 Some of the gases (such as propylene) are liquified and sent to storage while others are used to fuel the furnaces or are returned to the feedstock.The Unit is protected from overpressure by pressure relief valves which send gases into a flare system where the excess gases are burned at a distance from the Unit.However, on October 6, 2005, such valves were manually operated and could not be reached by workers in time to prevent the explosions.