DOT final rule on 6351 cylylinders

[CompuDude]

I think calling it a "bad habit of blowing up" is a poor analogy of it. But I understand your concerns though...

I think that out of the tens or thousands of the 6351 tanks made over numerous years, that only something like 12 of them have had a failure that resulted in some sort of crack or rupture of the tank. (someone correct me if I'm wrong).

Many people safely use these tanks daily. You will find however though than many shops have up signs saying they won't fill them.

Ok, I'll grant that it is rare for a blow-up to happen, and thus that wasn't the best phrasing, but yes, I've seen the signs up by the fill stations at a couple LDSs. I don't own any non-steel tanks myself, so it's a non-issue for me, I was just making sure I didn't need to worry about anything.

 

[SparticleBrane]

Tens? Thousands? Try millions...

3. How many aluminum cylinders have exhibited SLC?

* Out of a total population of approximately 1,073,000 Luxfer scuba cylinders made of 6351 alloy, only 1.25% have exhibited SLC.1
* Out of Luxfer's total 6.1-million population of 6351-alloy cylinders, the SLC rate is slightly less than 0.37%.
* While we do not have complete statistics on cylinders manufactured by other companies, industry experts estimate that out of a worldwide population of more than 30 million 6351-alloy cylinders, far less than 1% have exhibited SLC.

Note that those are the ones showing SLC--the number blowing up (12?) might as well be statistically insignificant. (Not saying exploding tanks are insignificant--just that the number is so low that most people wouldn't consider it a big deal)

With that being said, if I owned a LDS, I wouldn't fill them.

 

[DA Aquamaster]

Given the millions of tanks out there, a "large" number of 6351-T6 aluminum tanks are found to have cracks every year, but statistically, very few have SLC issues and of those with cracks very, very few catatrophically failed. More importantly, of those that did fail, none were properly inspected per the current industry standard.

Or in other words, no properly inspected (eddy current/visual plus) 6351 tank has ever blown up due to a sustained load crack.

The interim standard recommended by Luxfer was an eddy current inspection every 18 months, which in effect meant every year along with every VIP for a scuba tank. The permanant DOT rule would actually allow a return to a more normal cycle of an inspection every 5 years done concurrently with the hydro test.

If I owned an LDS I would fill them. The whole thing has been nothing more than a whole lot of chicken little hype since the problem was discovered and an effective inspection protocol developed.

Look at it this way...let's pretend I just discovered steel tanks rust and that left uninspected and unchecked it will over time proceed to the point where it can cause a steel tank to fail catastophically. OH MY GOD!!! WE'RE ALL GONNA GET BLOWN UP!!!...we better condemn all those steel tanks just for being steel.

Obviously that's an excessively conservative response as with an annual VIP (or a VIP every 5 years in the case of steel medical and welding tanks) any rust is detected, monitored and eventually removed via tumbling before it develops to the point where pitting or other stuctrual degradation occurs to the point where the tank will fail. We know that due to years of experience monitoring rust progression in scuba tanks and other types of steel tanks.

Unfortunately, this overboard response was exactly what happened with 6351 tanks and the SLC issue as the process was not as obvious or easy to understand as rust in a steel tank. Given the lack of knowledge initially, a conservative response by LDS owners was perhaps not out of line.

However at this point based on years of experience with 6351 tank eddy current/visual plus inspections and SLC crack propogation, the process is now well understood. It is now obvious that crack formation and propogation, if it does occur, will occur so slowly that the time required to progress to the point where failure is a risk is longer than the required inspection interval. Consequently, any cracks that pose a risk will be found and the tank condemned before a failure can occur.

And let's review...no properly inspected 6351 aluminum tank has ever blown up. None. Nada. Zip.

That means that your LDS really does not have a valid reason not to fill your properly inspected 6351 tank and if they persist in not filling it in light of the current and permanent DOT ruling, you have to consider that perhaps they are just a whole lot more interested in forcing you to buy a new tank and that perhaps you would be far better off dealing with a much more honest and/or knowledgeble LDS somewhere else.

 

[rjack321]

They have a valid reason - they don't trust other inspectors.

The chance of missing a significant defect in a steel or 6061 AL tank is (considered) less since substantive defects in those are (typically) more obvious. 6351 defects are much more subtle, eddy current machines can be operated by half trained nit-wits, etc.

All of these cylinders are at least 17 years old. They sell for about $20 at the scrap yard. Are they really worth it at this point?

 

[divinginn]

it only takes one out of the millons and i had one didnt blow but it had a crack in the neck,scared the crap out of me when the tech showed it to me. my advice get rid of them or at least get them properly inspected.

 

[DA Aquamaster]

They have a valid reason - they don't trust other inspectors.

In the past I there was perhaps some validity regarding concern over VIP's conducted by other shops as just about any smoe can get a PSI certification to do VIP's, but may not do many eddy current inspections and may not be very proficient at them.

However, at the same time we still have that irritating fact that there have been no explosions of properly inspected 6351 tanks - and you can extend that to and define "properly inspected" to mean any tank with a sticker on it inidcating a visual plus was done. So basically, even though shops don't trust each others VIP's with reagrd to visual plus inspections they all seem to be doing an adequate job. And in any event, given the likelihood that they will be filling the tank, I'd argue the majority of shops are conscientious about the inspection.

More importantly, the new DOT ruling specifying a 5 tear interval allows the inspection to be done with the hydrostatic test. A little acknowledged fact is that the hydro test facility must conduct a thorough and complete VIP after the bydro test - this would include the eddy current/visual plus inspection required for a 6351 tank. (Dive shops like to charge divers for VIP's on their just returned from hydro tanks, but it is redundant.)

Consequently this means that the DOT ruling ensures that a DOT licensed facility will have eddy current inspected it at hydro. This means that if it is in hydro the eddy inspection will be both current and will have been conducted by a licensed facility that has signed off on the tank and will have test records on file for the tank.

Now more than ever, "the don't trust other inspectors" argument just does not wash.

 

[jbd]

Clay's Notes version:

After much consideration and several comments, it has been ruled that SCUBA, SCBA, and O2 service 6351-T6 cylinders must now have an eddy current test (along with the already required visual) with each scheduled hydro from January 1, 2007, onward, due to the SLC issue. Any cracks reaching two thread lengths require the cylinder to be condemned. There will be no fixed service lifetime. The new rules don't apply to other uses of 6351-T6 cylinders. Oh, and you should know better than to use 6351-T6 cylinders for pyrophoric gases, but in case you didn't, now you do.

(Okay, so I'm not quite the Reduced Shakespeare Company...)

ClayJar---thanks for saving my stomach from the gallon of coffee and my eyes from the strain of all that reading.

 

[jbd]

it only takes one out of the millons and i had one didnt blow but it had a crack in the neck,scared the crap out of me when the tech showed it to me. my advice get rid of them or at least get them properly inspected.

I know of one that had been filled to 3000 psi, used for a dive, brought in for a fill from near 400 to 500 psi. Folks standing around talking while the tank was being filled and all of a sudden there was a loud POP and a loud hissing of escaping air. This occurred at around 1700 psi IIRC and from what I am told those standing around the tank scattered faster and in more directions than diarrhea hitting a high speed fan. Someone did have the thought to hit the kill switch during the scatter and the tank hissed its way down to ambient pressure. The crack was pretty impressive.

 

[Leadking]

I know of one that had been filled to 3000 psi, used for a dive, brought in for a fill from near 400 to 500 psi. Folks standing around talking while the tank was being filled and all of a sudden there was a loud POP and a loud hissing of escaping air. This occurred at around 1700 psi IIRC and from what I am told those standing around the tank scattered faster and in more directions than diarrhea hitting a high speed fan. Someone did have the thought to hit the kill switch during the scatter and the tank hissed its way down to ambient pressure. The crack was pretty impressive.

All cylinders are designed to LBB, Leak Before Burst. Cylinders fail all the time, but do so in a safe manner.
It appears the dive industry wants a cylinder that can take unlimited abuse, be filled to hydro pressure regularily, never be inspected, and never fail. I read all the chatrooms and it scares me as to what people do to cylinders.

 

[pescador775]

I've been trying to train a pigeon to inspect my tanks. So far, he can spot rust and pits but can not distinguish a crack from a normal thread; he can see them but doesn't always count the threads right. He has learned to peck a red button leading to a bell when he sees a problem. He still can't get the hang of the eddy current machine so I decided to dispose of my sole aluminum tank and sell the machine.