Newsgroups: rec.crafts.metalworking
Subject: Re: Compresssor Tank Ideas?
From: John De Armond
Date: Mon, 18 Oct 93 07:33:48 GMT

dparsons@plasma.ps.uci.edu () writes:

>Although I doubt that an air tank can level a house I agree that it is 
>dangerous
>enough that I wouldn't try cheating mother nature over it.  

Ignorance can be bliss (NOT!)  About 20 years ago one of my racing partners
lost his garage and much of his house to a jerry-rigged compressor
that used a 40 gal water heater for the tank.  The explosion lifted
the roof, blew the walls out and settled the roof down on what remained.

If you want to figure this stuff out yourself, compute the expansion
ratio of a 40 gal tank and a typical garage and see what the pressure
on the walls would be.  Then keep in mind that according to DOE,
about 0.2 psi will topple the walls of a wood frame house.

Not only would I not even think about making a tank at home, I won't
use one that isn't ASME certified and is within 5 years of a current
hydro test.  Just not worth the risk.

John

Newsgroups: rec.crafts.metalworking
Subject: Re: Compresssor Tank Ideas?
From: John De Armond
Date: Tue, 19 Oct 93 08:21:53 GMT

Jim Jaskie <jim_jaskie@tempeqm.sps.mot.com> writes:

>I took up you challenge on this one.  I opened up MathCad and did the cal
>on a forty gallon water heater, and in a garage of 20ft x 30ft x 10ft.  I
>don't know if this is typical or not, but the calculation was instructive
>nontheless.

>After the water heater blows up ( I assumed it went at 100 psi) the new
>*equilibrium* pressure increased by about 0.1 psi.  This is less than the
>0.2 psi that you mention, but note two items:
>	1.  I believe that you 0.2psi number refers to external pressure loading
>(as in an explosion nearby), internal loading ability may be much less. 
>Note how often houses near tornados explode rather than implode.

yes.  This is from the DOE book "The Effects of Nuclear Weapons".  0.2 psi
renders a wood frame house into toothpicks.  It is interesting
to note that on your 20 X 30 foot roof, 0.1 psi pressure will exert
a bit over 4 tons of force on it.  That would be pretty spectacular.


>So which is it?  Does ASME demand a brittle steel that can be proof
>tested, or a ductile steel that will fail graciously?

ASME does it the old fashined way - with brute force.  My 5 hp dual
stage compressor has an ASME 250 psi rated 60 gal tank whose walls are
almost 1/4" according to my little ultrasonic thickness gizmo.  The
safety margins are typically huge.  Don't shoot me if my memory is a bit
fuzzy because it's been a long time since I owned the welding supply
store and an employee did the hydro testing but as I recall, the
DOT/ASME hydro spec for welding gas tanks was for less than a specified
increase in girth at 2.5 times its rated pressure.  I know we used an 
air-driven hydraulic pump capable of 10,000 psi and a little gizmo that
involved a dial indicator and  a cable that wrapped a loop around
the midpoint of the cylinder.  Welding gas tanks are rated to 2500 psi
service so we probably tested to 6,250 psi.  That number rings a bell.

Before any pressure was applied, however, a visual test was performed.
ANY visible rust was cause to fail the tank.  If the rust was minor,
we'd send the tanks back to the mfr for cleaning and re-certification.
According to information we got from our tank supplier, the cause of 
most welding gas tank failures is internal rust/corrosion.  THIS is
what scared me about jury-rigged air tanks.  Most welding gases are
vaporized from cryrogenic liquid which means the dew point is very low,
typically -50 deg F or better.  Compressed air, on the other hand, is
typically saturated which is why water collects in the tank.   Rust
is inevitable which is why the water that drains from the water drain is
red.  A freon tank or water heater may be more than capable of holding
the pressure when new but how about after a few years of rust?  I 
suspect my friend's tank exploded for just that reason.  I know the 
insurance company impounded what was left of the compressor and 
determined that the pressure switch and safety relief were working
properly.

A "tank" that most people don't think of but which is safe and cheap is
the volume of the distribution piping.  I plumbed my house and shop 
for air using 1" copper tubing.  Using the pressure drop method, I 
estimated the volume of my piping by observing how much the pressure in my
60 gallon tank dropped when I valved the distribution system in.  
It works out to about 25 gallons.  That is a significant amount of storage
capacity.  The most catastrophic failure possible for this "tank" is a 
sweat fitting blowing off.  No big deal because the pipe friction slows
the discharge rate.

John