On October 1, 2020, at approximately 9:00 a.m., approximately 554 pounds of chlorine gas were accidentally released. Exposure to the toxic chlorine vapors seriously injured one employee. At the time of the incident, four (4) workers (two unit employees and two contractors) were trying to stop a chlorine leak (described as a fugitive emission) from a drain valve in the company’s methyl diisocyanate production plant. Because a chlorine leak was detected, the unit assumed that at least one of the drain valve’s connections in its flanges or bonnet was loose.
The four (4) workers held valid safe work permits to retighten the bolts using hand tools. However, the leak persisted after the bolts were retightened by hand. One of the contractors then tried to retighten the bolts with an impact wrench, a power tool that delivers short bursts of high torque. The vibrations from the impact wrench caused the connections on the drain valve’s bonnet to fail catastrophically, forcefully disconnecting the top half of the drain valve (closing element, stem, and handle) from the bottom half (body and seat) and releasing the toxic chlorine vapors.
The workers were wearing supplied-air respirators while performing the work. After the chlorine release, they evacuated the area. However, one (1) employee did not switch to their “escape bottle” before detaching the respirator from the stationary air supply source during the evacuation. The subsequent lack of airflow supplied to the respirator caused the employee to remove their mask during the evacuation, exposing them to toxic chlorine vapors. The employee was transported and admitted to a hospital for medical treatment.
The facility’s investigation found that the drain valve was severely corroded. Over time, chlorine being released from the drain valve had reacted with condensed water that had accumulated in the piping system’s insulation blanket, creating hydrochloric acid that had corroded the valve and deteriorated the bolts in the valve’s bonnet. After the incident, the facility discovered that three (3) other valves were similarly corroded.
The investigation also revealed that one of the contractors attempted to exercise their stop-work authority upon observing the dilapidated valve. The contractor brought concerns to one of the Unit employees. While the Unit employee sought a supervisor’s opinion, there was a miscommunication between the two parties about whether to continue working on the corroded valve. As a result, the work continued, ultimately resulting in the release of chlorine gas.
Probable Cause
Based on the facility’s investigation, the CSB determined that the probable cause of the incident was the catastrophic failure of corroded bolts in the drain valve’s bonnet. The corrosion was caused by prolonged exposure to hydrochloric acid, created by the reaction between chlorine and condensed water.
Contributing to the incident was the facility’s mechanical integrity program. Had the valve been inspected before authorizing this job, the extent of the corrosion could have been identified, and a shutdown could have been initiated to replace the drain valve rather than attempting repairs.
Contributing to the severity of the incident was the facility’s respiratory protection program, which did not ensure its workers could effectively transition to the escape bottle during an emergency. If the facility had effectively trained its workers to use their escape bottles during an emergency, the employee should not have been exposed to chlorine.
Source: https://www.csb.gov/assets/1/6/incident_reports_volume_2_2025-03-12.pdf