In both ruptures, a piece of the cylinder neck separated from the cylinder. RSPA estimates that approximately seven million cylinders have been manufactured using aluminum alloy 6351-T6. RSPA presently does not know which cylinders among this population have the potential for similar failure. Extensive research, testing and analysis have been performed on cylinders made of aluminum alloy 6351-T6 to determine any correlation between SLC and the probability of rupture. Findings indicated that cylinders with a marked service pressure below 4000 psig failing due to SLC would leak and not rupture.
Present data are inconclusive as to why the two cylinders noted here ruptured instead of leaked.
RSPA is continuing to investigate the incidents. To RSPA's knowledge, no cylinders have been manufactured under the exemption or special permit numbers listed above, except DOT-E 7235, since 1984. Any cylinder marked with one of these exemption or special permit numbers most likely is made of aluminum alloy 6351-T6. If in doubt, contact the cylinder manufacturer or distributor to identify the material of construction. Aluminum Precision Products Inc. Luxfer USA is the only manufacturer of DOT-E 7235 cylinders.