home made air tanks

[Tony Sobrito <tony@domain.elided>]

I snaked this off the J**p list. After some discussion on pros and cons
of home made tanks this reply was posted. 


> Another thing that still hasn't been answered is the strength of the
> average bumper air tank and its welds.  Do any of you engineers have any
> data on this?

Did someone ask for an engineering answer?

Ok, lets make some assumptions (yeah I know ass-u-me):
14 gauge (0.0785") wall thickness (pretty thin)
3" pipe diameter
150 psi max air pressure
No defects and good welds, etc.
AISI 1010 cold drawn steel (55ksi yield strength, 67 ksi tensile strength)

Equations for a thin walled pipe:
hoop stress = (pressure*radius)/(2*wall_thickness) = 1433 psi
longitudinal stress  = (pressure*radius)/(wall_thickness) = 2866 psi
combined stress = sqrt(h^2 + l^2) =   3,175 psi

That's a very low stress number.  The yield strength for the lowly
AISI 1010 cold drawn steel is 55,000 psi, giving you a factor
of safety of 17.  A 0.190" wall gives you a factor of safety of
over 42.  I consider yielding to be failure, but note that the
tensile strength.

You'd need about 2600 psi air pressure to rupture the 14 ga wall
pipe and 6300 psi to rupture a 0.190" wall pipe.

Again this is assuming a perfect pipe under no other stresses. I'd
think you have a big enough safety margin that you could beat the
hell out of the pipe and not rupture at 150 psi.


> So far, no one's mentioned hearing of an actual bumper air tank rupture.

I have seen two high pressure (400 psi) tanks fail, one got punctured
and the other split from internal rust.  In both cases, you simply got a 
semi-violent release of air which tended to kick loose objects around.
It would be pretty hard to get shrapnel from the tank.

- -Chris