The explosion of an auxiliary boiler occurred in the process of lighting, during plant start-up. In the 20 years of plant operation, the boiler has been operated in the same way and without any problems. The investigation showed that the explosion happened because the manual valves for burners were not sufficiently closed and that they allowed larger quantities of gas into the boiler; moreover, the dampers were not sufficiently open for air and the flue gas. The installation Of firing equipment, made according to the latest revision of relevant and recognized standards, which ensures automatic burner ignition, flame monitoring, and safety control equipment is one of the requirements to avoid boiler explosion. (C) 2004 American Institute of Chemical Engineers Process.