Overfilling Scuba Cylinders

[Doc Harry]

At operating pressure, modern scuba cylinders are designed to be completely elastic. Luxfer advertises that their cylinders have been tested through 100,000 fill cycles.

If you over-pressurize a cylinder, you are exceeding the fully elastic capacity of the material. The pressures used during hydro testing deliberately push cylinders into the inelastic expansion range. Take a look at your cylinder's last hydro. If your cylinder was completely elastic at hydro pressures, it would have had an expansion of 0%. But it wasn't 0%, was it? The permanent expansion ratio was more like 1.5%.

How many times can your cylinder tolerate over-pressurization before it expands inelastically in a brisk, dynamic manner (i.e., explodes)? Who the **** knows.

If you regularly over-pressurize your cylinders yourself in your garage, have a great time. But you owe it to fill station operators to leave your over-pressurized cylinders at home. You can play Russian Roulette with yourself, but don't endanger someone else with your stressed cylinders.

Bad cylinders can and do explode at less than 3,000 PSI. And I am not talking about the old 6351 alloy cylinders, either.

 

[jsado]

I dropped some tanks off at a shop for a fill, all AL80's. Loaded up my gear, went out on the boat and started to get ready for a dive ........ 4600 PSI!!!!! A buddy said if it fails the hydro in a couple of years, that'll be why.....

 

[dannobee]

If you regularly over-pressurize your cylinders yourself in your garage, have a great time. But you owe it to fill station operators to leave your over-pressurized cylinders at home. You can play Russian Roulette with yourself, but don't endanger someone else with your stressed cylinders.

The local shops around me have that angle covered by never filling up to the rated pressure.

 

[fire_diver]

I dropped some tanks off at a shop for a fill, all AL80's. Loaded up my gear, went out on the boat and started to get ready for a dive ........ 4600 PSI!!!!! A buddy said if it fails the hydro in a couple of years, that'll be why.....

Not that I recommend overfilling tanks, but your friend doesn't know what he's talking about. AL80's are hydro tested to 5000, and can sustain at least 10,000 hydro cycles. Your tank is fine.

 

[fppf]

Wow, lots of experts again pop out of the wood work.

Doc Harry nailed it. The cycles before failure will reduce in an exponential factor based on stress. The closer you take a material to yield the sooner it will fail. These are called fatigue curves. And even though a company may rate a cylinder for X amount of hydro cycles, those cycles are very short duration and in a controlled setting. Ever watch the news how a building is just fine busting at the seams with people until a few hours into the party, then it collapse.

As for "I"m an engineer", cylinders are not cast. Cylinders are mfgred by cold drawing. Basiclly they take a slug of aluminum, or a thick disk in the case of steel cylinder, and press it through a die. To form the neck the spin the cylinders and close it up. Then drill tap and turn the valve seat.

YouTube - making a scuba tank

YouTube - Making Worthing Steel Scuba Cylinders

As for regulators, almost all 1st stage regulators are upstream designs. This means the valve seat is held closed by pressure and the spring force is what opens it. The orfice is very small in the first stage and the seat is normally metal backed. So I highly doubt that the seat will be the failure point of a over pressurised regulator. My educated guess is that the threads on the Din/Yoke fitting into the regulator body would fail, or the threads holding the high pressure seat assembly would fail. If you try it make sure to do it using water or oil, don't use gas. That way it will pop rather than BOOM.

 

[jsado]

Not that I recommend overfilling tanks, but your friend doesn't know what he's talking about. AL80's are hydro tested to 5000, and can sustain at least 10,000 hydro cycles. Your tank is fine.

That's good news!

 

[Cave Bum]

At operating pressure, modern scuba cylinders are designed to be completely elastic. Luxfer advertises that their cylinders have been tested through 100,000 fill cycles.

If you over-pressurize a cylinder, you are exceeding the fully elastic capacity of the material.

Fascinating! And were did you take your class in metallurgy?

All material stretches. There is an elastic limit, known in engineering as ‘yield strength’, which is the limit of stretch beyond which the material will no longer return to its original shape and size (see stress-strain curve). General design criteria uses six tenths of the yield strength (.6 * yield) as the maximum allowable working stress. Pressure cylinders and other items that could constitute a hazard to humans have more stringent requirements.

The working pressure of the steel scuba cylinders currently being manufactured are not close to the pressure at which they will permanently deform. That is, however, part of the hydro inspection, to record volume displaced by stretch and to see if it returns to original size (if I understand correctly what I've been told about hydros).

 

[halocline]

Is it ok to fill scuba cylinders, specifically steel cylinders, I have heard of people filling LP cylinders (service pressure 2400) to 3600 and sometimes even 4200. Is this safe?

Thanks! It's been at least a couple of weeks before this topic has been endlessly beaten to a pulp and I've been missing it.

 

[jridg]

Don't forget, any corrosion inside the tank (or outside if bad enough) can create a stress-riser (beginning point for failure of the metal). So what works for a well-maintained, clean and corrosion-free tank may not work out so well for an older, possibly corroded tank.

Steel has pretty amazing elastic qualities, aluminum, less so - personally, I would NOT overfill an aluminum tank. I like to dive HP steel - on occasion I end up with a couple hundred extra PSI, it doesn't bother me......

 

[dmoore19]

I am not a metalurgist, do not profess to be. This is the way that things go where I work. Not just scuba tanks but everthing I work around has a maximum pressure and and working pressure. fittings on pipe and the pipe itself have both numhebers. IE 3000 psi working pressure 5000 Max. The fitting such as a union or an el or a T can continuously withstand 3000 psi. It is "rated" for 5000 psi Max. The 5000 psi max is OK but a constant working pressure of 3000 psi is what the piece is designed to be able to handle safely. I don't know about you folks but I have a lot of faith in how a design engineer and a metalurgist have designed the equipment, whatever it is, to perform safely. Just FYI I have seen things with a 3000 psi working pressure come apart and it is not pretty. Can't say I would want something that can fail like that strapped to my back with a cheap piece of padding or plastic or even stainless steel between me and it. I would prefer to just use the pressure the equipment is designed to be used at as my max pressure. After this thread I know I will not be buying any used tanks unless I know it's history. Thanks for the warnings.