning procedures may not suffice to remove these.  (Of course, this isn't a certainty.  A tank might be in very clean condition, and a simple detergent-water soaking might clean it up fine.)
> 
> I side with those who favor purging with inert gas before any sort of cutting of any sort of tank.
> 
> Inert gases you might have handy include carbon dioxide, nitrogen, and helium.  Steam should not be considered inert because it has a nasty tendency of reacting with metal and fuels at high temperatures and creating other fuels (carbon monoxide and hydrogen, for example).  
> 
> In a real low-tech situation, you can still easily generate carbon dioxide by reacting washing soda or baking soda with acid, or even by heating a solution of baking soda - the gas given off is carbon dioxide.  I don't suggest these things be done in the tank itself.  Do them in some external "retort" and run a tube into the tank to carry the carbon dioxide.
> 
> Bruce
> NJ
> 
> 
>>>>schade at acegroup.cc 3/30/2005 5:07:25 PM >>>
> 
> Are there really "pores" in metal?
> 
> If I take a "dry" piece of steel, let it sit submerged in a bucket of 
> water overnight, take it out and dry it with a towel (always carry a 
> towel) and weigh it (the steel), will it weigh more than before I 
> "soaked" it? How much?
> 
> Are some metals more porous than others?
> 
> I have some doubts.
> 
> Bob
> _______________
> 
> 
> 
> 
> On Mar 30, 2005, at 11:30 AM, Ron Childers wrote:
> 
>> the gasoline in the pores of the metal
>>
>>Ron C
>>
> 
> 
> On Mar 30, 2005, at 12:53 PM, Ralph Sproul wrote:
> 
> 
>> sweat the fuel out of the pores of the metal.  This
> 
> 
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