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

[Schade]

Bruce,

Thanks for your detailed analysis. I hoped you would comment.

Bob
_______


On Mar 31, 2005, at 7:45 AM, 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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