max pressure in (stage) tanks

Skipbreather

It’s been long time since I took my metallurgy courses so rather than attempt numerical explanations, I’d put it this way: Legal issues aside, while over pressurizing steel cylinders isn’t a smart idea, over blowing aluminum tanks is REALLY a very bad practice. The reason has to do with the materials themselves. Insulting aluminum w/ an overstress in not unlike shouting at your wife/ GF during an argument. Later when things quiet down, on the surface it may LOOK like you got away with it, but in reality the ill treatment is always remembered- and is cumulative. The chance of hearing about it years later is always there. Steel is a lot more forgiving material.

__________________
"Entropy RULES! Enjoy the interim."

Drmike

tanks are designed with a fatigue life (it can be 'infinite' life) based on the fill/working pressures and the regular req. inspection pressure tests


If you stick to the stated fill pressures you prity much can gaurantee between inspections its not going to suddenly fail - you CANT say the same thing if you have overpressurised the tank

Overpressurising will reduce the life of your tank and will increase risk of sudden failure.

AL is far worse than steel in fatigue so AL tanks are more sensitive to pressurising abuse.

Bottom line the more often you overfill and the higher the pressure the shorter the lifetime of the tank will be and the less predictable the tank will be.

failing tanks have killed people

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

jdicediver

Witnessed a brand new HP130 pressured to 5000 PSI in a LDS once about a year ago...Promptly opened the valve and bled it off, while letting the operator know that 3442 was the WP, not the TP of 5000+ (%##@ Some People shouldn't work at Dive Shops either)

Consequently, I won't buy a cylinder there again!!!

__________________
Regards;

"Alterations performed under controlled conditions, on a closed course by a qualified Closed Circuit Diver"

"Some people shouldn't dive...others shouldn't even get wet".

jdicediver

Someone would be replacing that bottle or wearing it had it been mine.

__________________
Regards;

"Alterations performed under controlled conditions, on a closed course by a qualified Closed Circuit Diver"

"Some people shouldn't dive...others shouldn't even get wet".

Ben Field

Using it to melt the hair off the gas monkeys hand worked

__________________

Beware Fridge Suck

jaap

Take a look at this (scroll down to the part about typical EU markings for SCUBA) : Luxfer: Serial Numbers & Markings

This one might be interesting too:
http://www.luxfercylinders.com/downl...ba_leaflet.pdf

Don't forget the soap & towel ;-)

__________________
My initials: JAAP

jaap

Hey, cannot agree more.

Question is how much does the risk increase, and from what level...?

As an engineer with only basic soild mechanics education I don't know. But what I do know is that I don't buy people chopping off discussions by saying "Is bad, dangerous, not good".

I'm not saying its fine to overfill, but I find it far more interesting to see what others do and to try and get a reading on the risks.

I guess one could take the fatigue life vs fill pressure and apply to any human activity...push you luck enough and. But none of us take any risks do we So we should be fine ;-)

And to try make my post even slightly serious I'l throw in a link that is at least slightly on topic:
Luxfer: Sustained-load Cracking FAQ

__________________
My initials: JAAP

Drmike

As an engineer you should appreciate that the tank was designed based on assumed freq of filling and assumed fill pressures and assumed hydrostatic test pressures and test frequency.

IF you do anything to the tank outside those pressures/freq then all bets are off. Nobody can then easily predict what will happen to the tanks life.

We just cant say what the effect will be for sure - its not a simple thing to determine all we can safely say for sure is just what I said - that it has a detrimental effect on life and predictability.

So its not a case of chopping discussion and I don't believe that's what I did (seeing as were still discussing) I just stated the facts as they are.

Overpressuring a tank (any tank) IS going to irrevocably DAMAGE that tank. You WILL shorten its working life and make it less predictable. By how much? Who knows - it likely could be determined by the mnf or someone else with time to kill and an understanding of the science - but they would need to know the exact pressurizing history and even then it would be a huge guess - your assuming its an exact science - when it isn't. Life analysis/determination is not an exact science - if I told you how we determine fatigue life on jet engine components you'd probably never want to fly again!

Bottom line is if you are going to overpressure - dont get carried away keep it to a min (for AL personally I dont go over 10%) given that if they are your personal tanks they wont be seening the assumed freq of pumping as they were likely designed around (dive shop use)

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

jaap

Cool, the chopping comment was not directed at any single post/person.

Again I'm not advocating overpressing in any way.

I just get hung up on talk about DAMAGE, FAILURE and so on making it sound like the end of the world. Without any good statistics about violent tank failures my _feeling_ is that limited overfilling of personal tanks does not have to be a big issue (compared to other risks, not saying its a risk worth taking).

Sure the safer aproach is always to assume everything will fail, take position behind the mnf recomendations and shoot from there. But would we get any discussion from that?

I at least find the stamped service pressure/temp (not working or test pressure) found on newer CE-marked SCUBA cylinders quite interesting.

I only recall bits of things from reading and talking to experts (MNF of SCBA/SCUBA cylinders) over the years about the various tests that are performed for new designs.

As I got it the designs are already cycled extensively well above the working pressure. In some cases several thousands of cycles far above the test pressure. The pressure could even be assumed to cycle quite high above the working pressure when in regular use, even if that user only filled it to the WP (hot days/cool nights in a car). Of course if the user overfilled and then heated the cylinder things could get more interesting.

The logic of how it could be "OK" with moderate overfills for steel but not Al beats me. But thats for later.

__________________
My initials: JAAP

Drmike

as has already been stated (and all other things equal) Steel is better than AL when it comes to fatigue and other material properties such as creap.

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment