Aluminum cylinders known to have been exposed to overheating or showing evidence of having been overheated, with overheating being a cylinder metal temperature in excess of 350°F, must be condemned per 49 CFR 180.205 (i) (viii). No testing for acceptance or re-heat treatment is authorized.
In CGA pamphlet C-6.1; Standards For Visual Inspection of High Pressure Aluminum Gas Cylinders, it states that cylinders heated to cylinder metal temperatures in excess of 350°F (176°C) must be condemned.
Common evidence of exposure to overheating that may heat the metal temperature to 350°F (176°C) include:
charring, blistering, or discoloration of the cylinder paint or protective coatings;
distortion of the cylinder;
melting of non-metallic (i.e. plastic, etc.) valve components;
charring or burning of labels;
activation of valve pressure relief device; or
activation of the heat indication system.
Catalina Cylinders recommends that if there is evidence, or it is believed, that a cylinder has been exposed to overheating but the temperature of metal of the cylinder is not believed to have reached 350°F (176°C) for any duration of time, the cylinder must still be subjected to hydrostatic testing or condemned. A cylinder that has been exposed to temperatures great enough to change the temper of the cylinder could show an increase in total or permanent expansion as measured during hydrostatic testing. Cylinders showing unusually high total expansion or exceeding the regulatory limits for the relationship of permanent expansion to total expansion (i.e. DOT = 10% and TC = 6%) should be condemned.