Two liquid leaks with a hydrogen chloride (HCl) release occurred on pipelines when restarting the silicon tetrachloride workshop (SiCl4) at a chemical factory. Both the SiCl4 and carbochlorination units were shut down and the factory’s internal emergency plan was activated. In assuming that a return of water in contact with SiCl4 was responsible for this acid release, the operator turned off the gas treatment plant’s scrubbing columns. This action did not stop the leaking. Three hours later, an analysis of unit parameters revealed an abnormal drop in the level of both SiCl4 storage tanks: closing a valve on the SiCl4 bleed circuit served to halt the leak.
Failure to close this manual valve had caused the accident: the SiCl4 transferred to the boiler was vaporizing and condensing once again in the ‘spent gas’ pipeline. The pressure rise in this pipe then triggered opening of the safety valve connecting it to the ‘gas-rich’ supply line running below. The weight of liquid in these pipes, which were solely intended to transport gas, led them to break and the SiCl4 to leak. The next day, an accident analysis revealed that the ‘gas-rich’ supply pipe still contained liquid SiCl4. Since this line had never been fitted with a bleed system, the operator decided to eliminate the SiCl4 through vaporization, by injecting nitrogen (N2) so as to draw the SiCl4 to the gas treatment plant. Once the injection process began, the leak on the ‘gas-rich’ supply line resumed: to avoid exposing the technician to the risk of contact with SiCl4, the pipe was not repaired. N2 injection would have removed a silica plug that had been clogging the leak, which however did not stop by suspending the injection. The installation 3 hours later of a specially designed funnel to recover the product in a vacuum retention made it possible to bleed the entire facility. The 1,300 litres of SiCl4 recovered were then stripped and handed over to a specialist firm, as was the heat insulation waste. Several corrective actions were implemented: identification of the bleed valve position, reinforcement of pipeline support structures, issuing of operating procedures for verifying the circuits, inspection scheduled prior to unit restart, reorganisation of personnel shifts as part of the restart protocol, and availability of equipment to collect potential leaks.