The U.S. Chemical Safety Board (CSB) recently released a safety video detailing lessons learned from the release of 32,000 pounds of anhydrous ammonia that occurred at Millard Refrigerated Services Inc. on August 23, 2010 (GM Aug. 30, 2010). The accident resulted in over 150 exposures to offsite workers, thirty of whom were hospitalized, with four winding up in an intensive care unit.
The video, entitled “Shock to the System,” is based on the CSB’s recent safety bulletin entitled Key Lessons for Preventing Hydraulic Shock in Industrial Refrigeration Systems, and includes a detailed 3D animation of the events that led up to the resulting ammonia release. It is available for viewing at www.csb.gov and on YouTube.
The CSB’s investigation found that the day prior to the accident the Millard facility experienced a loss of power, during which time the refrigeration system was shut down. The next day the system regained power and was up and running. 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.
The result was that both hot, high-pressure gas and extremely low temperature liquid ammonia were present in the coils and associated piping at the same time, causing the hot high-pressure ammonia gas to rapidly condense into a liquid. Because liquid ammonia takes up less volume than ammonia gas, a vacuum was created where the gas had condensed. The sudden pressure drop sent a wave of liquid ammonia through the piping, causing a sudden pressure surge known as “hydraulic shock.”
Hydraulic shock results in a sharp pressure rise, with the potential to cause catastrophic failure of piping, valves, and other components. In the Millard case a roof-mounted 12-inch suction pipe failed, releasing more than 32,000 pounds of anhydrous ammonia. The ammonia cloud traveled 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.
The video presents a number of key lessons learned from the CSB’s investigation, including avoiding the manual interruption of evaporators in defrost; requiring control systems to be equipped with password protection to ensure that only trained and authorized personnel have the authority to manually override systems; designing ammonia refrigeration systems so that each evaporator coil is controlled by a separate set of valves; and initiating an emergency shutdown in the event of an ammonia release if a leak cannot be promptly isolated and controlled.
Ammonia leaks caused problems at two other facilities recently. The U.S. Occupational Safety and Health Administration (OSHA) is investigating an ammonia leak at the Reser’s Fine Foods plant in Topeka, Kan., where some 100 employees reportedly had to be evacuated due to a leak from a refrigeration unit.
The Texas Commission on Environmental Quality (TCEQ) is investigating an anhydrous ammonia release reportedly caused when a forklift was driven into a supply line at the Pepsi bottling plant in Mesquite, Tex. One worker was taken to the hospital with respiratory problems and skin burns, and the entire Pepsi plant was evacuated while emergency responders spent an hour sealing the leak.