On November 5, 2005, two Valero employees died from nitrogen asphyxiation inside a hydrocracker reactor at the Delaware City Refinery in Delaware City, Delaware. The hydrocracker reactor had been placed under nitrogen purge as part of a turnaround maintenance preparation. A worker entered the reactor and was overcome by the oxygen-deficient nitrogen atmosphere. A second worker entered the reactor to rescue the first worker and was also overcome. Both workers died inside the reactor.

The incident is a classic "would-be rescuer" confined space fatality scenario — where a second person enters a hazardous confined space to rescue a victim without adequate respiratory protection, and both die. Nitrogen asphyxiation kills rapidly and silently: a worker entering a nitrogen-purged atmosphere may become incapacitated within seconds, without any warning sensation, because nitrogen is odorless, colorless, and does not cause the choking sensation that would trigger a self-rescue response.

The CSB found that the fundamental failure was inadequate training and awareness of nitrogen asphyxiation hazards among the workers involved. The workers did not understand that entering a nitrogen-purged reactor without supplied-air respiratory protection would cause instantaneous incapacitation. The rescue attempt without proper equipment turned a single fatality into a double fatality. The case demonstrates why confined space rescue must always be performed by trained, properly equipped rescue personnel — never by well-meaning bystanders.