Abstract: At 7:30 p.m. on July 27, 2021, employees were working on a valve when approximately 160,000 pounds of a mixture that included acetic acid, methyl iodide, methyl acetate, hydrogen iodide, and other materials were released. Employees were sent in to the unit to modify the control valve so that it could be used as a point of isolation for a repair project to fix a leaking pipe spool in the methanol supply line to the acetic acid reactor, scheduled for the next morning. The employees received a permit from the facility operations group to enter and perform the work, but no one from the host employer walked out to the job with the team or went up to monitor their activities. The employees were not informed that system was in an energized state, or that the reactor that the valve was immediately connected to contained hot and pressurized hazardous process materials in liquid state. The valve was the only flow restriction device between the reactor and the spool of pipe that was to be worked on. The employees removed several fasteners from the valve assembly, enabling the actuator to be removed. In the process, the fasteners that secured the bonnet of the valve in place were also removed. With these fasteners removed, the pressure boundary seal made by securing the bonnet in place was compromised. The fluids in the system pushed the stem out of the valve through the valve top cap opening and process containment was lost. Employees were exposed to the fluids that left the process. Two of the employees, a pipefitter and pipefitter foreman, died onsite due to thermal and chemical burns, and inhalation injuries. The third employee, a pipefitter, was rescued and transported to an offsite medical facility, where he was admitted for treatment and hospitalized for several weeks, before being transferred and subsequently released for continued recovery at home.