isolation canisters created a pressure spike of 4.4 pounds per square inch gauge (psig) inthe dust collector and caused the flame front to propagate counter-current toward themixing tanks. This rapid pressure rise (and the associated rapid flame propagation backthrough the ducting) triggered an initial flash fire at the bag dump station and within therubberized ducts above T-306 (where the second and more volatile flash fire occurred).After the incident, the CSB learned that the design total suppression pressure (maximumdesign pressure for suppressed explosion) predicted for the dust collector was 3.8 psig. Theincrease in the total suppression pressure occurred because of the higher rate of pressurerise of the dust mixture compared to the expected design KST of 165 bar-meters per second(bar-m/sec).37