Eddy current testing of Luxfer tanks made of 6061 alloy

[Bob DBF]

There was one posted on facebook in Jim Lapenta's accidents group last week.

The warning was about tanks without a VE stamp, why he filled the tank without a VE is beyond me. That was my read.


Bob

 

[rjack321]

There were pictures of a split open al80. Admittedly it should never have passed visual but it was 6351 alloy and it went kaboom too. At this point trying to keep them in service isn't really cost effective unless you are cutting corners on visuals or VEs. The thing that really gets my goat is people posting PSI's 6351 alloy webpage and citing that as evidence of "only a few explosions". That EXACT text was part of my PSI course back in 2006 hasn't been updated since.

 

[Bob DBF]

It didn't pass or have a visual VE on the last check but, the way I read it, he didn't check and filled it. His warning was to make sure there is a valid VE on the tank before filling.

I agree that having the tank and associated costs, for me, is not worth it considering how cheap I can pick up newer AL 80's, and the fact some shops won't fill regardless of hydro and VE.

I doubt if an update on the course would include explosions caused by not following proper inspection, hydro and VE procedures.



Bob

 

[JackD342]

First, I'll have to claim initial responsibility for apparently hijacking this thread from 6061 alloy to 6351 alloy. But I thought it was important for folks that have heard about "those bad old tanks" without knowing the exact alloy, to understand they were a different animal and still required additional testing.

There were pictures of a split open al80. Admittedly it should never have passed visual but it was 6351 alloy and it went kaboom too. At this point trying to keep them in service isn't really cost effective unless you are cutting corners on visuals or VEs. The thing that really gets my goat is people posting PSI's 6351 alloy webpage and citing that as evidence of "only a few explosions". That EXACT text was part of my PSI course back in 2006 hasn't been updated since.

If anyone has that Facebook post I would appreciate a link. I am very curious to know what part of the cylinder the failure occurred on. I know I will never know the answers to my next questions, but I'm gonna put them out there anyway - Was the cylinder being overfilled to a higher than rated pressure? Did the cylinder previously have a history of overfills?

Another clarification note - the eddy current test and VE mark have no relation to any testing of cylinder walls or bottoms. They are only looking for evidence of SLC starting in the cylinder threads.

Now, here are some very relevant and enlightening links to materials on the Luxfer website. I encourage anyone interested to click on every relevant link possible, and to read the full text and not the quick summaries. That way you won't be exposed to any bias of mine for what I think was most important to excerpt.
Nevertheless, I am going to include here in this post what I found to be very important excerpts, including one that does not regard 6351. Any red formatting you see is mine for emphasis.
Sustained-load cracking (SLC) in ruptured scuba cylinder made from 6351 aluminum alloy
and
FAQs

Excerpts:
"Research has consistently shown that sustained-load cracks grow very slowly, typically taking eight or more years to grow large enough to cause a cylinder leak or, in extremely rare cases, a rupture. Because SLC growth is so slow, properly trained inspectors have adequate opportunity to detect cracks and remove cylinders from service during normal annual inspections or as part of the DOT-required requalification process every five years. Both Luxfer and DOT have long maintained that proper, diligent inspection by correctly trained personnel is the best way to avoid cylinder-related accidents."

"How do I know if the person inspecting my cylinders is properly trained?

• Luxfer recommends taking your cylinder to an authorized Luxfer service center or to an inspector trained by Professional Scuba Inspectors, Inc. (PSI) or the Association of Scuba Service Engineers & Technicians (ASSET).
  
• It cannot be overemphasized that the quality of inspection is far more important than the frequency of inspection! An untrained or improperly trained inspector can look at a 6351-alloy cylinder numerous times without detecting SLC.
  
• Unfortunately, many untrained or improperly trained technicians continue to inspect cylinders, and no uniform standards for inspector training and certification exist among regulatory authorities around the world.
  
• Luxfer is working with industry groups and government agencies to help establish such standards."

"If there have been so few SLC-related cylinder ruptures, why is there so much alarming information about them on the Internet?

A great deal of misinformation and exaggeration about SLC is attributable to rumors and inaccurate reports spread by word-of-mouth and the trade press, but especially by the Internet, where the proliferation of inaccuracies is widespread and essentially unregulated."

"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.
• 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." (JackD342: exhibiting SLC is NOT the same as catastrophic failure)

"How quickly do the cracks grow?

• Very slowly, as extensive research by Luxfer and outside laboratories has shown.
• No scientific evidence supports rumors and claims of "fast crack growth."
• Cracks typically take eight or more years to grow large enough to cause a cylinder leak.
• Because SLC growth is so slow, properly trained inspectors have adequate opportunity to detect cracks during the normal requalification process."

"It’s really important that I have enough pure oxygen in my decompression cylinder when I need to use it after a deep dive. Is it okay to put extra oxygen in my cylinder to make sure that I don’t run out?

DOT regulations (see CFR 49) strictly prohibit over-pressurizing a scuba cylinder containing any kind of gas, but it is particularly dangerous to do so with high oxygen concentrations. The reason is: The higher the pressure and the higher the oxygen concentration, the higher the risk of a fire and explosion if a contaminant is present.

Luxfer has received numerous and persistent reports that technicians in certain sectors of the recreational diving community routinely over-pressurize scuba cylinders, including cylinders containing high concentrations of oxygen. This is sometimes described as “doing divers a favor,” offering “a little more down time” or “giving divers their money’s worth.” Not only is this an unsafe practice, it is against the law! Under no circumstances should you allow the gas pressure in your scuba cylinder to exceed the service pressure for which the cylinder is designed and stamped or marked. If a filler offers to over-pressurize your cylinder, you should not only refuse the offer, you should report the filler to the DOT. If you suspect that your cylinder has been over-pressurized, you should have it depressurized and have it inspected by a competent technician to determine whether it is fit for further service.

For pure oxygen, DOT mandates strict pressure limits: Gas pressure in an aluminum cylinder containing pure oxygen must never exceed 3,000 psi (even if the cylinder is stamped for a pressure above 3,000 psi).

If you are concerned about running out of oxygen, use a larger decompression cylinder filled to the proper service pressure—or carry more cylinders."

 

[rjack321]

There is no permalink to the FB post so I am guessing its not set to public?
Hopefully this link gets you there: <iframe src="Facebook" width="500" height="714" style="border:none;overflow:hidden" scrolling="no" frameborder="0" allowTransparency="true" allow="encrypted-media"></iframe>

If not you'll have to join the group and scroll to find it its actually dated June 18, 2017 but I guess got bumped up via someone posting about it.
Scuba Accidents and Risk Management Techniques for Divers

Based on the rip in the cylinder getting more ragged as it progressed down the sidewall to cylinder bottom, it does appear to me to have initiated in the crown area.

 

[rjack321]

• Out of a total population of approximately 1,073,000 Luxfer scuba cylinders made of 6351 alloy, only 1.25% have exhibited SLC.
• 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." (JackD342: exhibiting SLC is NOT the same as catastrophic failure)

I would love to know 1) how they complied these SLC visual or VE failure rates since there is no reporting requirement 2) the date this survey was conducted. At least as of 2018, I would guess the number of failures has gone up and the total population still in use has gone down.