Overfilling LP Steel Tanks -- How bad is it?

[bleeb]

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.

So as a rough WAG, that's a lifetime total of 337,500 commercial fills for North Florida, with an unknown number of 'private' fills, but which may be within an order of magnitude either way. While the privately-owned fills are probably almost entirely privately owned tanks, which means filling the same tanks over and over again, the commercial fills are split between privately owned tanks and rentals. The latter possibly see more use and thereby accounting for a larger fraction of fills, biasing downward the total number of tanks in the population. So, at least for commercial fills, it sounds like the number of unique tanks is maybe at most somewhere in the five figures, possibly towards the lower end. This would be considerably below the number of tanks it took to see a catastrophic failure of a 6351 tank (roughly one in a million tanks, and likely tens of millions of fills or more).

 

[H2Andy]

random thought:

if you're seriously worried about the risk of an overfilled tank getting medieval on you, then the mere thought of getting in your car must turn you into a quivering mass of jelly

 

[FritzCat66]

I'm just going to end my part by saying that it seems our definitions of "standards" are different.

Yes, I think you have summarized the two positions succinctly and will also let this go now, but I leave with this warning:

Ultimately, I think the risk of consistently and routinely overfilling LP steel tanks has nothing to do with damage or injuries from exploding tanks. No, the risk is more likely from sharks. Yes, the kind of well-suited sharks even freshwater, deep cave divers fear.

Let's say a cave diver with overfilled tanks dies for some reason in back of some cave. The family wants blood - they don't care if the diver screwed up, they just want someone to PAY, and they can afford exactly the kind of shark who is good at getting people to pay. It comes out in the investigation that the deceased diver had overfilled tanks. So they ask the shop, "Did you knowingly and willingly overfill his tank?" And they have to answer, "yes" (I mean, the service pressure is stamped right on the tank, they can't "not know" they're overfilling). Great - now the shop pays, whether or not the rest of us think that's right, because I'm sure lawyers can construe overfilling a tank as aiding and abetting a diver to go beyond normal limits or something. Probably a shutdown shop afterwards, if not then I'll bet at least they won't be overfilling anymore.

If the shop says, "Overfilling beyond spec is standard practice advocated by such-and-such cave dive agency that trained the deceased diver." OK, now they sue the agency and ask, "Is it true you knowingly and willingly train divers to use tanks filled beyond the manufacturer's listed spec?". I'll bet they won't be saying "yes" to that question very often.

They're going to ask the manufacturer if they condone overfilling and of course they're going to play stupid and say, "Gosh, no! We had no idea people were being so naughty with our tanks! We would never support filling beyond our listed capacity."

Legally, as a business or organization it's just a very bad move to openly advocate pushing hardware beyond published specifications. It really opens you up to essentially an indefensible position when it comes to lawsuits. I'm rather surprised this hasn't already happened regarding the overfilling issue.


But... that's a problem for the dive shops and agencies to deal with. As for the OP and the rest of us individuals overfilling; sure, go ahead while you can get it!

>*< Fritz

 

[tc]

 

[Greg Barlow]

Legally, as a business or organization it's just a very bad move to openly advocate pushing hardware beyond published specifications. It really opens you up to essentially an indefensible position when it comes to lawsuits. I'm rather surprised this hasn't already happened regarding the overfilling issue.

If we follow this logic then every motor vehicle sold in the country should have a speed limiting device installed that receives a GPS signal. That signal could then allow "Big Brother" to determine what the maximum legal speed is for that stretch of road's coordinates.

Heaven forbid that we start discussing "doubling-up" burst disk, or the thought of using stamped steel shims rather than the copper alloy disks...

As far as I'm concerned, I'll continue filling my LP steel tanks to the "needed level" for the dive's plan. Meanwhile, I think I'll go wind my Mazda out to around 7,000rpm on a long, deserted stretch of straight road...

Greg Barlow

 

[D_B]

note ... your tires speed rating changed and/or speed limiting software was installed on vehicles when manufacturers decided putting 85mph rated tires on vehicles that could do over 100mph was a bad idea from a lawsuit perspective, not from any tire durability standpoint

 

[RAWalker]

Ever wonder why it is a DOT spec and certification?
It's really very simple and has nothing to do with the tank exploding while some diver is submerged.
It is so in the event that during transport on our roadways the cylinder happens to be in a vehicle that is involved in an accident it doesn't act like a bomb.
There really isn't anything else that should be read into the spec or cert. The pressure rating is for the material the tank is made of to behave in an expected manner if it is crushed or punctured while pressurized.

 

[JDMerk]

Great - now the shop pays, whether or not the rest of us think that's right, because I'm sure lawyers can construe overfilling a tank as aiding and abetting a diver to go beyond normal limits or something.

>*< Fritz

WOW that's just plain nutz, by that flawed logic you would assume that selling someone a HP130 vs AL80 would open up the shop to litigation based on the additional capacity, and that is obviously not the case. Your attempting to turn this into something its not.

I thought you were going after the "look who gets sued if it blows up in a cave" angle but in order to award damages they have to prove a) that the negligence was intentional and b) that the negligence resulted in harm. I think its safe to assume that a SCUBA cylinder has never exploded underwater during a dive...no harm.

Now if it blew up in a car wreck on the way to the cave...

 

[ctcdg]

Holy smokes, guys. Here we go again - ignorance is bliss until it goes "BANG!"

Your 3AA and 3AL cylinders are designed for unlimited fills AT SERVICE PRESSURE (or for 3AA, the 110% fill, if marked with the "plus" sign). Once you go over service pressure , you are in danger of entering into the fatigue-cycle life of the cylinder. There are a limited number of cycles that a cylinder can withstand at test pressure (typically 5/3 times service pressure) - that's how they're designed. When you exceed service pressure, you are on your way up the cycle life curve to that limit, (at burst pressure, there is only ONE cycle!). For example, paperclip works in the range of a few pieces of paper indefinitely, but try bending it back and forth a few times and see what happens.

You will NOT see this in hydro!! If you cycle your cylinder to higher pressures and fatigue it beyond it's cycle life, the result will be a sudden and catastrophic failure - NO WARNING. I don't care what any of the "armchair experts" have said here. I do this for a living, I've watched cylinders fail after exceeding cycle life - it isn't pretty. In fatigue failure the cylinder has lost the properties that were built into it. It will most likely fragment - something that it's not supposed to do. Failing the hydrostatic test is a ductile failure. Fatigue failure is not.

If you're an engineer, you can go find some of this information in the design spec's in 49 CFR, section 178, or download and read the Special Permits for the E/SP cylinders. Otherwise, keep your speculation to yourself. Most of what I read here is so far off base I really didn't even want to reply.

"Leadking" got it right. Manufacturers do both cycle testing AND burst testing. These are two separate and distinct tests.

Unless you have read AND UNDERSTAND the specifications, you are playing Russian Roulette.


Darrell
CTC Seminars
(We teach cylinder regulations in more than a dozen countries.)

 

[Rick Murchison]

I can't count how many times I've gotten a '72 back with 3000 psi in it.
I fill 'em to 2475 myself.
Slowly.

Rick