(96b) Root Cause Analysis of a Cryogenic Refrigeration System Explosion

Emergency response personnel who responded to a smoke alarm at a manufacturing facility were injured when a cryogenic refrigeration system overheated and then exploded. The refrigeration system was a skid-mounted package unit that relied upon liquid nitrogen as the cryogen, a heat transfer oil as the secondary refrigerant, and electrical resistance heaters as a heat source to provide split-range control over the heat transfer oil temperature. The incident investigation revealed that the event was not the result of a simple single cause. Detailed root cause investigation and failure analysis was performed to identify the underlying causes.

The explosion was caused by the thermal expansion of trapped liquid. It was determined that several elements of good engineering practice that are considered for basic safety measures in chemical processing plants were not considered by the equipment designers. The following deficiencies were noted:

? The freezing point of the heat transfer oil was lower than the approach temperature of the nitrogen vapor in the cryogenic heat exchanger. If oil circulation stopped for any reason, the oil could freeze.

? In the heat transfer oil circuit, the electrical resistance heater was installed between a check valve and a manual valve with no pressure relief. The cryogenic heat exchanger was adjacent to the manual valve. If the oil in the cryogenic heat exchanger froze, it would behave like a closed valve.

? There was an interlock linking the oil temperature to the actuation of the electrical heater, but the temperature sensor was located far from the location of the heater. When the cryogenic heat exchanger froze, it isolated the temperature sensor from the oil in the immediate vicinity of the heating coils. This created the opportunity for a runaway heater.

? The design documentation and functional specifications provided by the refrigeration system manufacturer was inadequate for an independent hazard analysis.

The lessons learned from this incident can be applied by facilities to evaluate similar specialty refrigeration packages before integrating them into their plants.