The reduction stage of the powder production process of a chemical plant had been shut down, and the buffer tank of the line had been drained on the preceding day. After the shutdown, the batch to be reduced was normally processed in the autoclave and the process was duly acknowledged. The sequence opened the autoclave degassing operation via the cyclone up to the roof, as a result of which the pressure inside the autoclave started to fall. Soon after that, an explosion occurred in the buffer tank downstream of the autoclave and at the gas scrubber. There is no connection between the degassing line and the buffer tank, and the autoclave buffer line was not open.CAUSE The safety valves of the autoclave loading and additive line had been connected to the buffer tank vapour duct leading to the gas scrubber about one month prior to the incident because a lot of hot work was being performed in the hall during installations. The hydrogen extraction line added to the buffer tank vapour duct was implemented in a location different from the site indicated in the drawings so that a hydrogen pocket was left inside the line. Presumably, hydrogen had been released into the vapour duct through the afore-mentioned safety valves during reduction. Purging gas (N2, CO2) had been fed into the buffer tank on a continuous basis, but the carbon dioxide line check valve had been installed in the wrong way round. As a result, the flow rate of the gas supplied by the line was not sufficient to inert the entire tank volume (0.5 m3/h). Because the buffer tank overflow line was not furnished with a water seal, replacement air (oxygen) was able to enter through it into the vapour duct and the buffer tank. At the time of the incident, the vapour duct contained fine powder that is incandescent in an oxygen atmosphere. This could have caused the ignition of the gas mixture.