Tank experts only!

[OneBrightGator]

The tank wasn't dropped or any other nonsense being spread around, including Luxfer's claims.

Call it hysteria if you want, but maybe you should stop by Rennakers and talk to them about it or wait until someone you know is killed by one until you start labeling our reactions.

 

[freediver]

Isn't it true that a hydro is one of the hardest things on a cylinder, if that is the case would it be reasonable to suggest that a cylinder be taken out of service based on x number of hydros it has been subjected to? Asking out of curiosity, not argument's sake.

 

[gjmmotors]

I wasnt being unreasonable.
My opinion:
Why even take a chance? A new tank is cheap. Get rid of the old and get a new.
-g mount

I'll stand behind the statement that less than a dozen 6351 alloy tanks failed in service and none have failed catastophically since the eddy current/VIP requirement has been in place. But due to the unreasonable nature of some people who seem to like to ignore the obvious and confuse correlation with causation, I will amend the statement slightly:

"less than a dozen 6351 alloy tanks failed in service and no properly maintained, inspected, and handled 6351 tanks have failed catastophically since the eddy current/VIP requirement has been in place."

Truth is proper inspection procedures are catching any tanks with SLC's long before they become a threat. As long as a 6351 tank is properly inspected, maintained and handled, it presents no more of a threat than any other scuba cylinder.

 

[Ontario Diver]

Isn't it true that a hydro is one of the hardest things on a cylinder, if that is the case would it be reasonable to suggest that a cylinder be taken out of service based on x number of hydros it has been subjected to? Asking out of curiosity, not argument's sake.

No. The hardest thing on a tank is repeated rapid filling because the heat generated is bad for the tank, especially when combined with the pressure.

When a tank is hydro'd, it is in a water bath and water is used to pressurize the tank so the heat is not an issue. The hydro test checks the elastic properties of the tank. A tank still works like a ballon, it streaches under pressure and then relaxes back into its original shape. As long as the tank is "good" (maintaining its correct elasticity) the hydro test will not cause problems.

 

[freediver]

No. The hardest thing on a tank is repeated rapid filling because the heat generated is bad for the tank, especially when combined with the pressure.

When a tank is hydro'd, it is in a water bath and water is used to pressurize the tank so the heat is not an issue. The hydro test checks the elastic properties of the tank. A tank still works like a ballon, it streaches under pressure and then relaxes back into its original shape. As long as the tank is "good" (maintaining its correct elasticity) the hydro test will not cause problems.

This is what I was informed of during cylinder inspection training. The difference between aluminum and steel tanks is that steels have an "endurance limit". Stresses applied to the steel below this limit will never create fatigue. Aluminum, on the other hand, has no endurance limit, so even small repetitive stresses will eventually cause failure as the metal fatigues. They went on to say that a hydro is detrimental because the cylinder is stretched and that this repetition takes its toll on the cylinder. Hence the five year period for testing maybe. I agree with you that rapid filling can't be good either.

 

[Ontario Diver]

This is what I was informed of during cylinder inspection training. The difference between aluminum and steel tanks is that steels have an "endurance limit". Stresses applied to the steel below this limit will never create fatigue. Aluminum, on the other hand, has no endurance limit, so even small repetitive stresses will eventually cause failure as the metal fatigues. They went on to say that a hydro is detrimental because the cylinder is stretched and that this repetition takes its toll on the cylinder. Hence the five year period for testing maybe. I agree with you that rapid filling can't be good either.

First all metals have some endurance limit. Aluminum is just lower than steel. The endurance limit is the point just before the metal deforms (expands) under pressure. Anytime the tank expands, you are past the endurance limit. Do it enough times and the tank will fail. A hydro test is determental in exactly the same way a normal fill is detremental, you push the metal over its endurance level.

It is the nature of tanks to fail someday!

The life of a tank is calculated by both fill cycles and working pressure. A hydro test puts an excessive test load far beyond normal levels - if the tank passes that it is deemed to be safe at the lower working levels for another five years.

Don't worry about the hydro reducing the lifetime of a tank. Although what you have been told is technically "correct", the extra expansion that a hydro does is inconsequencial comparted to the damage from rust, mishandling, hot fills, etc, etc etc.

 

[DA Aquamaster]

I wasnt being unreasonable.
My opinion:
Why even take a chance? A new tank is cheap. Get rid of the old and get a new.
-g mount

Thats one way to look at it but how extreme do want to get with this approach? For the ultimate in saftey in terms of fatigue in AL tanks, you could consider them disposable and throw them away after the first cycle. But I think most people would agree this is would be a waste of a perfectly safe tank.

In the same vein throwing away a 6351 tank that passes it's hydro and eddy current/VIP is also arguably a waste of a safe tank. Again, with proper testing and maintenance 6351 tanks have not presented any elvation of risk compared to other aluminum tanks since the new test procedures were implemented. If anyone has heard of a verified case of a catastophic failure of a properly maintained, inspected and handled 6351 tank since the new inspection prodedures have been in place, I'd love to hear about it.

I initially had a lot of reservations about 6351 tanks when the concern first came to light, but since seeing how effectively defective tanks get identified with the new test procedures during a hydro test has alleviated those concerns. I am a whole lot more concerned about filling a freshly painted AL tank (and may require a new hydro before I'll fill it) than I am about filling a properly inspected 6351 tank.

Truth is you are more likely to get killed in a car accident on your way to the dive shop to buy a new AL tank than you are of getting killed by your old 6351 tank exploding. Risk minimization is a sound idea but it needs to be moderated with objectivity and common sense. Otherwise we would all take it to the extreme and never get out of bed in the morning. We would certainly never even consider going diving.

 

[DennisW]

DA, How many Aluminum tanks have you seen fail hydro? I would bet very few if any. The visual inspection and eddy current VIP is the only way to insure an older AL tank is bad. If it passes a VIP that has been completed by a certified inspector, not many of them out there by the way, then it is a good tank.

If someone wants to throw one away, give me a PM. Depending on the shipping cost, I'll pay the shipping to send it to me. It just needs to have a current hydro, and a VIP by either PSI or Diving Technologies International.

 

[Ontario Diver]

DA, How many Aluminum tanks have you seen fail hydro? I would bet very few if any. The visual inspection and eddy current VIP is the only way to insure an older AL tank is bad.

No. First of all, a hydro AND a good visual/eddy current is required to check to see if the tank is safe AND the tank has to have been handled safely and correctly since the tests.

Now remember that the test can only tell you if a tank is unsafe.... It cannot tell you that the tank is safe. A hydro and a visual can only tell you that there were no indications of being unsafe at the time of the test.

And I have seen a fair number of both steel and AL tanks fail hydro. I fail more steel tanks throught the visual though.

 

[DA Aquamaster]

A VIP (including eddy current if required) is a required part of the hydro test process and the hydro test facility is required to do it before putting their stamp on the tank.

A lot of dive shops do a VIP on tanks that have just returned from hydro testing and slap the shop's sticker on it, but that VIP is in fact redundant and has everything to do with generating revenue rather than increasing tank safety. If the tanks are shipped without valves, another VIP would be required to ensure no contaminats etc are in the tank, but other wise if the valve is re-installed at the test facility, the shops VIP is redundant.

What we have observed during hydro tests is that AL tanks with no observeable cracks prior to hydro will occassionally have one after hydro. The most interesting thing I have obvserved was an aluminum medical 02 tank that actually had a thin stream of water squirting from a crack in the neck after it failed the hydro and was being removed from the test tank.

The extra stress experienced by the tank during hydro is usually more than enough to aggravate any incipent crack and ensures it wil be detenctable during the VIP portion of the test. If an AL tank passes the hydro test and then also passes the visual and eddy current, then I am confident it will be safe in service until the next VIP/eddy current inspection(s) and to the next hydro test date.

The hydro test side of the shop where I work will fail 2-3 AL tanks per week (nearly all medical O2 tanks as they out number scuba cylinders about 100 to 1) but rarely does a steel tank fail a hydro portion of the test. Most of the steel tanks that are condemned fail the visual portion of the hydro, usually due to rust and pitting in the bottom of the tank.