Abstract: A double boiler type melting kettle was being used to melt asphalt-based waterproofing compound on the seventh floor of a hospital addition. Vent openings in the melting kettle oil jacket were plugged by cap fittings and crystalline asphaltic material. When heat was applied to the oil jacket the jacket ruptured, presumably resulting in a boiling liquid evaporating vapor explosion. Employees # 1 through #3, who were within the fireball, were burned. They were hospitalized for their injuries. Employee #4 may have been burned by hot material thrown outside the main flame. Employee # 1 was the job project manager, supervising employee #3 at the time of the accident. Employees #2 and #4 were applying waterproofing compound to flooring at the time of the explosion.