An abnormal transient hazard known as “hydraulic shock” poses a significant risk to industries that use anhydrous ammonia in bulk refrigeration operations, a safety bulletin by the U.S. Chemical Safety Board states.
Hydraulic shock refers to a sharp pressure rise with the potential to cause catastrophic failure of piping, valves and other components, the bulletin states. Often prior to a hydraulic shock incident there is an audible “hammering” in refrigeration piping.
The bulletin is based on an investigation into a 2010 anhydrous ammonia release that occurred at a refrigerated services company in Theodore, AL.
At Theodore, two international ships were being loaded when the facility’s refrigeration system experienced “hydraulic shock” which is defined as 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 are disturbed by a sudden change in volume.
A roof-mounted 12-inch suction pipe catastrophically failed, releasing more than 16 tons of anhydrous ammonia.
One employee fell while attempting to escape from a crane after it became engulfed in the traveling ammonia cloud. 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.
A total of 152 offsite workers and ship crew members reported symptomatic illnesses from ammonia exposure. Thirty two of the offsite workers required hospitalization, four of them in an intensive care unit.
Entitled “Key Lessons for Preventing Hydraulic Shock in Industrial Refrigeration Systems,” the bulletin describes that on the day before the incident the facility experienced a loss of power that lasted over seven hours. During that time, the refrigeration system was shut down. The next day the system regained power, 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.
Through an automatic valve, low temperature liquid ammonia was fed back into all four evaporator coils before removing the hot ammonia gas. Hot, high-pressure gas and extremely low temperature liquid ammonia came together in the coils and associated piping at the same time.
The hot high-pressure ammonia gas rapidly condensed into a liquid. Because liquid ammonia takes up less volume than ammonia gas, a vacuum was created where the gas had been, sending a wave of liquid ammonia through the piping.
The pressure surge ruptured the evaporator piping manifold inside one of the freezers and its associated 12-inch piping on the roof of the facility. An estimated 32,100 pounds of ammonia were released into the surrounding environment.
CSB investigator Lucy Tyler said one key lesson from the incident 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.
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