During degassing of a 107-m³ propane tank on an industrial site undergoing dismantling, some gas escaped from the reservoir and ignited, while at the same time operators were replacing flange bolts on the manhole. The three subcontractor employees experienced shock and were treated in hospital for relatively minor burns. The POI emergency plan was enacted, 100 employees evacuated and a safety zone cordoned off. Firefighters cooled the tank (the reservoir had not been operated since March 1998). The accident in fact had multiple causes: a defective manometer, which incorrectly indicated the absence of gas inside the tank; subcontractor procedural error by removing the manhole cover before filling the reservoir with water in order to speed reservoir draining; and subcontractor misjudgement – despite their experience with this type of operation – as evidenced by the failure to use explosion-proof equipment within a zone where their activity necessarily created an inflammable atmosphere.