In a chemical plant, washing hoses and a stationary steel (DN 80) pipeline broke at 12:15 pm during cleaning of an evaporator/liquefier using carbon tetrachloride (CCl4). A 1-m³ discharge of CCl4 and ferric chloride particles (FeCl3) formed a thick orange cloud visible beyond the site. The operator based the response on a “chlorine leak” scenario and notified the local population by siren. Safety teams verified that no one had suffered from intoxication. The leak was plugged around 1 pm, with no injuries reported and plant activity unaffected. The operator issued a press release.
A portion of the discharge evaporated, while another portion was capable of mixing with stormwater. Effluent from the channel that served as an outlet before discharge to the natural outfall for all of the site’s storm drains was channelled into the extinction water retention basin; the measures introduced revealed no significant CCl4 concentration in the effluent collected from this basin.
Over the previous period, the evaporator/liquefier (EVLQ) purge circuit had not been operating satisfactorily, an intermittent internal clogging of the frigorie exchange and recovery circuit had been suspected. A few weeks earlier, internal washing using dichloromethane or methylene chloride (CH2Cl2) had been (unsuccessfully) attempted in order to dissolve the clogging deposits, with these tests being conducted after isolating the capacity and re-circulating the solvent in a closed circuit with a mobile pump and armoured hoses. The energy provided by the pump raised the loop temperature, with washing operations being interrupted in order to avoid boiling of the CH2Cl2.
On Wednesday morning, the use of CCl4 instead of CH2Cl2 was authorised; the CCl4 capable of being introduced at a higher was in fact “saturated” and therefore less reactive with the chlorine (Cl2).
A washing was performed during the same day, then stopped in the evening and resumed the next day under normal conditions until the accident occurred. Bent at more than 90°, the steel pipeline was torn from its supports, and the washing mix was sprayed onto the internal traffic lanes and adjacent reservoirs. In the presence of anhydrous Cl2, FeCl3 formed in all steel capacities; this “passivation” layer protected the steel against Cl2 aggressions. Its orange colouring grew more intense in the presence of (even very small) quantities of water. The suspension of FeCl3 particles in the vaporized CCl4 fog actually formed the orange cloud. Pressure in the pipeline rose to nearly 80 bar, thereby causing an instantaneous and sudden loss of confinement.
At the scene of the accident, the classified facilities inspectors requested a detailed incident report from the operator. Placing the EVLQ back into service was contingent upon determining the exact causes and circumstances of this accident.
