The U.S. Chemical Safety Board (CSB) has released its case study titled “Key Lessons for Preventing Inadvertent Mixing During Chemical Unloading Operations”. The study examines a mixture of incompatible materials at the MGPI Processing Plant in Atchison, Kansas on October 21, 2016.
The mixture resulted in a chemical release containing chlorine and other compounds that traveled into the community. The CSB’s investigation examines several key issues including the design of chemical transfer equipment, automated and remote shut off systems, and chemical unloading procedures.
The MGPI facility produces distilled spirits and specialty wheat proteins and starches. The chemical release occurred when sulfuric acid was inadvertently unloaded from a tanker truck into a fixed sodium hypochlorite tank at the plant. The two materials combined to produce chlorine gas and other by-products that sent over 140 individuals, both workers and members of the public, to area hospitals and resulted in shelter-in-place and evacuation orders for thousands of local residents.
The CSB’s final report includes 11 key lessons and outlines clear safety improvements that can be implemented at similar facilities across the country. Among these are facilities should evaluate chemical unloading equipment and processes and implement safeguards to reduce the likelihood of an incident while taking into account human factors issues that could impact how facility operators and drivers interact with equipment.
Facility management should evaluate their chemical transfer equipment and processes and, where feasible, install alarms and interlocks in the process control system that can shut down the transfer of chemicals in an emergency.
The CSB’s investigation found that on the morning of the incident, a tanker truck arrived at the MGPI facility to deliver sulfuric acid. A facility employee escorted the driver to the locked loading dock and unlocked the gate to the fill lines and the sulfuric acid fill line.
But unknown to the operator, the sodium hypochlorite fill line was also unlocked. And the two lines, which were only 18 inches apart, looked similar but were not clearly marked. The driver inadvertently connected his truck’s sulfuric acid hose to the sodium hypochlorite line and sulfuric acid began flowing inside.
As a result of the incorrect connection, thousands of gallons of sulfuric acid from the tanker truck entered the facility’s sodium hypochlorite tank. The resulting mixture created a dense green cloud that traveled northeast of the facility until the wind shifted the cloud northwest towards a more densely populated area of town.