Overpressurizing / Overfilling steel tanks

[John C. Ratliff]

Someone above said:

So you agree that following the industry standard (it's a voluntary standard, not a regulation) of doing an annual VIP is important as is following the regulation requiring a hydro test every 5 years. However, I need to clarify this, you feel it is OK to ignore the regulation limiting the fill pressure to what is stamped on the shoulder plus 10% if the tank is plus rated just because they fill similar tanks approved in europe under different engineering standards to higher pressures? Sounds like picking and choosing your saftey rules to me.

DOT regulations are regulations, not voluntary, consensus standards. This means that it is illegal to transport pressurized cylinders across state lines which are deliberately overfilled. Let me give you a scenario:

A dive vehicle is transponting diving cylinders with a 40% overfill from New Jersey to New York for a dive (it could be Oregon to Washington, it makes no difference), and has an accident. That accident results in a fire, which the fire department responds to. They do not know you have a pressurized cylinder in the vehicle that is on fire, and go to put it out. This cylinder, in addition to having been overfilled, has had it's burst disc tampered with too. In the fire, the cylinder fails catestrophically, and the resulting explosion throws chunks of car and cylinder into the fire truck less than 100 feet away. A fireman is killed from flying debries. The driver, who is also an instructor and the diver who filled the scuba cylinder, was very seriously injured, but not killed. He will require years of rehab to begin functioning normally from his injuries.

An investigation begins. If the dive vehicle is a private vehicle, than probably the best they could do is to go after the people on a liability suit. The fire department's worker's compensation carrier would file the suit as a third party action, against the divers personally. What they would be able to get, who knows, but they would try.

Now let's say that the vehicle is owned by a dive shop, and a dive shop employee not only is the driver, but also an instructor hired by the dive shop to train students at a remote dive location. He is also the person who filled the tank, and tampered with the burst disc, and he did this either on instructions from the dive shop owner, or with the dive shop owner's knowledge. Here, we have a new ball game for the insurance company, and for the victim's family.

Because the dive shop owner knew about the overfills, and these led to a catestrophic accident, he could be both personally liable (possible criminally, I'm no lawyer, but I think that's within the realm of possibility), and subject to an OSHA citation for willfully disregarding OSHA pressurized cylinder regulations (yes, OSHA does have these regulations). The dive shop operator faces hundreds of thousands of dollars in claims costs for the compensible injury to his instructor. He also faces a possible third party suit from the fire department's worker's comp carrier, and potentially from the fire department's liability carrier too. After all this, the dive shop probably would have extreme difficulty getting workers comp and liability insurance, and may go out of business.

This is the type of situation you face when you deliberately overfill the tanks. It is not only limited to the tank blowing up in the shop, and causing the cylinder to weaken and not pass its next hydro. You face some potentially devistating legal action (at least in the USA) as a possible consequence.

Remember, whenever there is an employee/employer relationship, OSHA kicks in and does have regulations which cover these things (I'm about to start another thread about this aspect of technical diving, which I don't think most technical divers are aware of). Last summer, the Boy Scouts of America lost a scout when a cannon they were firing blew up. Oregon OSHA did fine the BSA just over $1000 for the accident, citing information that they should have known about, and a lack of training of the adults present (it was a high school kid who was killed), using the wrong gunpowder, using the wrong method of firing it off, and using a cast iron cannon (which is really made for ceremonial events, as opposed to actual firing) as the reason for the fines. That's not much for a death; I don't know whether there was any liability action going on, but that's usually done independently from any OSHA action.

I could see a similar line of reasoning for anyone deliberately overfilling air cylinders. This is just one possibility; there are many more.

By the way, if you really want to know the reason for the actual pressure rating on a cylinder, ask the manufacturer. These are performance regulations, and the manufacturers are required to have this data available to OSHA, DOT and others to justify their pressure rating. If there is a difference between two seemingly similar cylinders, again ask the manufacturer. They could be a different metal composition, different heat treatment, or they may be the same and complying with different regulations (as some have said earlier).

In any case, it is never o good idea to deliberately overfill a cylinder. You are then taking all the liability of that action on your own. The manufacturer will disown you in a liability situaton. Saying "everyone's doing it" doesn't cut it either, legally. You are responsible for your actions, and no one else will accept any liability for a product that has been deliberately abused.

For those who say they need the extra air, I say your dive planning needs better attention. I've heard dive shop personnel say that today's divers are sometimes so out-of-shape that they need huge cylinders simply to stay down the equivalent time as an in-shape diver with a regular cylinder. "I need a bigger cylinder" is the response, rather than "I'm really out-of-shape and need a personal fitness program" such as swimming a mile or two, three times a week.

SeaRat

 

[DFC5343]

Excellent. Your patience proceeds you.

 

[MikeFerrara]

:doctor: You know it just amazes me that people think engineers are wrong. Yeh I have no problem noting that the Titanic was built by professionals and Noah's Ark was built by a amature. (Granted that Noah had some divine plans and supervision to work with) There is a reason that the fill pressures are placed on cylinders and why we use a different standard here in North America than in Europe. [/B]

Engineers are often wrong especially when they're doing what an MBA tells them to do. Also they usually know it's wrong when they're doing it.

Actually PST told me that they tried to get the old tanks approved for higher pressure but they failed the DOT specified tests. The question is though how many cycles is the tank good for in actual use.

As an engineer I can tell you that testing often has little to do with how a device oporates in actual use. One of the things some engineers spend lots of time doing is argueing test specs with QA and sometimes even legal people.

While overfilling might cause a tank to have a shorter life (fail hydro sooner?) some might be able to accept that. Is the failure mode any more dramatic? I think not? Do these tanks blow up when they fail hydro? I don't think so.

But since we are looking at what overfilling a tank does and the risk involved what about looking at what your reg is doing when that cylinder is overpressurized. The reg is only set up for optimal performance at 300 bar. Excessive pressure on the seat places the reg into a potential failure possibility. I have yet to see where overfilling a cylinder to such high pressures has benefited the diver. I have yet to turn a dive using my hp 120's, my e-series 120's or even my 98's all filled to their rated psi when diving with people who overpresure their tanks. The reg will not perform as it should against that kind of pressure. Your attempt to dive longer using higher pressure fills decreases the regs performance and only causes the diver to breath worse. Maybe I'm just that good on my SAC rate? You want more dive time carry more tanks!

What is your RMV? I'll bet that if we do a dive and my 104's are pumped to 3500 (275 cu ft of gas) and you're 98's (almost the same as 104's) are pumped to 2640 (196 cu ft of gas), you'll turn the dive.

Also we'll do the dive in a Missouri cave on trimix. You'll have to carry all the gas you need because there isn't an air fill for 100 miles and no trimix fill for 300 miles. Oh, on a week end the closest fill is my house in Indiana period. LOL

You could just use an extra stage (for a total of three along with three bottles for decompression) but whatever tanks you carry need to be hauled down the side of a mountain (well a huge steep hill) to and from the site. You also need to carry them int he water and you'll notice each extra tank when you get in that flow.

We don't have any trouble with reg performance and we don't need to service the regs more often either.

 

[Rick Murchison]

What I find interesting in this thread is that the same arguments for overfilling tanks support decompression on 80% vice 100% oxygen - but many of the same folks who use the overfill instead of "just carrying bigger tanks" are just as vehement that "just carrying bigger tanks" is the solution to having that 100% deco gas.
Yep...
Interesting.
Rick

 

[MikeFerrara]

What I find interesting in this thread is that the same arguments for overfilling tanks support decompression on 80% vice 100% oxygen - but many of the same folks who use the overfill instead of "just carrying bigger tanks" are just as vehement that "just carrying bigger tanks" is the solution to having that 100% deco gas.
Yep...
Interesting.
Rick

I use a size tank for doubles that I can manage as far as carring, size and trim. Double 121's or bigger aren't going to work for me.

I can, however handle an AL 80 for decompression (or several of them) easily.

I think the most sensible argument against 80% is that the other 20 is nitrogen which is one of the gasses we're trying to get rid of on ascent. And of course I know that on some dives according to some tables switching to 80% at 30 gets you out faster that 100 at 20. We also know that the models don't have everything nailed down.

 

[GDI]

Engineers are often wrong especially when they're doing what an MBA tells them to do. Also they usually know it's wrong when they're doing it.

Actually PST told me that they tried to get the old tanks approved for higher pressure but they failed the DOT specified tests. The question is though how many cycles is the tank good for in actual use.

As an engineer I can tell you that testing often has little to do with how a device oporates in actual use. One of the things some engineers spend lots of time doing is argueing test specs with QA and sometimes even legal people.

While overfilling might cause a tank to have a shorter life (fail hydro sooner?) some might be able to accept that. Is the failure mode any more dramatic? I think not? Do these tanks blow up when they fail hydro? I don't think so.

What is your RMV? I'll bet that if we do a dive and my 104's are pumped to 3500 (275 cu ft of gas) and you're 98's (almost the same as 104's) are pumped to 2640 (196 cu ft of gas), you'll turn the dive.

Also we'll do the dive in a Missouri cave on trimix. You'll have to carry all the gas you need because there isn't an air fill for 100 miles and no trimix fill for 300 miles. Oh, on a week end the closest fill is my house in Indiana period. LOL

You could just use an extra stage (for a total of three along with three bottles for decompression) but whatever tanks you carry need to be hauled down the side of a mountain (well a huge steep hill) to and from the site. You also need to carry them int he water and you'll notice each extra tank when you get in that flow.

We don't have any trouble with reg performance and we don't need to service the regs more often either.

:doctor: Mike maybe I'm sure you are right if I use my 98's. But I would most likely do that dive with my 120's @3500 psi rated. I'm lazy when I'm in the water. My RMV on dbls's is just over 0.5, I use 0.8 for conservatism.

 

[Norm]

Wasn't it Bill High's company that said overfilling LP steels, to the pressure that most of us do, will shorten the fill cycle of a typical tank to failure by Hydro testing from 30,000 cycles to 10,000?

I say, who's gonna do 10,000 dives on the same tank?

Very few.

Double disk them, and have at it. OK OK, I mean REASONABLY have at it. Say, 3400 - 3500 PSI tops.

Some guys are touching 4000 PSI, now that scares me a tad, even though I'm reasonably sure it ain't gonna blow.

Of course, if you need more gas, get a bigger tank?

OR

What do you need that much gas for in the first place?

All valid questions.

"In the good old days......." I have seen many of the old 104's double-filled. I have also seen aluminum 80's regularly filled to 3800 PSI. It is not snake oil, if you will. I DO NOT advocate it but it has/can/will be done. This is another reason we are so lucky to live here in the good old USA where safety factors are so large.

 

[d33ps1x]

I wasn't trying to start a major argument on here. My apologies if this thread caused any more bad feelings than there obviously already are. We'll settle those arguments with split fins at 30 yards tomorrow morning.

This would happen even if you hadn't asked a question...It's why the board entertains.

 

[budgy]

In the UK your not supposed to overfill a cylinder, the US dot system that allows 10% overfilling doesn't exist here. Sometimes places overfill by around 10% its unlawful, but many do it. I'm told rightly or wrongly, overfilling by over 10% permanently stretches the cylinder. On a fill, cylinders heat up even on a slow fill or a decant. Some shrinkage in pressure as the cylinder cools down is normal, I often see 20/30 bar lost, I think a number of shops fill to 250 bar instead of 232 to cover complaints of low fills. As to effeciency or effectiveness of higher pressures and particularly with regards to 300 bar cylinders I seen some information that suggests your no longer getting linear increases in volume when the pressure is over 200 bar.

 

[DA Aquamaster]

Compression of various gases is not exactly linear. O2 for example is more compressible than nitrogen between 2000 and 4000 psi. Consequently for maximum accuracy you have to correct for this increased compressibility when mixing nitrox in a tank with a residual pressure over a 1000 psi or so.