Abstract: Operators in the cat cracker (vapor recovery) unit were preparing to remove and repair a heat exchanger with leaking tubes. Day shift operators had blocked in the exchanger, bled the propane in the exchanger to the flare, and drained the water. The night shift operator was going to connect a steam hose to the exchanger, and he re-opened the bleed valves to double-check. When he opened the water drain valve, propane in the exchanger sent a flammable vapor cloud out of the drain line. The vapor cloud was ignited, apparently by a heater approximately 150 feet away. Employees #1, #2, and #3, all machinists working on a pump, sustained burns in the approximately 3 second flash fire. The block valve was leaking, allowing propane to enter the exchanger.