Max bursting pressure within an aluminium container

[srosefx]

Hello, this is my first post, i hope somebody can help me.
I have searched for the answer myself all over on the web and here, but I am stuck.

Im trying to work out the maximum pressure (psi) that i can get into an aluminium cylinder that's 80mm wide and 250mm tall and 4mm thick

ive found calculators that work out the max bursting or working pressure of aluminium but not able to work out from my dimensions the optimum working pressure.

i hope you can help

 

[mgb_phys]

For a thin walled cylinder the stress in the walls is approx Pressure/2
So if you know the ultimate tensile stress of aluminium you can work out the maximum pressure.

BUT in the real world you have to worry about the effects of micro-cracks, welds, joints etc. The are probably specific regulations for your country/industry about the maximum allowed pressure. Which is likely to be only a small part 10-50% of the theoretical maximum

 

[srosefx]

Hi,
thankyou very much for the very swift answer but there are a few details i lack.
i found that the tensile strength of aluminium is 145.037738 lbf/in2 (psi)
what i couldn't find was the variable of thickness as quite obviously 1mm of alu is weaker than 4mm thick although the area would be the same i tried working out by myself using the inverse square rule- (i have to note that i received a very generous 'D' in maths gcse that's it)
i would enjoy finding out this myself if someone could point me in the right direction ie book or right subject I am guessing metallurgy is a start.

i feel a little fustrated as an answer will always result in another question :)

thanks

simon

 

[mgb_phys]

The term you are looking for is 'hoop stress'

BUT nothing you are told on here is professional advice. Compressed gas cylinders are dangerous and are usually heavily regulated, if you actualy want to store compressed air you are much better off using a commercial scuba or similair cylinder.

 

[srosefx]

i quickly looked at hoop stress and its perfect for what i need, I am not familiar with the mechanics of these formulas but it seems pretty straight forward.

Im only working with pressures of upto 200psi within a small container as an air mortor for fx in a film, but i need to run up the risk asessments of max working pressures over wall thickness for min wieght/max safety.

these safety measures will be engraved on the cylinder.

thanks very much for the help, I am facinated by physics i wish i had a contact here in london to chat too about some questions i have about other things too, but i will post those when the time comes.

cheers
simon

 

[Q_Goest]

Hi simon,

i found that the tensile strength of aluminium is 145.037738 lbf/in2 (psi)

simon

Your tensile strength is wrong. It should be many thousands of psi, not 145 psi. In the US, a small diameter vessel (less than 6") doesn't need to meet ASME BPV code. Not sure about Europe where the PED is applicable.

Since your pressure is very low (200 psi) you can estimate hoop stress using a very simple equation: Stress = Pr/t
where
P= pressure (psi)
r = inside radius (in)
t = wall thickness (in)

If you have no other information regarding the material, I'd suggest using an allowable stress of 5000 psi. That should be fine for even the worst aluminum, but I suspect it's much better than that. Regardless, that means you can pressurize the aluminum container you have to ~500 psi.

Note that any pressurized container should really be subjected to a hydrostatic test prior to use. Fill it with water, make sure there's no air bubbles, then increase pressure to 1.5 times your working pressure. ex: If your working pressure is 200 psi, make sure the vessel is hydrostatically pressure tested to at least 300 psi.