Another dirty little secret: when I built a larger tank for my
Roundhouse engine, I made it from a 2-inch copper pipe cap, with
a 1/16" copper base and a stay screw through the middle, all silver-
soldered. Actually, I made two.
A well-known small-scale live steam authority (who made me swear
never to tell who gave me this idea) told me how to pressure test
my tank the simple, empirical way. You take a large stew pot and
put a couple of gallons of water in it, then heat the water to boiling
on the backyard barbecue grill. Fill the tank with gas, then dump it
in the water. If it explodes, it wasn't safe (but all that it will do is
spray water into the air). If it is safe, use it.
There is a snag in the method, however. Silver soldering softened
the copper sufficiently that the sudden change in internal pressure
made my tank balloon like a dead toad on hot asphalt. But is still
worked fine. After determining that the method was fundamentall
sound (I used the tank for several weeks) I refined the testing
methodology and built another tank. This time, I filled it with gas
and put it in the cold water. As the water heated and the gas
pressure rose, the tank was work-hardened gradually; when the full
pressure was reached the tank was fully hardened and retained its
shape perfectly. This method also lessens your chances of getting
wet, should the tank fail when you drop it in. :-)
I believe, but did not record and do not remember, that I was using
butane-propane mix in this tank. I haven't been able to find pure
butane in years, and use the mixture in all our locomotives on the
FH&PB. So the pressures in my test would have been
correspondingly higher than those on the LocoSteam graph.
Bottom line is this: with the standard 1/16" thich copper tank, you
have nothing whatever to worry about. Those tanks will withstand
pressures far, far in excess of anything you're going to encounter in
real life. Check out the graph at
<http://www.nmia.com/~vrbass/images/ButanePressure.gif>. Note
that the pressure rise with temperature is geometric, so that the
pressure at twice the temperature is something like the square of
the lower pressure. Yet my boiled tanks withstood the brute-force
test fine (once they were work-hardened again :-).
I can imagine that someone could easily construct an exploding gas
tank, so I don't think the concern is unfounded. But if you overbuild
it, like we do our boilers, there's nothing to worry about. I'll gladly
draw up and post my tank design, if anyone is interested.
regards,
-vance-
Vance Bass
Albuquerque, New Mexico, USA
Small-scale live steam resources: http://www.nmia.com/~vrbass
