HAVE YOU IDENTIFIED ALL POSSIBLE SOURCES OF CORROSION?

On October 1, 2020, at approximately 9:00 a.m., approximately 554 pounds of chlorine gas were accidentally released at a facility in Louisiana. Exposure to the toxic chlorine vapors seriously injured one employee.

At the time of the incident, four workers (two employees and two contractors) were trying to stop a chlorine leak (described by as a fugitive emission) from a drain valve in the company’s methyl diisocyanate production plant. Because a chlorine leak was detected, the company assumed that at least one of the drain valve’s connections in its flanges or bonnet was loose.
The four workers had a safe work permit 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 using an impact wrench, a power tool designed to tighten and loosen bolts with 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 to perform the job. After the chlorine release, they evacuated the area. However, one employee did not switch to the “escape bottle” for their respirator before detaching the respirator from the stationary air supply source during the evacuation. The subsequent lack of supplied airflow in the respirator caused the employee to remove their respirator mask during the evacuation, resulting in the employee being exposed to the toxic chlorine vapors. The employee was transported and admitted to a hospital for medical treatment.
The company'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 company discovered that three other valves were similarly corroded.

The investigation also revealed that one of the contractors tried to use their stop work authority after seeing the dilapidated valve. The contractor brought concerns to one of the employees. While the  employee went to get a supervisor’s opinion, there was a miscommunication between the two parties about whether or not to continue working on the corroded valve. As a result, the work continued, ultimately leading to the chlorine gas release.

Probable Cause
Based on the company'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 company's mechanical integrity program. Had the company  inspected the valve 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 instead of attempts to repair it. Contributing to the severity of the incident was the company's respiratory protection program, which did not ensure its workers could effectively transition to the escape bottle during an emergency. Had the company effectively trained its workers to switch to their escape bottles during an emergency, the employee should have not been exposed to the chlorine.

Source: CSB.gov