A Tragedy When Preparing for Valve Maintenance

A catastrophic explosion and major fire occurred within a major refinery as operations prepared a system for valve maintenance. This refinery stored a flashing flammable fluid (isobutane with a boiling point of 11° F or —12° C) in two spherical tanks. The spheres connected to an alkylation unit via a 10-inch (25 cm) line. Operating line pressure was about 50 psig (345 kPa gauge) and one of the valves in this underground system was in an open pit. [9] 

A refinery operator noticed bubbles in the water that covered the 10-inch (25 cm) valve in the pit. The employee proceeded to try to clear the underground pipeline by flushing the line with a 110 psig (760 kPa gauge) water supply from the unit Sphere No. 1. When the operator determined that water had flushed the flammables from the pipeline and into 

In 1967, it was believed this incident was the largest vapor cloud explosion reported to date. A company spokesman presented details of this incident to the American Institute of Chemical Engineers Loss Prevention Symposium. [11] The next four paragraphs are direct quotes from that technical paper  

The explosion occurred about 4 45 AM. Residents of the area were awakened by a frightening roar to see smoke and flame filling the western horizon. With dawn, the destruction was more apparent three refining units were afire, tankage was ablaze, and columns of black smoke spiraled upward.. . .  

The explosion had all the characteristics of a detonation. A plot of overpressure destruction vs. distance statics fits exactly on known TNT detonation curves. Based on the stoichiometric combustion, my calculations indicate that the explosion had the force of 10 to 12 tons of TNT. 

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