U.S. Chemical Safety Board Issues Bulletin on Hydraulic Shock

The U.S. Safety Chemical Board, or CSB, issued a safety bulletin on Jan. 15 that provided recommendations for industries utilizing anhydrous ammonia in bulk refrigeration operations.

The bulletin focused on how to avoid a hazard called “hydraulic shock,” which is defined as a sudden, localized pressure surge in piping or equipment that often occurs 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.

In pointing out lessons to prevent hydraulic shock, the CSB report noted that it is vital to avoid a manual interruption of evaporators in defrost and to ensure that control systems are password protected so that only trained, authorized personnel have the authority to manually override the system.

The report also suggested that each evaporator coil in an ammonia refrigeration system should be controlled by a separate set of valves.

It went on to say that an emergency shutdown should be activated in the event of an ammonia release if the leak cannot immediately be isolated and controlled.

The report also stressed the danger of bypassing safety controls put in place to prevent an ammonia release.

“This issue really comes back to IIAR safety standards,” IIAR president Dave Rule said. “It’s always important to identify any procedures and standards that aren’t being followed correctly, which could result in ammonia accidents.”

However, IIAR said it does not agree with all of the conclusions reached in the CSB report, which was issued following a recent ammonia release resulting from hydraulic shock.

The report suggested that a freezer with four coils running on one bank of valves can cause hydraulic shock, and recommended that every evaporator have its own bank of valves.

“That might be good practice in very large, low-temperature scenarios, but it’s not necessarily a panacea,” said Eric Smith, IIAR vice president and technical director. “There are some instances where grouping evaporators is entirely appropriate.”

IIAR said it will continue gathering information on the CSB report and hopes to submit a response in the near future.

“It was good to have a report that gives an overview to help prevent this type of accident in the future,” Rule said. “The goal is to have everyone working together for a safer industry by following IIAR standards. IIAR and the CSB are working toward the same goals in following appropriate safety standards and design guidelines, and by making sure operators are properly trained and adhering to the safety protocol.”