Two employees were exposed to Hydrogen Fluoride (HF) after a release occurred during maintenance works. Following 2 accidents in less than a month (ARIA 23694 and 23707), plant activities were limited to maintenance and inspections. Some equipments, such as an HF vaporizer, were left in an abnormal shutdown state (i.e., it contained liquid HF) after the unit emergency shut down of July 20.
A permanent system for removing liquid hydrogen fluoride from equipment after shutdown had recently been installed at the facility, but the system, used for the first time, did not function properly. Therefore, operators began using a venturi stick to remove liquid HF from the vaporizer on August 12. Personnel fed plant nitrogen under 13 bar to the equipment upstream of the vaporizer then attached a venturi stick with copper tubing to the vaporizer, inserted it into the plant sewer, and attached a water hose to the end of the stick to draw the liquid hydrogen fluoride to the sewer. The venturi stick was tied off with a rope. The draining operation continued into the next day (13/08). At 9 am, after checking the progress of the venturi process, the operator suspected a problem with nitrogen flow to the vaporizer, and opened and closed some valves to create a surge in the system and clear any blockage. Flow to the sewer rapidly increased, causing the venturi stick to lift out of the sewerwhich created a cloud that likely contained HF. The operator instructed a nearby contractor to leave the area and then stopped the flow to the sewer by closing a valve at the vaporizer. Noticing a red mark on his arm, he immediately got under a nearby safety shower and remained there until assistance arrived. A maintenance supervisor walking through the area aided the operator in moving to first aid, then experienced a coughing spell suggesting he may have been exposed to HF vapours ; both were sent to the hospital and released the next day.
The company was faced with a non-routine situation after the 20/07 accident and lacked formal procedures for identifying and planning non-routine activities. Given the highly hazardous nature of HF, more specific procedures and job planning would have been necessary to ensure the operation was safe (such as appropriateness of using 13-bar nitrogen, adequate means for securing the venturi stick, individual protective equipments required ). The US CSB that investigated the 3 accidents pointed out the need for the company to improve its management system, in particular hazards analysis, non routine situations (degraded mode) and written operating procedures.
