I was curious where things were at with regards to the DOT granting service life extension beyond 15 years for carbon tanks so I did some googling. I found an interesting study done in cooperation with the DOT in 2015. A company took 50 expired cylinders and split them into two groups, each getting between 10k-25k additional load cycles depending on the test profile. These cycles also included 1.5x service pressure loading at appropriate intervals to simulate hydro testing. At the end of the test, all 50 cylinders were burst tested and ALL passed, exceeding burst thresholds applied to new tanks. Good news: the test indicates current carbon cylinders could safely be used for an additional 20+ years or even indefinitely as long as they pass hydro. Whitepaper for the test: http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Files/FinalReport_012016_SCBA_ReAutofrettage.pdf
Unfortunately, that's where the good news stops. Back in the 90's when the DOT introduced the standards for composite tanks, they also put a process in place for manufacturers to apply for life extension. Luxfer and other companies jumped through all the hoops, but the DOT denied all applications. The cylinder industry then tried a group approach to gain life extension approval, but was also denied. Around 2006 Luxfer started making some '30yr' cylinders to an International ISO standard that is above and beyond typical cylinders with the hope of getting extension to 30 years. This is a bit confusing, but it seems that the 30 yr lifespan is not guaranteed and Luxfer will still have to get DOT extensions for these new cylinders after they reach 15 years. Maybe they hope they will be able to sway the DOT with the even stronger design. Here is Luxfer doing a little venting (har har) about their struggles to attain life extension: http://www.luxfercylinders.com/press-releases/concerning-life-extension-of-dot-cffc-carbon-composite-cylinders
So far only the Navy has received DOT life extension permission. It looks unlikely that we'll see retro-active life extension on current cylinders. In fact, Luxfer will be lucky to gain extension on their new '30yr' cylinders. In the event that an extension is granted, it involves SPECIAL testing, not just a normal Hydro. This will likely involve the fancy acoustic testing being pioneered by the company that did the study in the whitepaper linked above. I'm guessing it won't be cheap.
Good news for people who home fill (continued cheap tanks), bad news for people who store-fill.
Two more notes - The same company intentionally damaged tanks by notching them and it did not lower the burst pressure very much. They also noted that typical failure mode of composite tanks was liner leakage (not considered dangerous).
Unfortunately, that's where the good news stops. Back in the 90's when the DOT introduced the standards for composite tanks, they also put a process in place for manufacturers to apply for life extension. Luxfer and other companies jumped through all the hoops, but the DOT denied all applications. The cylinder industry then tried a group approach to gain life extension approval, but was also denied. Around 2006 Luxfer started making some '30yr' cylinders to an International ISO standard that is above and beyond typical cylinders with the hope of getting extension to 30 years. This is a bit confusing, but it seems that the 30 yr lifespan is not guaranteed and Luxfer will still have to get DOT extensions for these new cylinders after they reach 15 years. Maybe they hope they will be able to sway the DOT with the even stronger design. Here is Luxfer doing a little venting (har har) about their struggles to attain life extension: http://www.luxfercylinders.com/press-releases/concerning-life-extension-of-dot-cffc-carbon-composite-cylinders
So far only the Navy has received DOT life extension permission. It looks unlikely that we'll see retro-active life extension on current cylinders. In fact, Luxfer will be lucky to gain extension on their new '30yr' cylinders. In the event that an extension is granted, it involves SPECIAL testing, not just a normal Hydro. This will likely involve the fancy acoustic testing being pioneered by the company that did the study in the whitepaper linked above. I'm guessing it won't be cheap.
Good news for people who home fill (continued cheap tanks), bad news for people who store-fill.
Two more notes - The same company intentionally damaged tanks by notching them and it did not lower the burst pressure very much. They also noted that typical failure mode of composite tanks was liner leakage (not considered dangerous).
