Abstract: Employee #1 was standing on the top of a "gun barrel" tank (heater settling tank) checking the fluid level through an open hatch as the contents waste, water and product were pumped out into adjacent tanks. When approximately 3 to 4 feet of liquid was left in the tank, it was discovered the product was crude oil and the employees changed from pumping it into the adjacent tank that ran to the disposal system to pumping it into the stock tank. At that time, there were two employees on the catwalk adjacent to the stock tanks, one employee on top of the gun barrel and two employees across the road checking equipment. Through interviews and witness statements, Employee #1, who was on top of the gun barrel, lit a cigarette as coworkers called to him to stop and he waived it over the open hatch of the tank igniting the flammable vapors causing an explosion which blew off the top of the tank and instantly killing Employee #1.