In a chemical plant, an increase in pressure occurred in an atmospheric pressure tank storing effluents while the site was in the production phase. The cap of the liquid seal located at the tank’s high point was thrown a distance of 77 m. The tank’s shell was deformed (curved downward) but was not leaking. Employees noted a similar noise when a valve was opened, and a low-pressure alarm in the tank appeared in the control room, although the level and temperature were normal. The facility had been operating in the same configuration for roughly 30 years. The tank is fed by two main flows of aqueous effluent. The first effluent is a solution containing an organic component that generates hydrogen gas under normal storage conditions. The second effluent consists of a solution of formic acid and a minor by-product of the process. This by-product is 2% hydrogen peroxide (a strong oxidant) which, under normal storage conditions, generates carbon dioxide as a result of decomposition, with traces of oxygen. The tank is maintained under positive nitrogen pressure to inert its vapour space owing to the presence of highly flammable hydrogen in the air. Two days before the incident, the second effluent sent to the tank was slightly contaminated with suspended solid particles. These particles, in small quantities (< 10 ppm), consist of palladium on carbon and serve as a catalyst in the workshop that produces the effluent. The presence of these particles is abnormal and results from a process malfunction. Palladium catalyzed the decomposition of hydrogen peroxide in the tank to produce water and additional gaseous oxygen. The hydrogen and oxygen mixture in the vapour space then exploded in contact with an undefined heat source, causing an overpressure in the tank resulting in its deformation and the ejection of the liquid seal cap. Despite the presence of nitrogen in the vapour space, the additional supply of oxygen (oxidizer) as a result of the decomposition of hydrogen peroxide, lowered the lower flammability limit of the hydrogen (fuel), which is naturally low in air (4.1%) and the ignition energy of which is so low (about 0.02 mJ) that it ignites almost systematically when mixed with air. A post-accident study showed that, even in the absence of palladium catalysis, oxygen production in the tank through decomposition of hydrogen peroxide was higher than initially estimated. The manufacturer is considering the installation of a permanent nitrogen sweep in the tank, with the removal of the hydrogen at the high point. The date and location of the accident are unknown.