Identify the initiation, propagation, and termination steps for the following accident report. suggest ways to prevent and contain the accident. b. an alkylation unit was being started up after shutdown because of electrical outage. when adequate circulation could not be maintained in a deisobutanizer heater circuit, it was decided to clean the strainer. workers had depressurized the tank and removed all but three of the flange bolts when a pressure release blew a black material from the flange, followed by butane vapors. the vapors were carried to a furnace 100 ft away, where they ignited, flashing back to the flange. the ensuing fire exposed a fractionation tower and horizontal receiver drums. these drums exploded, rupturing pipelines, which added more fuel. the explosions and heat caused loss of insulation from the 8-ft x 122-ft fractionator tower, causing it to weaken and fall across two major pipelines, breaking piping - which added more fuel to the fire. extinguishment, achieved basically by isolating the fuel source, took 2 7 hours. the fault was traced to a 10-in valve that had been prevented from closing the last 3/4-inch by a fine powder of carbon and iron oxide. when the flange was opened, this powder blew out, allowing liquid butane to be released.