A hydrochloric acid (HCl) leak was detected shortly after 1:00 p.m. in the recently installed HCl distillation unit of a chemical plant. The unit was still in the test phase before going into production. Toxic gas detectors detected the leak at around 1:10 p.m. The installation was secured, and the site’s internal safety department secured the affected area. The unit was shut down at 1:50 p.m., after a total of 11 kg of HCl had been released. The operator had to wait for the distillation column to empty because it was not equipped with an emergency decompression system.
The leak was caused by a hole in the tantalum membrane on the stainless steel plug of the pressure sensor positioned on the gas-liquid interface of the column to be distilled. The pressure sensor is located downstream from a valve. Incorrect assembly of the membrane by the subcontractor responsible for the manufacture of this equipment caused the tantalum membrane to crack. This cracking would have allowed HCl to flow between the membrane and the plug, causing it to corrode rapidly and become pierced.
In addition, the accident analysis revealed other problems, particularly concerning the valve upstream from the sensor. Deterioration of its PFA coating was noted during examination of the pressure sensor. Following discussions with the valve’s supplier, it would appear that the valve manufacturer had noted MFI (Melting Flow Index) values above the acceptable limits. The shift of this parameter indicates that a coating quality problem had occurred, making it increasingly fragile under high temperature and pressure conditions. These elements were either unknown to the supplier of the distillation unit’s skid or to the operator. The operator reviewed all his valves in order to verify the coating; some valves were changed. Following the incident, the facility operator also installed up an emergency decompression system on the column. A system of water curtains was also installed to contain any possible leaks.
An initial incident had occurred at this unit on the first day of testing just 1 month earlier (ARIA 51003).