Steel tanks, overfills, hydros and plus

[Gombessa]

I've never seen any hard evidence of the effects of overfilling one way or another, so I thought this might be insightful...maybe it'll also spur someone more knowledgeable to tell me what I'm doing wrong before something goes boom:

Over the last 6 months or so, some friends and I have each happened upon three sets of near-identical used doubles - all low pressure 80s. So that's six tanks. They're all the same brand, all have serial numbers starting 77x,xxx, all with birthdates between 2002-2003. All were out of hydro (their original and only hydro) when we got them. Four of the six tanks came overfilled and stored with 3550-3600psi for the past 14-16 months. Since they came from local tech divers, I can only assume they've been dived at these pressures for most of their working lives. The other two were also stored filled (pressure unknown) and had not been dived for several months.

So over the last few months, we've sent all six in for hydro testing. Happily, the local facility does plus testing. Long story short, all six tanks passed hydro, and all six retained their plus ratings. I was a little concerned about the ones that had been stored at overfill pressures for so long, but apparently it wasn't an issue.

While the n is admittedly small, the pressures at which the tanks were used and stored, the 100% pass rate, and particularly the 100% plus rate, has given me at least real first-hand evidence that overfilling and storage at "moderate" pressures of 3,200, and occasionally to 3,500, is not appreciably affecting the structural integrity of the tanks. If steel tank lives are really measured in terms of 10,000s to 100,000s of fills, and, say, routine overfilling brings that down from 100,000 fills to 50,000, or even from 10,000 to 5,000, they'll still well outlive my expected scuba career, and likely that of anyone else I might pass them on to.

My thought is also that if it were detrimental to the tank, the first diagnostic indication (beyond maybe something showing up during a vis) would be that the tank would exceed the REE on hydro and thus fail the plus. If that had happened to any of these tanks, I'd be a little more wary of the fill pressure. But it seems to me, and apparently to most of the tech divers in the area, that cave/over-fills are not an imminent threat to otherwise healthy tanks.

So...thoughts? I obviously can't be the only person who has sent a number of storage-overfilled tanks in for plus testing, so I'd love to hear what others have found.

 

[Brewone0to]

I have my steel 120's filled to these pressures regularly.
I am a firm believer that the pressure stamped into the tank is not a max fill pressure, but rather the proper turn pressure for when to turn the dive.
Although I don't store them with what we here in Florida refer to as a "proper fill".(3600)
I would imagine that this sort of thing done regularly could reduce the lifespan of the tanks more than just to fill and use that day.
I have witnessed on a number of occaissions when tanks have been filled into the 4200-4500 range with no imediate issues with the tanks.
This being said, there have been a number of times I have heard of Steel tanks not getting past their 1st or 2nd hydro.
I would only buy tanks with an agreement that they would pass hydro,if not then seller pays for failed test since they would be the one responsible for it not passing.
It should also be noted that these pressures are not to kind to the HP seat of the attached regs, and they will require more frequent rebuilds.
Thus, one should plan their dive and not run these extreme pressures (4000+) if there isn't a defined need for the extra gas to acomplish the objective safetly.

 

[Gombessa]

Thanks Brew. While I wouldn't want to store the tanks long term at high overpressures, the way I dive may have the same effect--I typically fill the tanks right after my last dive so they'll be ready for the next weekend. In essence, the more I dive, more I'd be "storing" the tanks at overfill pressures.

I've heard of these 4,000+ psi cave fills, and my immediate thought is that this exceeds the 5/3 test pressure for the tank. If you regularly use/store a tank at hydro pressure, I'm not too surprised that detrimental effects show up sooner. The big question then is, how big of a difference is there between 3,500psi and 4,000 psi? Or 3,200 and 3,500 for that matter?

 

[captain]

Pressure it self over a period of time does not affect the strength of a tank, the number of expansions and contractions cycles from filling and emptying are what does. Just as you can take a piece of wire and bend it back and forth until it breaks, the same process is at work in a tank.
You could bend the wire a little for a long time or bed it a lot for a short time before it breaks. Yes the over fill probably shortens the tanks life but the lifetime is so great that it will probably out live you.
What you are doing is reducing whatever safety factor the tank was designed to have. If it is a 3000 psi tank and was perfectly made with no flaws and has a burst pressure of 7000 psi than you have a safety factor of 4000 psi. If there is an unknown flaw in the tank that could cause it to burst at a lower pressure than you are cutting into that safety factor by the overfill amount.

 

[battles2a5]

This issue has been discussed over and over again and I have never seen a consistent result. Some people say overfilling is dangerous, others say it's fine. Some say that the exact same tanks with the same metalurgy are rated for much higher pressures elsewhere in the world, others say that the US tanks are designed specifically for the US market. Some say that HPs and LPs are the same metal, others disagree. All I know is that I see LP tanks WAAYYY overfilled all the time and I've never heard of an issue first-hand. I routinely fill my LP tanks to 3700 or so and have never had an issue. There are hundreds of sets of LP tanks in cave country that see a lot of abuse and never seem to have issues. I'm not saying it's ok, or that you should do it, I've just never seen any issues.

 

[matt t.]

You only need to see the "issues" or results 1 time to regret it. If you need more gas, either buy bigger tanks or learn to proficiently use a bottom stage.

 

[SparticleBrane]

Fill 'em till they're round.


Here's my feelings on the subject:

• I have no issues filling X/E/FX series cylinders (3442 rated) and HP cylinders (3500rated ) to 3600 routinely. Doesn't bother me in the slightest -- why? Because they're essentially rated for it. Note that to get these pressures while cold you'd need to hit ~3800 or perhaps a bit more during the fill process, but after cooling for a few minutes they'll be down to ~3600. If I don't get 3600, or even 3442 for most of my diving -- big deal, I usually don't need that much gas anyway. If I'm just messing around at the quarry or doing student checkouts I'm not going to need that much gas anyway.
  
  
• For LP cylinders I don't have issues bringing them up to 3600 on a cold fill, but I prefer to only do that when needed -- cave dives, longer deco dives, etc. For a lot of the diving I do, double LP80s at rated pressure gives plenty of gas. No need to possibly stress the cylinder if you don't have to.
  
  
• For 3000psi aluminum cylinders I generally don't overfill them but if I do, 3600 or less (just like in the LP category -- only when needed). For Al80s there really isn't much point due to the low cf/psi ratio. I've only had 3600 in my double Al80s a few times, although once they were in my trunk on the hottest day of the year and were at 4000 when I brought them inside...hehe whoops.

 

[widget]

Its a long debated question, and no real firm answer. Personally I think its a good standard, and although its routinely ignored, its a standard from which manufacturers limit liability.

I visited Jos Heiser cylinders in Austria before Worthington bought them out, and their standards were so high their cylinders were 5 kgs heavier than any other manufacturer and consequently more expensive (more material) for the same rated cylinder. When asked they were quite prepared to admit that they "overmanufactured" their cylinders, and were happy to do so, their opinion was a DOT or TUV standard was just a minimum and their internal standards were far higher.
My opinion was, this is fine if the end user is not paying extra for this unnecessary safety margin, but thats seldom the case.!

All their cylinders had a TUV rating (as well as various others) and a certain number of cylinders were required to be tested to destruction on runs, generally destruction was well over 500 bar on a 230 bar rated cylinder, and they would cycle it to 500 bar repeatedly tens of hundreds of times before destruction or over expansion.

So, yes, steel cylinders from this manufacturer anyway had a huge safety factor built in, which, is very unlikely to ever effect a scuba diver marginally overfilling his cylinder, but, obviously, this may not apply to ALL manufacturers, so I think a rating is still there to be acknowledged.

So should you overfill, - general concensus is NO, - or if you do, not to exceed 10% of its working pressure rating.

Will it weaken the cylinder - probably, eventually, but not sufficiently enough to cause any test issues in its normal workinglife.

 

[cool_hardware52]

Pressure it self over a period of time does not affect the strength of a tank, the number of expansions and contractions cycles from filling and emptying are what does. Just as you can take a piece of wire and bend it back and forth until it breaks, the same process is at work in a tank.

Not really. If you do not exceed the elastic limit the life of both the tank, and wire is essentially infinite, damn handy thing about steel.

Say we start with a long thin piece of steel, picture a steel yard stick. And we flex it slightly over it's entire length. No permanent deformation takes place, and we can continue to flex the yard stick effectively forever.

If, OTOH we grab the yard stick with two vice grips a couple inches apart and bend the steel to the point where there is permanent deformation, soon the steel will fail.

Hydro's are looking for permanent deformation, that's what REE is all about.

A tank in good condition will not exceed it's elastic limit even at the hydro pressure, that's what the "REE" is all about.

If the hydro pressure does not create stresses that exceed the elastic limit, pressures below the hydro pressure certainly won't either, regardless of the number of cycles.

I am not recommending overfilling tanks. Much of the conservatism built into the working pressures vs the elastic limit is to provide a large safety margin for tanks that are not in good condition.

Tobin

 

[pescador775]

Well, the bent wire is just an example of metal fatigue. Steel tanks are not made of spring tempered metal like a meter stick. Tanks will fatigue and will fail REE without ever being pulled beyond their elastic limits or reaching 100,000 cycles. Should a tank be stretched beyond its elastic limit it would fail hydro, not just the "plus" test. The REE is a measure of permanent expansion and retained elasticity, not yield (elastic limit). Practically, the life of a steel tank should be quite protracted but not infinite. Hydro testing weeds out the numerous exceptions to the theoretical lifespan. The current generation of LP tanks seem to have exceptional reliability over their useful life often passing the REE, or so I have heard.