???tank overfilling info???
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ucfdiver
Contributor
I'll give it a less sarcastic approach...every weekend easily 100+ sets of doubles are filled at each of the more popular shops in cave country, most all being 3600psi fills in LP tanks, and no one on this forum can quote a single accident because of it.
If you're THAT worried about not filling your tanks beyond working pressure, I'd suggest that you warn the shop operator, because if you don't ask they're gonna be at 3600.
You'll find some retarded OW shops who don't have a clue about it and tell you people die all the time. I had a shop tell me "we don't kill people for a little more air like they do in cave country either". Complete BS. If a shop tells you this, say thanks and walk out.
If you're THAT worried about not filling your tanks beyond working pressure, I'd suggest that you warn the shop operator, because if you don't ask they're gonna be at 3600.
You'll find some retarded OW shops who don't have a clue about it and tell you people die all the time. I had a shop tell me "we don't kill people for a little more air like they do in cave country either". Complete BS. If a shop tells you this, say thanks and walk out.
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When you overfill any steel cylinder all you are doing is cutting into the safety factor. In an ideal world we don’t need a safety factor and the structural geometry of a simple cylinder is close to ideal. If it is a perfect cylinder, calculating stresses is very predictable and doesn’t even require a calculator.
From the experience with overfilling in Florida we can see that the real world of steel cylinders seems to follow the theoretical ideal world… but, they are still cutting into the safety factor. All you need to ask is “do you feel lucky today?” The odds are that you will be OK.
I may sometimes overfill steel 72 to about 2800 psi max (technically that is not much of an overfill if they are hot). Personally, if I am going to take an unnecessary risk I want to at least get an adrenaline rush
…cutting into the safety factor of a pressure vessel, just doesn’t do it for me. 
I am totally in favor of educated risk taking as long as you don’t hurt someone else…that is why I am opposed to totally disabling burst discs, but that is a different subject. Using a higher pressure burst disc is kind of a reasonable compromise (I have done some high temperature structural calculations on the subject).
Be aware that hydro test only tests for one thing, material elasticity (yield strength limit). It does not test for cracks, pits, or any other defect. Those are hopefully caught during a visual. I have seen both aluminum and steel tanks pass hydro with a crack on the inside of the neck. That is one reason we have safety factors…hydro tests and inspections are not always perfect.
From the experience with overfilling in Florida we can see that the real world of steel cylinders seems to follow the theoretical ideal world… but, they are still cutting into the safety factor. All you need to ask is “do you feel lucky today?” The odds are that you will be OK.
I may sometimes overfill steel 72 to about 2800 psi max (technically that is not much of an overfill if they are hot). Personally, if I am going to take an unnecessary risk I want to at least get an adrenaline rush
…cutting into the safety factor of a pressure vessel, just doesn’t do it for me. I am totally in favor of educated risk taking as long as you don’t hurt someone else…that is why I am opposed to totally disabling burst discs, but that is a different subject. Using a higher pressure burst disc is kind of a reasonable compromise (I have done some high temperature structural calculations on the subject).
Be aware that hydro test only tests for one thing, material elasticity (yield strength limit). It does not test for cracks, pits, or any other defect. Those are hopefully caught during a visual. I have seen both aluminum and steel tanks pass hydro with a crack on the inside of the neck. That is one reason we have safety factors…hydro tests and inspections are not always perfect.
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ucfdiver
Contributor
There have been cases well documented where burst disks have failed underwater and in overhead environments. I'm not a risk taker, so I'm not all excited about leaving a failure point in my primary air supply. Isn't the US the only country that requires burst disk anyways?
You're saying it's a risk, but can you mention ONE SINGLE ACCIDENT that ever happened because of it? I spent hours and hours of searching before I ever asked to get a tank overfilled, and couldn't come up with anything.
I have heard of one case were a burst disc did let go underwater. I don’t know any of the details, but would like to know more about it. IMO it must have been a very marginal disc with poor maintenance to start with. There is no real trigger to induce a burst disc failure underwater…the only possibility that I can think of is a sudden change in temperature.
They don’t use burst disc in some European countries, but my impression is that they are not as concern about fires as we are. I don’t think they have as many wooden houses and buildings as we have. I may be wrong, but I think they use a lot more cement and bricks. I know in Puerto Rico where I grew up the houses are made out of concrete and I can only recall seeing one fire station in the San Juan metropolitan area. I am sure there are more in a city of over 3 million, but in general fire is not a big deal.
I am not aware of any single documented accident that can be traced to multiple overfilling cycles, but if you think that the relatively short previous history of overfilling is a guaranty that an accident will never happen…you are dreaming. It may never happen or it may never happen in our life time, but there is a big reason we do calculations to try and predict events that have never happen before.
If we relied purely on previous history most engineers (including myself) and many other professions would be out of a job. If everything that happens tomorrow is the same as yesterday…well you get the point.
We have only being building high pressure cylinders for about 100 years, but as I mentioned, they are structurally very predictable, if they have no major flaws. The limited previous history of overfilling (or even extensive history) is no guarantee of future events. It just gives you a nice warm false sense of security.
I can guaranty you the odds (the probability) are very good that it will not fail…so do you fill lucky today?
They don’t use burst disc in some European countries, but my impression is that they are not as concern about fires as we are. I don’t think they have as many wooden houses and buildings as we have. I may be wrong, but I think they use a lot more cement and bricks. I know in Puerto Rico where I grew up the houses are made out of concrete and I can only recall seeing one fire station in the San Juan metropolitan area. I am sure there are more in a city of over 3 million, but in general fire is not a big deal.
I am not aware of any single documented accident that can be traced to multiple overfilling cycles, but if you think that the relatively short previous history of overfilling is a guaranty that an accident will never happen…you are dreaming. It may never happen or it may never happen in our life time, but there is a big reason we do calculations to try and predict events that have never happen before.
If we relied purely on previous history most engineers (including myself) and many other professions would be out of a job. If everything that happens tomorrow is the same as yesterday…well you get the point.
We have only being building high pressure cylinders for about 100 years, but as I mentioned, they are structurally very predictable, if they have no major flaws. The limited previous history of overfilling (or even extensive history) is no guarantee of future events. It just gives you a nice warm false sense of security.
I can guaranty you the odds (the probability) are very good that it will not fail…so do you fill lucky today?
ucfdiver
Contributor
The chance is greater that a cylinder will fail at 2650 than 500psi as well, where do you draw the line?
As for the burst disk, there's several accounts on TDS where people have posted about it, a quick search will reveal them.
As for the burst disk, there's several accounts on TDS where people have posted about it, a quick search will reveal them.
While I predominately get all my fills in FL cave country, let's not forget that there is a whole big world out there and it doesn't revolve around cave country.
Personally, I think overfills are a matter of your risk tolerance. For some with a high risk tolerance, no big deal. For those with a lower risk tolerance, maybe more of a deal. No preference is right or wrong, it's a matter of what you are willing to accept.
No different than most of diving. Some won't even consider overhead environments, while others won't even consider low viz, etc etc.
Well the lines are drawn at different levels for different type of designs depending on many factors including: predictability, risk, etc.
For example for steel construction based on AISC (American Institute of Steel Construction) codes it would be 60% of yield (for allowable working stress), but that is a very short answer since it can vary to 66% depending on a number of factors.
For life support equipment it is often taken as a 5 to 1 safety factor again depending on types of materials and how predictable is the material and geometry.
For DOT 3AA cylinders (per CFR49section 178.37) it is a specified 70,000 psi max stress based on a specific equation per the code. Pressure vessels built under this particular section of the code use a well specified family of materials.
There are also codes from ASME (American Society of Mechanical Engineers) also have a different criteria for stationary pressure vessels.
How conservative a safety factor is will depend on risk and predictability among other factors. At times it may seem as it is arbitrary, but it is not intended to be.
Believe or not, this is just a short answer to give you just an idea that these criterias are not just trivial.
There is a lot of work behind the codes, regulations, and even just good engineering practices. And yes there are some errors and mistakes they are all written by humans.
Do you just Google Scuba burst disk failure or is there a particular place or words that will get me there faster?
A burst disc can fail for a variety of reasons.
It may be installed incorrectly in the assembly, it may be corroded, it may be old have seen many many cycles and be fatigued, it may have been removed and re-installed. All of the above can weaken a disc and or cause it to fail. In rare cases, the disc is defective and recalls of burst discs are not totally unknown.
In general though if you replace the burst disc at every hydro test, you will never see a burst disc fail in service. 95% of the time, when a burst disc does fail, it fails during the tank fill as the pressure increases to a point near the service pressure. It is very uncommon for one to fail during a dive where the tank pressure is falling - not unheard of but very uncommon. It is not a big deal on an air fill, but is a real pain in the wallet on a trimix fill - although you can close the isolator and save half the gas, just like you would in the water.
If you are really concerned, replace them annually with the VIP, they run $7 to $15 and that is small potatoes for a technical diver and a lot cheaper than paying 150% of the normal price of a trimix fill.
It may be installed incorrectly in the assembly, it may be corroded, it may be old have seen many many cycles and be fatigued, it may have been removed and re-installed. All of the above can weaken a disc and or cause it to fail. In rare cases, the disc is defective and recalls of burst discs are not totally unknown.
In general though if you replace the burst disc at every hydro test, you will never see a burst disc fail in service. 95% of the time, when a burst disc does fail, it fails during the tank fill as the pressure increases to a point near the service pressure. It is very uncommon for one to fail during a dive where the tank pressure is falling - not unheard of but very uncommon. It is not a big deal on an air fill, but is a real pain in the wallet on a trimix fill - although you can close the isolator and save half the gas, just like you would in the water.
If you are really concerned, replace them annually with the VIP, they run $7 to $15 and that is small potatoes for a technical diver and a lot cheaper than paying 150% of the normal price of a trimix fill.
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