In an industrial gas production plant, a 2.5-litre steel cylinder (34CrMo4), containing an 18% CO2/argon mixture, exploded at around 8 a.m. at the end of the filling operation. The pressure was between 178 and 200 bar. The filling module and the ceiling of the packaging hall were damaged. The cylinder underwent expert assessment.

Manufactured in 2007, the cylinder, designed for an operating pressure of 200 bar and a test pressure of 300 bar, had never been inspected. The gas mixture is used as a shielding gas in professional welding applications for carbon-manganese and stainless steels. The SMT valve without RPV (residual pressure value) broke suddenly when the cylinder ruptured.

According to the results of the expert assessment, the explosion was the result of the spherical-bottom cylinder being stored in the laying-down position. The introduction of a corrosive chemical agent, different from the fluid contained in the cylinder (no crater corrosion obtained with carbonic acid), would have led to the formation of corrosion along a longitudinal generatrix on the inside of the cylinder. An orange iron oxide deposit was present in the bottom of the cylinder, which is characteristic of the presence of humidity and a 40 mm-wide drip mark on the inside surface with significant etching of the macro relief by a stagnant, highly corrosive product. This stagnation line along the generatrix is responsible for the multiple initiations of a progressive cracking phenomenon parallel to the fracture surface.

The operator set up a process to check for the absence of liquid before filling (adapted mirror test), and plans to equip the cylinders with special valves (RPV type = residual pressure valve to maintain a residual pressure / NRV = non-return valve for the anti-return function), in order to prevent the accidental introduction of foreign media (liquids, gases), likely to react with the contents and/or the container, into the cylinders. Operator protection was also installed at the filling station.