Why not overfill Aluminum cylinders?

[Compressor]

Thanks for the link lowviz. Same here: I won't ever ask that any of my 2250 steel tanks get pumped to over 2400 psi but the shop routinely fills them to near 3000 psi even though I wrote with a sharpie on my tank that its a 2400 psi tank. I know they can read it but they think its steel; lets do it anyway to keep customer happy.

 

[Compressor]

This is a good place to start if you don't want to accept the enormous number of steel overfills that have been done over the years: Vintage Double Hose • View topic - Vintage tank hydros and "+" stamping
.

Impressive!!!

 

[matt_w]

This is a good place to start if you don't want to accept the enormous number of steel overfills that have been done over the years: Vintage Double Hose • View topic - Vintage tank hydros and "+" stamping

For the record, I won't ever ask that any of my 2250 steel tanks get pumped to over 3K.

That thread will make you think twice about hanging around an aluminum tank being filled to any pressure.

 

[lowviz]

...//... the shop routinely fills them to near 3000 psi ...

Sounds like a good shop to me. They fill to three grand and you lose a bit when it cools. You can ask for a last minute top off to 3K for your bigger dives. Why pay for a short fill?

...//... keep customer happy.

So did mine. They closed (retirement) and looking for a new shop. First question out of my mouth was will you fill my 2250's to 3000? Answer: "Only if they are in hydro." -looking good so far...

BTW, nobody I know fills past the stamped working pressure for pure O2.

 

[lowviz]

Impressive!!!

That thread will make you think twice about hanging around an aluminum tank being filled to any pressure.

Wow, you guys actually read it.

 

[Compressor]

Why pay for a short fill?

The tank is rated to 2400 psi. In fact they fill them in a water tank (39 deg F cooled). When I take it outside where its 85. the tank actually heats up. They are careful to have no water on their Whips and the tank valve.

Wow, you guys actually read it.

Of course I did.

 

[NAM001]

Thanks for the response. I love it when the answers are in picture form and that is a very nice graph. I always thought that aluminum had an elastic limit, but it's been a while since I studied and I could be wrong. Could you have meant endurance limit instead of elastic limit?

Based on the graph, let's select a design value of 20 ksi for the tank. Using that value, it does appear that the steel tank will last indefinitely; however, the aluminum tank will fail after approximately 100,000,000 cycles. Did I read that correctly?

Seems like I have some time. Or am I missing something?

I would read it as a tank with 100,000 cycles on it can only handle a stress level of 20. also if you put a stress of 45 on a tank you have cut the life to 10,000 cycles

 

[rjack321]

Thanks for the response. I love it when the answers are in picture form and that is a very nice graph. I always thought that aluminum had an elastic limit, but it's been a while since I studied and I could be wrong. Could you have meant endurance limit instead of elastic limit?

The elastic limit for aluminum is much lower than for steel. Which is the point at which deformation is permanent.

There's ample evidence of the impact of 'overfilling' on aluminum cylinders from Australia where they have to hydro them every year. And just one 5k "fill" (hydro) annually and they don't last as long down under as they do in the rest of the world.
Aluminum is also roughly 1/3rd as strong as steel and thus needs walls about 3 times as thick. Those thicker walls don't "stretch" evenly so that's another factor in the lower tolerance of aluminum to pressure in general and especially to overfilling past design pressures.
The short version, aluminum is brittle.

 

[Compressor]

The elastic limit for aluminum is much lower than for steel. Which is the point at which deformation is permanent.

There's ample evidence of the impact of 'overfilling' on aluminum cylinders from Australia where they have to hydro them every year. And just one 5k "fill" (hydro) annually and they don't last as long down under as they do in the rest of the world.
Aluminum is also roughly 1/3rd as strong as steel and thus needs walls about 3 times as thick. Those thicker walls don't "stretch" evenly so that's another factor in the lower tolerance of aluminum to pressure in general and especially to overfilling past design pressures.
The short version, aluminum is brittle.

Very scary read. Take home message: Try not to overfill your AL tank if you can help it.

 

[kelemvor]

BTW, nobody I know fills past the stamped working pressure for pure O2.

I've never had my deco bottle even close to the working pressure of the tank. Granted, I've only had it filled at one shop. He once told me why he only fills it to around 2200, but I don't remember. It hasn't mattered to me, it's been far more gas than I've needed for any accelerated deco I've planned so far.

This is the same shop that fills my Worthington lp's to 3600 regularly.