[PSUBS-MAILIST] Commercial Grade O2 vs. Medical

[Steve McQueen via Personal_Submersibles]

All, I was able to find data regarding the minimum design criteria of the aluminum medical O2 cylinder I purchased without relying on the vendors response.  The secret was noting that they advertise they where made to DOT-3AL and for our Canadian friends CT-3ALM specs.  Google these and you get all the details about the minimum structural design specifications. These should be helpful to us as references (when applicable). 

With this I can feel I understand the "numbers" and run some calculations for my personal experience even if the real concern/risk is minimal for my application.

Thanks,
Steve

---- "Sean T. Stevenson via Personal_Submersibles" <personal_submersibles at psubs.org> wrote: 
> Alan, I'm afraid that this is a dangerous misconception. Internal and external pressure are very different load cases. The reason you are seeing similar results is that you are:

1) evaluating a strongly isotropic material,
2) evaluating using a thin wall assumption whereby it is assumed that the net stress in the cylinder wall is evenly distributed throughout the cross-sectional area of the wall,
3) evaluating a material which exhibits effectively identical compressive and tensile material yield strengths, and
4) ignoring buckling failure modes, or acknowledging them to be secondary to a dominating material strength failure.

So, the results CAN be similar for internal and external pressure, but only under specific circumstances. It is dangerous to consider this to be a rule of thumb. As a general rule, internal pressure capacity is greater than external due to buckling failure modes.

Any material defects or deformation from applied loading only complicates the situation, as with internal pressure, any out-of-round deformation is self-limiting, whereas in the external pressure case such deformation will serve as a buckle initiation site which will propagate unchecked.

That said, I think that collapsing an aluminum gas cylinder with external pressure is unlikely at PSub depths, although I might be inclined to select shorter, stubbier cylinders if that is a worry, to ensure that buckling will not dominate (or at least, if it is dominant it is so at much higher pressure than the intended service pressure) as could be the case with a long, slender cylinder. Regarding flat bottoms, gas cylinders are typically extruded using a die, and the internal shape of the cylinder bottom is actually a 2:1 semi-ellipse, or at minimum, generously filleted. Extra material is added in design / to the minimum head thickness to accommodate standing the cylinder upright, creating the flat bottom. Evaluating the cylinder as an unreinforced cylinder subject to external pressure, with hemispherical or semi-elliptical end caps as appropriate, seems to me to be a reasonable approach.

Sean

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-------- Original Message --------

On Jan 4, 2018, 13:12, Alan via Personal_Submersibles wrote:

Steve, I just ran a couple of scenarios on tubes with differing aluminium grades in the free "under pressure" program. The results were exactly the same for internal pressure as external pressure. So if your medical tank is safe for 2000 psi internal it will be safe for 2000 psi external. plus there is probably at least a 100% safety margin so more likely it will crush at 4000psi or 8000ft. Perhaps Sean could confirm that with aluminium it is at least as strong under external pressure as it is under internal, then you won't need to do any calculations.

Cheers Alan