The event occurred at an ethylene production plant. The plant was shutdown at the time as the supply of compressed air to the gauges had inadvertently been switched off. This failure of the air supply was not immediately recognised because of the low pressure alarm was installed before the isolation valve. Shortly after the plant was restarted an unusually high temperature was observed in an acetylene hydrogenation column. Probably the hydrogen supply had not been turned off completely during the shutdown due to a leaking isolation valve and the continued passage of hydrogen through the column led to a gradual increase in temperature. The column thermometers were all located on the outside of the column and registered 120 degrees C when the interior temperature was probably about 400 degrees C. As the plant operator was unaware of this situation, he attempted to reduce the column temperature by passing in additional ethylene. At this high temperature the ethylene was hydrogenated to ethane and the column temperature rose further to about 950 degrees C as the hydrogenation reactions are exothermic. Also the ethylene decomposed into methane, carbon and hydrogen. The hydrogen supply was turned off and the temperature fell to 700 degrees C. The high temperature and pressure generated by the above reactions caused a leakage of flammable gases at a valve flange on a pipe close to the column. An explosion and fire followed.