DuPont had long-standing issues with vent piping to an incinerator installed in 2011. To deal with these problems, daily instructions had been provided to operations personnel to drain liquid from these pipes, which were located in another section of the plant.DuPont’s instructions did not specify additional breathing protection for this task. On the night of the incident, not realizing the original blockage was cleared, workers went to drain the other piping. They did not know that high pressure in that other piping was related to the fact that liquid methyl mercaptan was once again flowing through the original, now unblocked pipes.