TANK OVERPRESSURE INCIDENT

On June 22, 2023, at about 9:10 p.m., a 5,200-gallon polyethylene storage tank (“tank”) ruptured at a facility in North Carolina. The rupture fatally injured one employee.
At the time of the incident, the tank needed to be refilled with aluminum chloride; however, sulfuric acid was added instead. Approximately 80 gallons of 93 percent sulfuric acid were added to the 60 gallons of aluminum chloride remaining in the tank, resulting in a reaction that caused the tank to rupture. The tank rupture fatally injured the employee who was refilling the aluminum chloride tank.
The company's investigation appeared to focus on the incorrect reaction rather than a reaction scenario consistent with the available evidence. Its research materials pointed to the reaction between sulfuric acid and aluminum metal that generated hydrogen gas. The company concluded that the hydrogen gas was released at its autoignition temperature (over 1,000 degrees Fahrenheit (°F)) and ignited, triggering an explosion. Post-incident photographs of the damaged tank do not appear to support a scenario where the internal tank temperature reached 1,000 °F, however. The melting temperature of the polyethylene is approximately 260 °F. Therefore, the company’s reaction scenario appears inconsistent with the available evidence.
The CSB concluded that the reaction most likely generated hydrogen chloride vapors. The more likely scenario was that the reaction between aluminum chloride and sulfuric acid produced enough hydrogen
chloride vapor to increase the pressure within the polyethylene tank, likely resulting in an overpressure that separated the tank’s body from its base.
The CSB estimated that approximately 130 pounds of hydrogen chloride vapors were accidentally released.
Probable Cause
Based on the factual information from the company's investigation, the CSB determined that the probable cause of the overpressure was the increase in the tank’s internal pressure resulting from the hydrogen chloride vapors created by the reaction between sulfuric acid and aluminum chloride.

Source:CSB.gov