An explosion and fire occurred at the City Wastewater Treatment Plant (WWTP), killing two employees
and severely burning a third that stayed 4 months in hospital.
The WWTP additional process to remove nitrogen and phosphorus compounds requires the addition of methanol (highly flammable), which is stored in an aboveground storage tank. The methanol storage, installed in 1993, consisted in one 38 m3 steel tank, with plastic piping. An aluminium flame arrester had been installed on the vent of the tank, but had not been changed or maintained since and was heavily corroded by the methanol vapours. A metal roof 9 m above ground shaded the tank ; it was damaged by hurricanes in 2005 and required repair. It was decided to remove the roof using a city-owned crane and a rented man-lift. The job was planned but no specific risks analysis was carried out.
At 11:15 am., maintenance workers were using an oxy-acetylene cutting torch just above the methanol storage tank; sparks accidentally ignited vapours coming from the tank vent on that hot day, creating a fireball on top of the tank. The flame flashed back into the storage tank, causing an explosion inside the tank that precipitated multiple methanol PVC-piping failures and a large fire (11,35 m3 of methanol) that engulfed the tank and workers.
The U.S. Chemical Safety and Hazard Investigation Board (CSB) investigated the case and showed a lack of methanol hazard recognition, lack of safety and hazard review in job planning, a lack of communication regarding risks and employee training, an ineffective flame arrester (lack of maintenance / knowledge of safety measures). Plastic piping (instead of steel) that ruptured contributed to the aggravation of the consequences.
The plant operator was advised to implement policies, practices, and procedures concerning safety and health in the workplace (including hot work procedures and chemical hazard communication) and improve the safety of flammable liquid storage tanks, including appropriate piping and flame arresters.