New requirements mandated, proactive approach to operation and maintenance urged
By Simon Blake
Technical Safety BC has moved quickly to ensure that necessary safety procedures for ammonia refrigeration systems are in place following two significant leak incidents.
These incidents included a system failure at the Fernie Memorial Arena on Oct. 17, 2017, in which three workers died after ammonia leaked into the brine system, pressurizing it, and resulting in a rapid release of ammonia into an enclosed area when a coupling in the brine system separated. The ammonia leak also resulted in the evacuation of 95 residents from 55 nearby homes.
On Oct. 24, 2018, an ammonia leak occurred at a pet food manufacturing plant in Langley, B.C. An evaporator ruptured, releasing an estimated 485 to 1,500 lbs. of ammonia into a freeze-drying production chamber and from there into the environment.
There were no injuries, but the mostly industrial area in which the plant is located had to be evacuated for 48 hours.
Technical Safety BC released its investigation report for the Langley incident on March 19, 2019. It blames the rupture of the evaporator on an overpressure condition “that developed as a result of trapping an excessive volume of liquid ammonia between the valves.”
The responding refrigeration mechanic attempted to improve system performance by closing two hand expansion valves followed by closing the suction valve to generate pressure within the evaporator to move or dislodge potentially trapped oil. The pressure rose rapidly once the evaporator was isolated, breaking the pressure gauge needle and rupturing the evaporator. Cold Dynamics Ltd. of Calgary, of which P&HVAC columnist Greg Scrivener was an integral part of the investigation team and produced the engineering analysis that is included in the Technical Safety BC investigation report.
The investigation found that oil was not being sufficiently removed from the refrigeration system following a change in compressor output setting prior to the rupture. The previous day, an overload failure of the ammonia pump occurred and was suspected to be a result of oil contamination. Once service was restored, the performance of the evaporators was suspected to be affected by oil contamination.
The report also noted that the system was operating with the hand expansion valves “fully open, contrary to design specifications which resulted in the evaporator becoming flooded with ammonia and leaving no room for expansion.” Closing the suction valve without first pumping down the liquid ammonia level for the evaporator increased the risk of hydrostatic expansion.
“In this case, the system wasn’t operating properly and the valves were being manipulated in an effort to correct that situation,” noted Tony Scholl, Technical Safety BC technical leader for boilers and pressure vessels, in an interview with Plumbing & HVAC Magazine. “The over-pressurization and subsequent rupture of the cold trap (evaporator) occurred only after both the expansion and suction valves were closed.”
The ammonia level in the evaporator, normally about 20 to 50 percent by volume, was estimated to be as high as 96 percent at the time of the incident. “A relatively small increase in heat transfer resulted in a tremendous increase in the ammonia pressure within the cold trap (evaporator) which resulted in the rupture,” he added.
The Technical Safety BC investigation report for the Fernie incident was released last summer and was reported in the September issue of Plumbing & HVAC.
Issue not addressed
In the Langley incident, the report noted that there was no standard operating procedure specific to the procedure that was being attempted by the technician and that there was no designated chief operating engineer to oversee ongoing maintenance and monitor performance and trends at the time of the incident.
“(A standard operating procedure) may not have actually addressed this specific situation – the over-pressurization – but in the events leading up to the situation there was an existing procedure that could have been followed to drain oil from the cold trap (evaporator),” remarked Scholl.
Manipulating manual valves is not uncommon, however, a standard operating procedure would have provided guidance to technicians and operators in when and how much to manipulate them. Other options could include installing warning signs or locking the valve in place with a car seal.
The locking device may not have prevented the incident but would tend to prevent casual manipulation of the valves. “You would have to go and get a tool to remove the seal which might cause a person to give it some extra thought and determine what the actual control point for that valve should be,” said Scholl.
There was a qualified person in charge of the plant prior to the incident, but not at the actual time of the rupture, he added. And while having a chief engineer in charge of the system is required, it is just one part of overall refrigeration system management.
Oil and ammonia
In an ammonia refrigeration system, oil can migrate through the system. As a result, there are oil traps at various locations to collect the oil so that it can be removed.
“Excessive oil will have a detrimental effect on heat transfer,” noted Scholl. That means the plant has to work harder to maintain a set temperature, resulting in reduced performance and higher operating costs.
The oil lubricates the refrigeration compressors and as it migrates into the system, oil levels in the compressors need to be topped up to prevent damage.
This is why an oil management program is critical. “Once the plant is commissioned and up and running, you are going to get a really good idea of what oil consumption should be,” he added. If there are anomalies, such as more frequent draining and adding more oil than usual, this is a strong indicator of a condition that must be addressed.
Safety measures put in place
Immediately after the Fernie incident, Technical Safety BC issued two province-wide safety orders. The first required the testing of secondary cooling systems, like the brine systems in arenas, for ammonia contamination. “That’s an indicator that the chiller may have failed and is leaking ammonia,” noted Scholl.
All brine test results had to be reported to Technical Safety BC. “We did find brine systems that were contaminated with ammonia.” Systems with leaking chillers were shut down until repairs were made.
The second safety order reminded owners of their responsibility to staff and operate the refrigeration plant only with qualified personnel.
Technical Safety BC also provided its safety officers with further training on ammonia arena systems. Every public ice rink using ammonia – about 180 in total – was inspected.
“Before the Fernie incident, we were certainly tracking ammonia incidents, but after Fernie, we investigated every reported ammonia release by dispatching a safety officer and going to the site,” said Scholl.
Following the Fernie incident and investigation, Technical Safety BC made 18 recommendations to further improve refrigeration system safety.
And, as of this year, Technical Safety BC is recommending that ammonia refrigeration system operators have a maintenance program in place.
Standard operating parameters must be established and implemented, along with an emergency shutdown procedures. In the Langley incident, oil had been drained from different parts of the system on a number of occasions, and oil was added to the compressors, however, there was no evidence that an effective monitoring program was in place, said Scholl.
System operators need to establish an oil management system that tracks how much oil is drained or added, where it’s added or removed in the system and when.
The maintenance program, which is not limited to ammonia refrigeration systems, should include standard required maintenance along with identifying issues that can occur and can be identified through routine maintenance and inspection – corrosion, ageing components, servicing of valves, etc. It would also include standard operating procedures, maintenance requirements and an emergency operating procedure to shut down the plant.
“All these things need to work together,” added Scholl. “(Facility operators) need to develop a proactive approach to proper maintenance and procedures to ensure the safe operation of refrigeration plants.”
For more information please visit www.technicalsafetybc.ca.