A tank undergoing cleaning exploded after a 15-day production run. Along with a damaged drum dryer, the tank had been collecting washing water with an excess concentration of 2′-azobis (2 methylbutyronitrile) – AIVN (a by-product of hydrazine hydrate). In anticipating the difficulties involved in pump drainage, a technician used a hose and heated the vapour mix at 128°C for 15 min, despite a recommended temperature of 40°C (washing with hot water), inducing AIVN decomposition as of 50°C. The mix was decomposing more quickly and after 70 min of latency, with no technician present (due to a shift rotation), the tank lid fastened by 50 bolts was torn off and ejected. The gases ignited and flames entered the unit’s combustion chamber. The internal emergency plan was activated and the fire extinguished 5 min later; the total response only lasted 35 min. Some facilities were damaged (tank, drum dryer, filter, ducts); production was suspended. Operating procedures and protocols were revised and personnel training expanded.