Incident due to improperly ventiliated confined space

When I was a shift engineer, I entered a confined space, a pressure vessel, after obtaining necessary work permit. After I entered, I climbed up the internal fixed ladder to inspect a demister located at the top. As I was climbing up, the CO alarm in my personal gas monitor went off and I evacuated the vessel. Investigation determined that  during the purging process after plant shutdown, one part of an isolated pipeline connected with the vessel had not been been purged. When a valve connected to this pipeline was opened by an operator when I was inside, the pocket of trapped gas entered the vessel.

Lesson: Confined space entry can pop up surprises, even after receiving work permit. Ensure your personal gas monitor is working properly before you enter a confined space. It saved my life!