Overfilling LP Steel Tanks -- How bad is it?

[FritzCat66]

If overfilling is really as safe as everyone says (and I don't necessarily disagree about that), then it should be a simple matter to get the tank manufacturers to release an official statement to that effect, right? Faber, Worthington, et al just need to come out and say that tanks may be overfilled by such-and-such percent over the DOT rating for personal use.

Then instructors could happily practice overfilling without all the slippery-slope issues I brought up - because they would essentially still be "in spec".

Otherwise, the argument for overfilling is still just a version of, "everyone violates spec, so it must be OK", which is alright for an individual, but pretty weak, lame, and unacceptable for anything considering itself a professional organization.

Why would the tank manufacturers NOT do such a thing?

 

[Gombessa]

If overfilling is really as safe as everyone says (and I don't necessarily disagree about that), then it should be a simple matter to get the tank manufacturers to release an official statement to that effect, right? Faber, Worthington, et al just need to come out and say that tanks may be overfilled by such-and-such percent over the DOT rating for personal use.

I heard one of the manufacturers (Faber?) tried doing just this a few years back, and got smacked back to the stone age by DOT.

Then instructors could happily practice overfilling without all the slippery-slope issues I brought up - because they would essentially still be "in spec".

Without getting into what is or isn't official policy with any agency, I think what you are missing is the fact that dive agencies are not there to enforcement other people's standards. It's not their job to say "Worthington rates these tanks for 2640psi, so we won't let anyone fill higher than that" or "The US Navy tables only allow for xx depth for yy minutes, so none of our instructors may exceed such profile." They set their own standards based on a number of criteria, including goals, acceptable safety, revenue, whatever. Some of these standards can be much more conservative than what is "allowed" in the industry or regulation (limiting recreational depth to 100ft, for instance), and others may be more liberal, based on experience, differing practices, contravening data, etc. If you want to talk slippery slope, you should be looking to agencies that break or fail to be internally consistent with their own standards, not ones that have reasoned policies contrary to the norm.

 

[Cave Bum]

What I'm wondering is how large a sample size is the number of these 150% Florida cave fills, both in number of fills, and in the total number of tanks that have been regularly exposed to these conditions over all of recorded history. How does this compare to, say, the number of 6351 tanks? One possible way to estimate is to compare the number of Florida cave divers to 'regular' OW divers, and presumably, there are orders of magnitude (thousands) of the latter for every one of the former. There may be other ways. Thoughts? WAGs?

When I worked in a fill station down here in North Florida, we were pumping about 20,000 cu ft of nitrox a month and 8,000 cu ft of trimix. Since tanks usually come back with a third of their gas in them that equates to around 150 fills a month. There are probably about 25 shops in North Florida (that's just a guess) doing these fills with varying monthly volumes. If the average is (and this is another guess) half of what we were doing that would still be amost 22500 fills of doubles per year. Additionally, we have a lot of folks with their own compressors.

This has been going on since the mid 80's but the popularity of cave diving has increased greatly in recent years, say since around 2000 when "The Last Dive" came out. Still, I'd guess we've seen 10 years at the higher rate and 20 years at maybe a quarter of that.

Another piece of the puzzle is that most people don't bring doubles down here, they rent them (it's only $10/day, for gosh sakes!) so the tanks that have been seeing these overpressures are the same small group of tanks for the most part.

 

[Anti-Hero]

I regularly get my double LP85's filled to > 3400 psi. which is why I went with LP tanks. My fill guy had me replace the LP burst disks when I bought the tank, with 5000psi rated ones.

 

[FritzCat66]

Poking around prior threads on this issue, I found a comment from a tank vendor in a similar thread back in 2006 that seems very germane:

As I stated in my prior post: They are no more safe or no more unsafe to pressurize beyond 2640 psi (2400+ psi) then any other 3AA 2400+ psi scuba cylinder.

None of the DOT3AA scuba cylinders are "overbuilt" beyond the DOT 3AA requirements. It is a myth that any manufacture's cylinders are actually "high pressure" cylinders with an underrated working pressure. Why would any manufacture do that? If that were the case, I would have the cylinders stamped with a higher working pressure (increasing the cylinder capacity and value) or use less steel & reduce my cost.

Is your argument, that your knowledge of past experience of overfilling is greater then the knowledge of DOT regarding safety margin and high pressure vessels? If that is the case why not grossly overfill 3AL (aluminum) cylinders? Why not grossly overfill cylinders with a DOT exemption? All scuba cylinders manufactured to meet DOT specifications have essentially the same safety margin.

My point is simple, we manufacture high pressure cylinders designed to meet the DOT requirements for a working pressure of 3442 psi. These are not low pressure cylinders which we have stamped with a higher working pressure. These cylinders are designed and manufactured with a high tensile steel alloy to meet the DOT requirements of 3442 psi. If you want a cylinder that is safe for high pressure, then purchase the cylinder that is designed for that purpose.

So, there you have it from the manufacturing side. Rated pressure is rated pressure, it's what the cylinder is designed for. Go over rated pressure, and you're exceeding the specs. If you do this, you're saying you know more than the DOT and the manufacturer about cylinders and cylinder safety. If you exceed specs for one kind of hardware, why not any other?

Professionally, you just don't exceed manufacturer's specs for equipment, regardless of who else is getting away with it - even if you get away with it just fine on your own. I know in my industry (medical imaging systems), training people to routinely exceed manufacturing specs would definitely lead to lawsuits. I am surprised fill shops who regularly overfill LP cylinders don't get tagged for that on their insurance.

And yes, I still find it ironic and somewhat hypocritical that any agency that would tend to focus on technical diving, diving well within their own established limits like their rules of thirds, etc. would nevertheless turn around and advocate blatant disregard for manufacturer's rated limits, condoning regularly stressing a piece of critical diving hardware well beyond rated capacity.

 

[Gombessa]

So, there you have it from the manufacturing side.

XS Scuba isn't a manufacturer, and they don't design or make tanks. They do distribute and sell them though, so from a liability perspective of course they (as well as Worthington Cylinders, the actual manufacturer) will not condone exceeding the rated pressure. If you want to be talk potential liability, that's a different thread. But here, I'm sticking with your original arugment about how agencies are allegedly on some slippery-slope towards lax and loose standards becasue they don't follow DOT-required stamps on tanks.

In 95% of the circumstances we face in everyday life, I'd agree with you--there's no need or real benefit to exceed the manufacturer's specifications. However, there are circumstances where you can do so with a high degree of confidence as to safety/reliability, and in those cases, we sometimes know that the rated specs are extremely conservative.

Look at CPU overclocking. We know that a single CPU design and production run is manufactured, tested, and binned for numerous speeds and heat tolerances to price at different points. We know that oftentimes, CPUs easily exceed the top speed at which they are binned. We also know that oftentimes, demand for lower-cost, lower-speed processors result in high-speed CPUs only being tested and binned at the lower speed in order to meet that demand. We also know how to deal with voltage regulation, proper cooling of CPUs and thermal monitoring of CPUs. So hardcore PC enthusiasts (a bit like cave divers as compared to Joe Blow computer user, right?) regularly overclock, with an extremely high degree of success and extremely low rate of catastrophic failure. Do we KNOW more about these chips than Intel or AMD? Of course not, but we know more than what they tell us. And for entire classes of chips, we know that the number stamped on the front is not an indication of maximum capability of the processor.

And it even seems that just as the CPU makers have toyed with the idea of selling unlocked CPUs to allow the enthusiast market to overclock, some tank manufacturers have tried to hint at the true capabilities of their cylinders; but there's no federal authority regulating the maximum clock speed of chips, so the results are different.

You never need to overclock your computer if you don't want to. But I'm sure you can understand that there are people out there who really know what they're doing, and have been doing it for a lot longer than you or I, and just because they're exceeding the stated manufacturer's specifications doesn't automatically mean they're doing so in a cavalier or slipshod manner.

 

[Mark Michaud SELAUSAR]

So I live in Florida & I've noticed my LDS routinely overfills tanks...I wanna buy something like an LP80/85 to be overfilled. My question is..how bad/dangerous is it really to do this?

:focus: Back to the original question...........I think it has been reasonably answered that it is a common practice with no real-world proof of danger. Is it legal, more than you personally are willing to do, or more liablilty than you wish to accept? That is up to each individual to answer for themselves. For those like myself, I will fill my tanks to cave fills and carry them home without filling after a dive and that is the end of this. I personally don't store mine at a higher pressure, mostly for convenience. If you feel this is not in your comfort level, than don't do it.

Some of the arguements made about DOT standards, manufacturer rating, etc., is similar to bulletproof vests. They have a 5 year date of "safe usage". That is because the manufacturer will be ultimately responsible (deepest pockets) if it fails. Even getting wet through sweat or water, the manufacturers feel comfortable that they will withstand five years without failing. Most will go twenty years without failing but there is a date due to liability. Personally I feel the stamped pressure on steel bottles have the same reasons, and that seems to be proven by the years of fills.

The bottom line is that we have to each decide what benefits outweigh the risks and live accordingly. :focus:

 

[FritzCat66]

OK, this is fun: let's run with the CPU overclocking analogy - because I think that is a very good one for these purposes.

I have certainly overclocked both my GPU and CPU - that is, pushed them beyond their rated manufacturer's specification - in order to get increased performance out of them. Regularly, even. I have helped my friends do the same with their personal machines. Everybody is fine (well, with chips you very often push them beyond the breaking point and then back down, so the analogy isn't perfect for scuba cylinders - I hope! - but for sake of the analogy let's presume we don't push them that far).

However, in my official position as an imaging systems manager, when I'm training my techs to setup and deploy computer systems to be used in a professional capacity - to read and interpret medical images in my case - I most certainly do NOT train them to overclock those CPUs and GPUs beyond their rated spec! Even if I really know that they probably could be pushed to gain performance. Furthermore, I would go so far as to discipline a tech whom I discovered was clandestinely overclocking those professional systems.

But of course if he bought an identical model and took it home, I wouldn't mind him overclocking - in fact, I'd likely join him for some after-hours gaming. This by itself is not hypocritical - the crucial difference being between the professional world and the enthusiast world.

But a professional dive instructor is in her professional world when instructing - even if her students are non-professional enthusiasts. Training students to disregard manufacturer's specs is irresponsible, whether it's a dive instructor training students on the proper setup of their scuba equipment or a systems manager training techs on the proper setup of their computer equipment.

What would be hypocritical for me as a systems manager would be to train my techs to stay in spec, then wink at them while we crank up the GPU. I think it would also be problematic to be part of a professional organization that advocated that practice - I would protest that either the official position needed to be brought in line with the manufacturer's specs OR that the manufacturers open up their specs to allow for pushing the performance of their units (and I might use the collective power of my professional organization to push for just that).

So, this analogy dramatizes my point that scuba instructors should not promote a disregard for manufacturer's specifications by overfilling their tanks as part of regular training. At best it implies that the instructor does not recognize the line between professional and personal behavior; at worst it trains the students to be two-faced about understanding and honoring professional, rated hardware limits placed by the engineers who design, build, test, and spec out their hardware the way they do for good reason - that is, so every piece of hardware shipped is guaranteed to perform at least as well as specified.

 

[Leadking]

Let me explain a little about the "overbuilt" theory. First, every cylinder design, and particularly E cylinders, are designed to fail. They are designed to fail by splitting and not fragmenting or causing a "catastrophic failure". If a cylinder is "overbuilt" a manufacturer is adding more metal than required and one would expect to see a very heavy cylinder in comparison to others of the same manufacturing process. In the case of E cylinders, they MUST be tested until failure and must fail by splitting.

I have spent more time than 99.9% of the posters on this forum asking the questions that appear here of cylinder engineers and then posting detailed and accurate information. You have never seen me write "I think" or "I believe" when it comes to these issues. I worked diligently to give the diving public accurate information that they could take to the bank. Posters who state anything otherwise are doing so so that the "rules" now conform to their opinion.

 

[Gombessa]

I'd like to respect the request to get this discussion back on its intended topic. So I'm just going to end my part by saying that it seems our definitions of "standards" are different. If you'll allow, yours seems to be, "Do your professional job by adhering strictly to all manufacturers' guidelines; that way you're more likely to have a better-functioning, lower-failure system that operate within warrantied design limits, and can rely on such for purposes of liability or failure management."

The numerous cave agencies' standards are more like, "Regardless of what a manufacturer may claim, leverage the benefits of independent experience, history, and knowledge of design and manufacturing limitations to satisfy the highest levels of safety and functionality."

The fact is, sometimes this will be more conservative than manufacturer standards (e.g., someone replaces valve o-rings before every dive), sometimes this will be more liberal (overfilling tanks, going more than a year between regulator inspections), but it doesn't mean that the stated standards are low, slipping, or ignorable.