In an ethylene oxide production unit of a chemical plant, one worker was killed and another severely injured, asphyxiated by nitrogen venting through the large open pipe where they were working on during major maintenance works.

The 48-inch horizontal pipe had been shut down and opened to the air at one end but was still connected into the process unit. The two victims, both skilled and experienced workers, were inspecting the inside of the open pipe end to gauge the effectiveness of an earlier cleaning effort using a black light, which causes grease, oil, and other contaminants to glow in the dark. As the midday sun made it difficult to see, the workers asked two contractors to hold a black plastic sheet over the open pipe end while they crouched just inside. While the men worked, nitrogen gas was being used to purge air and moisture from the unit, protecting the chemicals inside. Unknown to any of the workers, the plastic sheet created a dangerous enclosure where nitrogen gas could accumulate, displacing oxygen and causing asphyxiation.

The two contract workers holding the sheet became concerned when they had not heard from the workers inside for some 15 minutes. When there was no response to their calls, they pulled the sheet away and found one worker unconscious and the other in a daze.

The U.S. Chemical Safety Board investigated the accident and found that inadequate confined-space warnings and entry procedures were at the root of the tragedy. Although the plant did have procedures for entering confined spaces, they did not cover temporary enclosures : no permit was issued prior to the workers’ entry into the enclosure, nor were any precautions taken to protect the men from the risk of asphyxiation.