Safety Plan: Preventing hydraulic shock
Shortly before 9 a.m. August 23, 2010, it was just a normal work day at Millard Refrigerated Services Inc. in Theodore, Alabama. Then everything changed.
Two international ships were loading when the facility’s refrigeration system experienced "hydraulic shock" a sudden, localized pressure surge in piping or equipment resulting from a rapid change in the velocity of a flowing liquid. The highest pressures often occur when vapor and liquid ammonia are present in a single line and end up disturbed by a sudden change in volume.
As a result of a massive leak, over 150 workers suffered injuries from the incident and the U.S. Chemical Safety Board (CSB) issued a safety bulletin to inform industries that utilize anhydrous ammonia in bulk refrigeration operations on how to avoid a hydraulic shock hazard.
This abnormal transient condition results in a sharp pressure rise with the potential to cause catastrophic failure of piping, valves, and other components. In addition, often prior to a hydraulic shock incident there is an audible "hammering" in refrigeration piping. The incident at Millard caused a roof-mounted 12-inch suction pipe to catastrophically fail, resulting in the release of more than 32,000 pounds of anhydrous ammonia.
The release led to one Millard employee sustaining injuries when he fell while attempting to escape from a crane after the traveling ammonia cloud engulfed it. The large 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. Overall, 152 offsite workers and ship crew members reported symptomatic illnesses from ammonia exposure. Of those with illnesses, 32 required hospitalization, four of them in an intensive care unit.
"The CSB believes that if companies in the ammonia refrigeration industry follow the key lessons from its investigation into the accident at Millard Refrigeration Services, dangerous hydraulic shock events can be avoided—preventing injuries, environmental damage, and potential fatalities," said CSB Chairperson Rafael Moure-Eraso.
"Key Lessons for Preventing Hydraulic Shock in Industrial Refrigeration Systems" describes that on the day before the incident, on August 22, 2010, the Millard facility experienced a loss of power that lasted over seven hours. During that time the refrigeration system shut down. The next day the system regained power and was up and running, though operators reported some problems. While doing some 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. The control system reset caused the freezer evaporator to switch directly from a step in the defrost cycle into refrigeration mode while the evaporator coil still contained hot, high-pressure gas.
The reset triggered a valve to open and low temperature liquid ammonia fed back into all four evaporator coils before removing the hot ammonia gas. This resulted in both hot, high-pressure gas and extremely low temperature liquid ammonia present in the coils and associated piping at the same time. This caused the hot high-pressure ammonia gas to rapidly condense into a liquid. Because liquid ammonia takes up less volume than ammonia gas—a vacuum ended up created where the gas had been. The void sent a wave of liquid ammonia through the piping—causing the "hydraulic shock."
The pressure surge ruptured the evaporator piping manifold inside one of the freezers and its associated 12 in piping on the roof of the facility. An estimated 32,100 pounds of ammonia released into the surrounding environment.
"The CSB notes that one key lesson is to avoid the manual interruption of evaporators in defrost and ensure control systems are equipped with password protection to ensure only trained and authorized personnel have the authority to manually override systems," said CSB investigator Lucy Tyler.
The CSB also found the design of the evaporators at the Millard facility were such that one set of valves controlled four separate evaporator coils. As a result, the contents of all four coils connected to that valve group were a part of the hydraulic shock event—leading to a larger, more hazardous pressure surge.
As a result, the CSB said when designing ammonia refrigeration systems each evaporator coil should have a separate set of valves.
The CSB found immediately after discovering the ammonia release, someone made a decision to isolate the source of the leak while the refrigeration system was still operating instead of initiating an emergency shutdown. Shutting down the refrigeration system may have resulted in a smaller release, since all other ammonia-containing equipment associated with the failed rooftop piping continued to operate.
A final key lesson from the CSB’s investigation is an emergency shutdown should end up activated in the event of an ammonia release if a leak cannot be promptly isolated and controlled. Doing so can greatly reduce the amount of ammonia released during an accident.