Abstract: On June 21, 2017, two employees were performing maintenance on a check valve that malfunctioned. Employee #1 entered a pit where the valve was located. After approximately 15-20 minutes, Employee #1 began exiting the pit, when he was overcome with gas and fell unconscious. Employee #2, who was serving as an attendant, tried to pull Employee #1 from the pit. However, he was overcome by the gas fumes and fell unconscious. Emergency services were contacted and, upon arrival, took Employee #2 by helicopter to a nearby hospital, where he was hospitalized and received treatment. Employee #1 was determined dead onsite.