Key lessons to prevent ammonia hydraulic shock&nbsp;

The US Chemical Safety Board (CSB) has released a safety video highlighting key lessons to prevent hydraulic shock, namely the type of incident that saw 14,500 kg of anhydrous ammonia released into the atmosphere at a refrigeration facility in Alabama in August 2010.

The safety video, titled ‘Shock to the System’, was commissioned in response to an investigation into the release at Millard Refrigerated Services Inc., which hospitalised 32 offsite workers.

There, hydraulic shock ruptured the evaporator piping manifold inside one of the freezers and caused a 12-inch suction pipe mounted on the roof to explode, and the resulting ammonia cloud to travel 400m towards an area where 800 contractors were working outdoors.

CSB initially investigated the incident and also released a safety bulletin, in January 2015, titled, ‘Key Lessons for Preventing Hydraulic Shock in Industrial Refrigeration Systems’. The paper is designed to advise facilities that use anhydrous ammonia in industrial quantities on how to avoid hydraulic shock in refrigeration systems.

“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,” CSB chairperson Rafael Moure-Eraso said.

What happened at Millard?

The paper, like the video, answers key questions as to what happened at Millard’s frozen poultry export facility in Mobile, Alabama. The facility operated a 65,000 kg capacity refrigeration system with eight separate freezers and just a day prior experienced a power outage that lasted over seven hours, during which time the refrigeration system was shut down.

The following day the system regained power and was operational again, yet operators reported some abnormalities. While trouble shooting with the control system, an operator unwittingly reset the refrigeration cycle on a group of freezer evaporators that were in the process of defrosting, in effect, causing the freezer evaporator to switch directly from a step in the defrost cycle into refrigeration mode with the evaporator coil still containing hot, high-pressure gas.

The reset triggered a valve to open, feeding low temperature liquid ammonia back into all four evaporator coils before removing the ammonia gas. This caused the hot, high-temperature ammonia gas to condense into a liquid once it merged with the low temperature liquid ammonia in the coils and piping. Given liquid ammonia’s lower density, a vacuum was created with the void sending a wave of liquid ammonia through the piping, causing the hydraulic shock.

Key lessons learned

The Millard incident highlighted the need to avoid the manual interruption of evaporators in defrost and ensure control systems are equipped with password protection to ensure only trained and authorised personnel have the authority to manually override systems.

The facility was designed so that one set of valves controlled four separate evaporator coils, leading to a larger, more hazardous pressure surge. CSB notes that to avoid this occurrence in ammonia refrigeration systems, each evaporator coil should be controlled by a separate set of valves.

After the release at Millard was discovered a decision was made to isolate the leak while the refrigeration system was still operating, rather than initiating an emergency shutdown. If an ammonia leak cannot be promptly isolated and controlled, the CSB advises that an emergency shutdown should be activated to minimise the amount of ammonia released in the event of an accident.

What is hydraulic shock?

A sudden, localised pressure surge in piping or equipment resulting from a rapid change in the velocity of a flowing liquid. The highest pressures often occur when vapour and liquid ammonia are present in a single line and are disturbed by a sudden change in volume. This 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’ sound in the refrigeration piping.