USA: The U.S. Chemical Safety Board (CSB) has released a safety video following the release of over 14.500 kg’s of refrigerant at the Millard Refrigeration Plant in 2010, which resulted in over 150 workers being exposed to anhydrous ammonia, thirty of which were hospitalized and four admitted to intensive care.
The safety video details the events that led up the resulting ammonia release and key lessons in order to prevent a similar accident from occurring due to 'Hydraulic Shock in Industrial Refrigeration' systems.
The investigation found that the day prior to the accident the Millard facility experienced a loss of power that lasted more than seven hours, during which the refrigeration system was shut down. The following day the system regained power and was up and running, though operators reported certain problems.
Whilst troubleshooting an operator cleared alarms in the control system, which reset the refrigeration cycle on a group of freezer evaporators that were in the process of defrosting.
This resulted in both hot, high-pressure gas and extremely low temperature liquid ammonia to be present in the coils and associated piping at the same time, causing the high-pressure ammonia gas to rapidly condense into a liquid.
The sudden pressure drop sent a wave of liquid ammonia through the piping, causing a sudden pressure surge known as "hydraulic shock," resulting in a sharp pressure rise and a catastrophic failure of piping.
The release formed a large cloud of Ammonia which travelled a quarter mile from the facility south toward an area where 800 contractors were working outdoors at a clean-up site for the Deepwater Horizon oil spill, resulting in a total of 152 workers reporting symptomatic illnesses from ammonia exposure.
CSB Investigator Tyler said, "The CSB's animation details how the pressure surge ruptured the evaporator piping manifold inside one of the freezers causing a roof-mounted 12-inch suction pipe to catastrophically fail, resulting in the release of more than 32,000 pounds of anhydrous ammonia and its associated 12-inch piping on the roof of the facility."
The video presents the key lessons learned from the CSB's investigation including avoiding the manual interruption of evaporators in defrost and requiring control systems to be equipped with password protection to ensure only trained and authorized personnel have
the authority to manually override systems.