O-ring fails, causes refrigerant leak

An O-ring that had been missed during major maintenance two years earlier was the cause of a refrigerant leak at a plant. 

The leak took place at a plant with three individual refrigeration units, all indirect closed loop systems with R-134A (Tetrafluoroethane) refrigerant. Two of the units are 740 kw each with 4,900 lbs of refrigerant, identical in configuration and function as heat pumps, and the third unit is 235 Kw with 2,300 lbs of refrigerant, used only for cooling.

Although the plant is monitored and managed by a Direct Digital Control (DDC) system, a 4th class power engineer continuously attends the plant, plus there are control and safety devices for each unit.  

The refrigerant leak detector set off an alarm and the shift engineer shut down the appropriate unit before the concentration of refrigerant exceeded the threshold limit value, avoiding a potential total loss of refrigerant.

The refrigeration unit involved in the incident has one condenser on the high pressure side, one evaporator on the low pressure side and one intercooler. BCSA’s investigation showed that the O-ring for a plug at the bottom part of the condenser had failed under operating pressure when the chiller unit started. Further investigation showed that this particular O-ring, unlike the others, was not replaced during major maintenance two years earlier, and was hard and brittle

A refrigeration unit  condenser goes through temperature changes as it is used for cooling in summer and heating in winter. O-rings that are subjected to these temperature changes will eventually become hard and brittle and cease to seal properly, so need to be replaced regularly. If this particular O-ring had been replaced at the last service with the others, this incident would most likely not have happened.

The investigation is documented in BCSA’s enhanced incident investigation report format.

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