Me personally, I would just scrap it, but that is me.
It was made under a CTC Special permit 922, which is still listed on the Canadian Transportation site. It is a 3000 PSI cylinder. If there are no other stamps in it, the tank needs to be hydro tested. After that it will need a visual inspection and very possible a mandatory eddy current test. Before doing a hydro on it, the facility must contact the CTC to get a copy of the permit for testing proccedures.
Even if it passes all the tests, there are many dive shops out there that are refusing to fill older 6351 cylinders.
Slonda828
June 26th, 2009, 02:57 AM
It all depends on where you live man, but down here in the south nobody will fill 6351 alloy tanks. If it were a steel tank, I'd tell you to keep it until you grow old and crotchety. I'm pretty adventurous and I will not use 6351 alloy tanks. There's plenty of other aluminum tanks around for good prices. I picked up a 90's aluminum tank the other day for 30 bucks. I'd scrap it, or make wind chimes out of it.
jbrians
June 26th, 2009, 08:30 AM
I have 4 AL tanks that are of 6351 alloy. All are in current hydro and viz and I'll take my chances as long as someone will fill them. I believe there have been something like 34 tanks out of 20,000,000 that have failed. When they fail a hydro, I'll stop using them...or when they are refused at the shops I deal with...
Having said that, I'm only using them now because they are my double 50's which I dive vintage with.
I wouldn't buy them now for anything above scrap value.
elan
June 26th, 2009, 08:41 AM
So far I know that G&S at Tobermory refuses filling them (Al tanks older than 89) I have not been to many shops here but I have not seen any other refusing older ones.
Luis H
June 26th, 2009, 08:43 AM
I have 4 AL tanks that are of 6351 alloy. All are in current hydro and viz and I'll take my chances as long as someone will fill them. I believe there have been something like 34 tanks out of 20,000,000 that have failed. When they fail a hydro, I'll stop using them...or when they are refused at the shops I deal with...
Having said that, I'm only using them now because they are my double 50's which I dive vintage with.
I wouldn't buy them now for anything above scrap value.
Make sure they are also eddy current tested (Visual Eddy or Visual Plus). A hydro test only detects the metal elasticity. It does not detect cracks. Many aluminum tanks will pass hydro test perfectly fine with very visible cracks on the neck.
The statistic you quoted is correct but it can be very misleading. The reason more are not failing is because they are being pulled out of service (in very large numbers yearly). Many cracked tanks are constantly being detected by the inspection methods that have being put in place.
Obviously, not all tanks that are pulled out of service have crack, but the percentage of aluminum 6351 tanks that will end up developing cracks eventually (even according to Luxfer) is kind of high for my comfort.
They are fine to use as long as they are carefully inspected to confirm that there are no cracks, but personally I prefer to just avoid them if possible. That is just a personal choice.
Again don’t get a false sense of security from passing a hydro test... it doesn’t test for cracks at all. The hydro will expand the cracks and make them easier to detect after the hydro, but it can pass hydro perfectly fine.
jbrians
June 29th, 2009, 08:10 AM
Make sure they are also eddy current tested (Visual Eddy or Visual Plus). A hydro test only detects the metal elasticity. It does not detect cracks. Many aluminum tanks will pass hydro test perfectly fine with very visible cracks on the neck.
The statistic you quoted is correct but it can be very misleading. The reason more are not failing is because they are being pulled out of service (in very large numbers yearly). Many cracked tanks are constantly being detected by the inspection methods that have being put in place.
Obviously, not all tanks that are pulled out of service have crack, but the percentage of aluminum 6351 tanks that will end up developing cracks eventually (even according to Luxfer) is kind of high for my comfort.
They are fine to use as long as they are carefully inspected to confirm that there are no cracks, but personally I prefer to just avoid them if possible. That is just a personal choice.
Again don’t get a false sense of security from passing a hydro test... it doesn’t test for cracks at all. The hydro will expand the cracks and make them easier to detect after the hydro, but it can pass hydro perfectly fine.
I hear and agree with what you are saying. The shop does an eddy current vis and passes the tank. The guy that does the vis also fills the tanks, so he knows he is in the line of fire!
At some point, I won't be willing to spend the money to get them tested anymore as the eddy current vis can only go up in price. Maybe by then I'll have a steel set of vintage doubles? Besides, I'd far sooner dive steel. AL just doesn't work for me as well for weight reasons.
jbrians
June 29th, 2009, 08:18 AM
So far I know that G&S at Tobermory refuses filling them (Al tanks older than 89) I have not been to many shops here but I have not seen any other refusing older ones.
Just got back from Toby. Friday and Sat were amazing weather but rain on Sunday...what is it about me that I hate getting wet before I get into the water??? LOL. 7 dives in 3 days. Water temp was 46F and pine tree pollen had been stirred into the water so the vis was down to only 60 feet or so.
About G&S...they won't fill any AL tank older than 1990...same with the other 2.
On the other hand, they will all fill 72's in date. Matter of fact, they give you really good fills if they are distracted. I put the gauge on mine when I picked them up saturday morning and one was sitting at 4000psi. If the burst disc had of let go, someone would be needing to change!
WD8CDH
June 29th, 2009, 08:40 AM
My personal opinion of 6351 alloy tanks is while they are safe if properly tested, I would only keep VIS and eddy current testing current until next hydro is needed and then retire them. The cost of the hydro just doesn't seem to justify the LDS hassle.
Luis H
June 29th, 2009, 10:37 AM
About G&S...they won't fill any AL tank older than 1990...same with the other 2.
On the other hand, they will all fill 72's in date. Matter of fact, they give you really good fills if they are distracted. I put the gauge on mine when I picked them up saturday morning and one was sitting at 4000psi. If the burst disc had of let go, someone would be needing to change!
If you had a steel 72 filled to 4000psi, you should at least have the burst disk replaced... and maybe even have it rehydroed... hopefully it will do well on the next hydro... it probably will, but this is not a good habit.
That tank was taken a bit beyond its elastic limit (or yield strength).
jbrians
June 29th, 2009, 01:02 PM
If you had a steel 72 filled to 4000psi, you should at least have the burst disk replaced... and maybe even have it rehydroed... hopefully it will do well on the next hydro... it probably will, but this is not a good habit.
That tank was taken a bit beyond its elastic limit (or yield strength).
I am heading into my LDS tomorrow to get the 4 tanks filled. I'll ask about a new disc.
Isn't the hydro pressure for a 72, 4000? If so, why redo it then?
I'm not arguing, I'd like to know why is all.
jbrians
June 29th, 2009, 01:03 PM
My personal opinion of 6351 alloy tanks is while they are safe if properly tested, I would only keep VIS and eddy current testing current until next hydro is needed and then retire them. The cost of the hydro just doesn't seem to justify the LDS hassle.
I can certainly agree with that statement!
couv
June 29th, 2009, 02:00 PM
If you had a steel 72 filled to 4000psi, you should at least have the burst disk replaced... and maybe even have it rehydroed... hopefully it will do well on the next hydro... it probably will, but this is not a good habit.
That tank was taken a bit beyond its elastic limit (or yield strength).
Luis, (or whoever)
I know that the burst disc must be designed to blow at X amount over the tank's working pressure, but is there a formula? Years ago, at Morrison Springs, FL. a gentleman blew 2 disc in 2 days on my steel 72s. He seemed to be in a hurry and would forget to reduce the fill pressure after filling AL 80ies. Of course this would clear the shack out as people would scatter in a hurry when the water volcano would blow….except for the fill guy, who seemed to be quite used to it :-)
At any rate, back then, most fill stations would hardly get much over 3000psi, so it makes me wonder why those disc gave way so easily.
Thanks,
Couv
Luis H
June 29th, 2009, 02:28 PM
The hydro pressure for any 3AA is 5/3 of the working pressure.
For a steel 72:
2250 * 5/3 = 3750 psi
The burst disk is supposed to let go at the hydro pressure with a tolerance of + 0, - 10%
In other words if it had a proper burst disk it should had let go somewhere from 3375 psi to 3750 psi.
The fact that it didn’t let go lets me to believe that it probably had the wrong burst disc.
In any case at 4000 psi that tank was exposed to 250 psi higher that a normal hydro test and 150 psi higher that a hydro test redone due to equipment malfunction.
That tank has seen some plastic (permanent) deformation. Plastic (permanent) deformation is what occurs when a metal is stressed beyond its elastic limit (yield point).
When steel is taken into the plastic deformation range it can work harden and become more brittle.
Is it likely that the amount of over stress caused to become very brittle… probably not? That is why you can permanently bend a coat hanger a few times before it fracture, but it doesn’t take many times and it will fracture. That is why I do recommend to have it retested (at the LDS expense) and if it fails they should replace them with some other nice steel 72’s.
Higher strength steel as the type found on 3AA steel tanks are more susceptible to work hardening and becoming brittle than the very mild steel found in a coat hanger.
The main purpose of a hydro test is to test for the material elasticity to make sure it has not become brittle. A brittle pressure vessel could in theory fracture like a hand grenade.
On the other hand if you stay below the yield strength then the number of stress cycle that steel can endure increases exponentially (as the level of stress goes down). I tend to laugh when some people talk about overfilling (to 90% of hydro test pressure) for 20 years is significant proof on anything…it is not, but this is a separate subject.
couv
June 29th, 2009, 04:13 PM
…..so in the case of Jbrians, the bust disc was for a higher rated tank, and most likely in my case the bust discs was either old, too low a burst pressure or the fill station was really cranked up-not likely as this happened in the mid 1980ies if memory serves me correctly.
Thanks Luis,
couv
Luis H
June 29th, 2009, 04:54 PM
…..so in the case of Jbrians, the bust disc was for a higher rated tank, and most likely in my case the bust discs was either old, too low a burst pressure or the fill station was really cranked up-not likely as this happened in the mid 1980ies if memory serves me correctly.
Thanks Luis,
couv
The burst disk could have been corroded, over torqued, faulty, the screw/ cutter could have been faulty, etc., etc.
A buddy of mine, in PR back in the 70's, had a set of double US Divers manifold that I had to change the burst disc about 1/2 dozen times before I got it to work... I never figured that one out. It could have been something on the valve interface surface or a bad batch of burst disc... I never figured it out.
jbrians
June 29th, 2009, 07:56 PM
…..so in the case of Jbrians, the bust disc was for a higher rated tank, and most likely in my case the bust discs was either old, too low a burst pressure or the fill station was really cranked up-not likely as this happened in the mid 1980ies if memory serves me correctly.
Thanks Luis,
couv
We know why it held now.
the burst disk was replaced with one from an AL80 early this year. I started a thread back then on VDH about having a tank hydroed assuming it was a 3000psi tank. It passed then but now with this HP fill, i'm now thinking it's luck is running out... and mine too.
Over the years, "I was told" the hot fills were causing disk failures...they'd pump them over so they'd cool down to the right PSI.
