Abstract: At approximately 9:00 p.m. on August 8, 2006, Employees #1 and #2 attempted to clean out a heat exchanger. The heat exchanger was part of a natural gas piping system in the power generation facility of a wastewater treatment plant. The heat exchanger contained methane and natural gas, pressurized to approximately 300 pounds per square inch. Approximately 200 parts per million of hydrogen sulfide contaminant was present in the natural gas. The natural gas piping system contained two compressors, labeled Compressor A and Compressor B. The heat exchanger that Employees #1 and #2 were to clean out was connected to Compressor B. Compressor B was off and Compressor A was running. Employees #1 and #2 incorrectly assumed that since compressor B was not running, it had already been isolated from Compressor A by a closed valve. However, the valve between compressor A and B was in the "open" position. Employee #2 began removing a plug on the Compressor B heat exchanger, with a pneumatic impact gun, while Employee #1 was standing directly behind him. When the heat exchanger plug was removed, the pressurized natural gas came out of the 0.5-inch plug opening and blew Employees #2 and #1 back. The natural gas did not ignite. Employee #1 was rendered unconscious from the impact. Employee #2 initiated emergency shutdown of the natural gas system and notified other employees about the emergency. An ambulance was called, but neither employee was hospitalized.