In a chemical plant, failure of a sensor resulted in disturbances in a boiler supplying a hydroquinone/catechol workshop. Two boilers were shutdown under emergency conditions. During their re-start, a vapour surge led to the opening of a valve whose tare pressure was set at 45 bar and the release of a large vapour cloud along with a loud noise that lasted 20 min. The police and the firemen were on site to gather information on the situation given the proximity of the accident site to a high speed traffic lane.