Let me just say that I have no issues with some degree of over filling, as long as the wall stress is below the material yield strength. All you are doing is eating into the safety factor. How much safety factor is really needed is the only real subject for debate.
I am just going to try to explain few facts. Not intended to change anyone's practice (maybe just to be a bit careful on how they do it).
DOT cylinders are designed for transportation over highways, railways, aviation and marine situation. An over the highway accident is not the most severe secondary load that a cylinder can experience. Railway or aircraft accidents can much more severe, but under the right condition over the highway could be bad.
The stress due to pressure is very predictable. A cylinder as a pressure vessel is a very simple structure and the stresses can be easily calculated. What is not as predictable is other secondary loads like those encountered in a transportation accident.
There are also several other reasons for the safety factor. Structure and events are not always perfect and predictable. I will expand on some of the other risks when I have more time.
Assuming that there is absolutely no risk is a total fallacy. Even when filling to the stamped pressure there is always some risk. The risk may be very small and therefore very acceptable, but risk is not binary (on or off). It is all about degrees of risk and what the individual is willing to accept.
The fact that no incident has been documented to this date just gives an indication that the risk may be low, but by no means does it predict the future. Remember that anything that has ever happen, always had a first time. Just because we have not seen a first time yet it doesn't guaranty that it will not happen.
We have some very good experience and data from the cave fills in Florida, but extrapolating that into saying that we have a huge experience is not accurate.
The number of cycles most scuba tank experience are always very low. Even rental tanks in the Caribbean only see about 360 fills a year. In ten years that is only 3,600 cycles. It is not the same, but just to give a little perspective, that is roughly the number of cycles an engine or compressor cylinder sees in one minute (running a 3600 rpm)
If someone are assuming that there is absolutely no risk, that person is either delusional, ignorant, or a combination.
When I have a lot more time I will be glad to write about all the reasons fro the safety factors, about the levels of risk, and most important about risk mitigation.
Just a quick advice. If you are going to over fill:
- Try to keep a margin below the low yield strength (below hydro pressure, see note)
- Inspect for corrosion a lot more often (I would recommend 6 months)
- Always make sure your tank is very dry inside and be extremely careful that moisture never gets in. Carefully dry the connection (both sides) before filling. I will expand a lot more on this later.
- Be very careful when transporting cylinder. Never overfill if you are going to transport on private aircraft (personal experience story later).
- Always restrain the cylinders or make sure they do not fly of the vehicle when transporting on the highways.
- Be extremely careful with potential exposure to heat or fire. Specially with compromised safety burst disc. The temperature inside a vehicle in Florida can be an issue. I can expand a lot more on this later (with facts and numbers)
More later.
Note: there is a huge difference between filling a 2400 psi cylinder to 3500 or even 3600 psi, that to filling them to 4000 psi (or higher). I can expand with a lot more of the physics behind this another time.
A quick comment about rust. The study (I think it was University of Rhode Island) showed that under the right conditions (water inside and under pressure) a new cylinder (with good hydro and VIP) can rust enough to compromise its structural integrity in as little as six month. The test was extreme and can be mitigated, but it brings one of many possible scenarios that shows the potential risk.
Under the right condition, rust can happen a lot quicker than most people realize and it is accelerated with the higher pressure, but it can be mitigated if you are very careful.
I am totally supportive of risk taking. I have jumped out of airplanes, flown ultra-light aircrafts and sailplanes, done a lot of white water kayaking including class 5, done fair amount of rock climbing and some ice climbing, just to mention a few... I even rode a motorcycle on the highways with cars around for a while (now that is
But my point is that there is risk on anything and knowing the risk is important, mitigating the risk is the only smart thing to do.
Taking calculated risk is not only acceptable in some cases can even be an adrenaline rush. But taking risk under the illusion that there is not is right down...
I can easily expand with a lot more facts and the physics of pressure vessels when I have more time, if anyone is actually interested.
