Avoid the manual interruption of evaporators during defrosting, and ensure that control systems are equipped with security devices so that only trained and authorized personnel may manually override programmed operations. Also, after an unintended interruption of service at a facility that utilizes anhydrous ammonia in bulk refrigeration operations, hydraulic shock can be avoided by programming the control system to automatically remove the contents from the evaporator coils prior to restarting refrigeration.
These are among the key takeaways of the U.S. Chemical Safety Board’s investigation and final report on a 2010 anhydrous ammonia release that occurred at Millard Refrigerated Services Inc., Theodore, Ala.
The accident occurred on August 23, 2010, while two international ships were being loaded at Millard’s frozen poultry export facility. The 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 — that caused a roof-mounted 12" suction pipe to catastrophically fail, resulting in the release of more than 32,000 pounds of anhydrous ammonia.
As a result of its investigation, the CSB produced a safety bulletin to inform industries that utilize anhydrous ammonia in bulk refrigeration operations on how to avoid hydraulic shock. Along with “Key Lessons for Preventing Hydraulic Shock in Industrial Refrigeration Systems,” the government agency also produced a seven-minute safety video, “Shock to the System,” that includes a detailed 3D animation of the events that led up the ammonia release.
With hydraulic shock, 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. This abnormal transient condition results in a sharp pressure rise with the potential to cause catastrophic failure of piping, valves and other components. Often, prior to a hydraulic shock incident, there is an audible “hammering” in refrigeration piping.
The CSB’s bulletin notes that on the day before the incident, the facility experienced a loss of power that lasted more than seven hours. During that time, the refrigeration system was 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 via hot gas. 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 was fed into four evaporator coils before the hot ammonia gas was removed. 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 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 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" piping on the roof of the facility, which released the ammonia into the surrounding environment.
The CSB also found that the evaporators at the Millard facility were designed so 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 involved in the hydraulic shock event – leading to a larger, more hazardous pressure surge. Noting that four evaporator coils were controlled with a single set of refrigeration valves, the CSB recommends that when designing ammonia refrigeration systems, each evaporator coil should be controlled by a single set of valves.
The CSB found that immediately after discovering the ammonia release, a decision was made 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 because all other ammonia-containing equipment associated with the failed rooftop piping continued to operate. As a result, the CSB recommends that an emergency shutdown should be activated in the event of an ammonia release if a leak cannot be quickly isolated and controlled.