Abstract: Employees #1 and #2, maintenance workers, were cleaning a Y-strainer on a natural gas line for furnaces in a secondary copper smelter. The main natural gas line valve was closed. The furnaces have a back-up system that uses propane. The propane valve closest to Employees #1 and #2 was open. Another employee opened the main propane valve to switch the system over from natural gas to propane. The propane gas then seeped back through the open valve into the area in which the employees were working. It ignited, causing a flash fire. Employee #1 was seriously burned; Employee #2 was also burned, but not as severely as Employee #1. None of the valves was locked out or tagged out.