What is a safe pressure?

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Fishkiller

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Ok say I have a AL80 that is rated to 3000psi, fill it hot at 3250psi and cold it drops to 3000psi. That is possible correct?

OK I now have a LP steel that is rated to 2400+ that can be filled to 2650. Why can I not go to 3000psi??
 
That's about right....

If you remember some of your Chem 101 classes, as you increase the pressure for the same volume, the temp rises. Especially when this is done in a 'relatively' short amount of time.

In my (short) experience, all tanks when filled will be a bit warm, and as such, the heat will increase the pressure slightly. But these will come back to "normal" 3000psi when the cool.

I don't have any experience with the steel tanks yet, so I will have to defer to someone else.



What I'd like to know is how 3000psi became the magic number in the first place. Anyone able to answer this?
 
3000 is NOT just a "magic number". It has been empirically determined.

Steel has two ways of accommodating stress (or yielding) before an actual failure. The first is “elastic deformation” where the steel acts like a real stiff rubber band. It returns to it’s natural position after the stress is relieved. The second (it comes after the elastic deformation)is “plastic deformation” where the steel is permanently deformed or stretched. It can't bounce back. While both of these “deformations” cause work hardening (where the alloy becomes brittle), plastic deformation is BY FAR the most damaging to your tanks.

So the trick is to put in only enough air to keep the tank in the “elastic deformation” range and still allow for variations in temperature. Do this and your tanks will last a LOT longer. This range will be determined by the particular alloy, wall thickness, and the method of rolling the tank. Obviously, low pressure tanks have weaker walls than a high pressure tank. Its not just some sort of voodoo...

Consequently, your tanks really have two "limits". The working pressure (stamped on your tank) and a max pressure... which is not stamped on your tanks because you monkeys would exceed that too! The burst disk in your valve is designed to keep your tank away from that plastic deformation limit. It's limit is higher than the working pressure to allow for temperature variations and such, WITHOUT letting it get into plastic deformation. It will pop first, to protect the tank. Is it clear as mud yet?

BTW, Aluminum and its alloys have COMPLETELY different characteristics; very little elastic deformation. That’s why they have far thicker walls, and why the AL tanks are so heavy.
 
FishKiller,Here in Fla Cavedivers regularly over-fill tanks and to my knowledge ,no one has been hurt .There is great potential for damage,but in reality a burst disc will pop.There are even some fellas who double up their burst discs so they can fill lp tanks as high as 3500 and hp tanks as high as 4500.For the reasons stated I would be wary of overfilling al tanks.I do see a lot of 3600-3800 fills in my 80s tho.Leave it to an OCD to buy bigger tanks and still overfill them.
 
I have AL tanks and was just wondering what the actual weight of a comparable STL (say 80cf) would be. Keeping in mind the weights are null and void once in the water, I also wonder which one is less prone to corrosion?
 
Aluminum forgiving on wallet,Hp steel forgiving on back.Still need to compensate for a 7lb bouyancy shift from full to empty.Al win hands down on corrosion.
 
It's pretty common among technical divers to over pressurize steel tanks as long as they plug or double the disks. Theoretically, you'll reduce the tanks life by continually over pressurizing. Instead of 20,000 fills, you might only get 15,000 fills.

The weak link in the tank (other than the disks)is the neck O-ring anyway. In the event of a fire or severe over filling, the O-ring will go before the tank explodes. I've never heard of a steel tank exploding while being filled. I suspect you'd be hard pressed to find a compressor cabable of extruding the O-ring in scuba shop.

As already discussed, aluminum tanks are a slightly different story, but there is a significant safety margin in newer aluminum tanks too. There's some older ones that are quite dangerous as their alloys weren't too good and have been recalled. I've seen both aluminum and steel tanks pumped well beyond their "recommended" pressures.

I suspect it's another prime example of a law suit skittish industry over protecting itself, for which I can't blame them. As I recall, the Europeans don't even use burst disks. Perhaps some they can shed some light on their practices.

Mike
 
Originally posted by Lost Yooper
I suspect it's another prime example of a law suit skittish industry over protecting itself, for which I can't blame them. As I recall, the Europeans don't even use burst disks. Perhaps some they can shed some light on their practices.

Mike

The situation in europe:-

1) Tank material - Aluminium tanks aren't unheard of here, however, they represent only a tiny percentage (about 1 to 2% I would guess) of the total. Steel rules. Aluminium was popular for a time in the 70's but no longer.

2) Valves - The UK and the Scandinavian countries use single valves, France, Switzerland, Germany Insist on double valves (ie Y or H valves) - the rest I'm not sure about.

3) Burst disks - Never been used, never likely to be used. This is partly explained by their relavent modes of failure. With such a low percentage of AL cylinders (industrial cyliners are not allowed in Al in the UK for some reason) the risk is even lower - Look at an exploded steel tank and an exploded aluminium. They have very different failure modes. Al literally explodes, whereas steel 'unzips'. No-one that I know of has ever been killed by a failing steel tank. A few years ago, a steel tank let go in my LDS, apparently it made a high pitched squeeking noise as the failure started, which turned into a tearing/gas rushing sound as the failure grew. In total it left a gap in the cyliner of less than an inch long, and about 1/4 wide, and several people with ringing ears, no injuries and no bits of metal flying everywhere.

4) Regulations - all the european countries have their own specifications for tank manufacture and testing. A tank made to french rules with a NF (norm français) stamp won't be filled in the UK, neither will a BS5045 be filled in france, and both won't be filled in germany where it has to be marked TUEV. This is being overcome by the new EN144 (european norm), However, as I have expensively discovered if it is marked EN144 NF (ie complying to the two) it won't be filled in the UK, whereas it is was just marked EN144 it would have no problems! - it is getting confusing at this point!

5) tank measurement and classification. - At least this is all agreed on. Tanks are sold by their internal water volume (ie a 12 L tank, if you took the valve out would hold 12L of beer/wine/water. This is then combined with the WP. So, a 12L tank will come in LP and HP. They are the same physical size, so the HP will just have more air 'squeezed in'

6) Overfilling - (the + sign for the US) - this doesn't exist here at all. Totally Banned / Interdit / Verboten. It is normal that if you have the time to leave your tank for a while it will be filled, allowed to cool and then 'topped off' so when you collect, it is also reasonably common (although illegal) to overfill by 20 bar and let the tank stand to cool down to its WP.

7) Working pressure - European tanks seem to be at slightly higher working pressures, a LP tank is usually 232 Bar (3300PSI), and a HP tank is usually 300Bar (4300Bar), although I might be wrong on this last point.

8) at european prices the statement 'Aluminum forgiving on wallet,Hp steel forgiving on back' by 100days is not true. Al tend to be the same price, if not slightly more expensive.

9) Weight - Since practically all european diving is done Dry, or with 5mm of neoprene minimum Steels hold the advantage. It is quite normal to see drysuited divers with Twin steels as backgass, and two stages made of steel, with still a weightbelt of 5 - 10 Lbs


HTH on giving the European picture.

Jon T

PS, as a european, looking at what happens in the states, The regs are all perfectly logical, but not always necessary (IE a bit OTT, or 'Over The Top') and often appear to be designed to protect people from lawsuits rather than provide a suitable level of public protection.
 
turnerjd...sorry to hear al. is so expensive there.In the states al. is roughly 1/2 the cost of steel,with HP being a little more costly than LP.I assume the cost is due to sheer volume.A recent foray to local shops turned up these prices out the door .Al80 $150,Lp80$300,Hp80 $350.Sorry to hear about the rules there.Must be drag to travel.Our biggest worry is getting fills done before we return from shopping.
 
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