Case Study: BP Texas City Explosion 23 March 2005
Light raffinate vapors flowed up the tower and down a 148-foot long section of pipe before they were condensed by the air-cooled fin fan condensers and then deposited into a reflux drum. Liquid from the reflux drum, called “reflux,” was then pumped back up into the raffinate splitter tower.
To protect the raffinate splitter tower from overpressure, three parallel safety relief valves were located in the overhead vapor line 148 feet below the top of the tower. The outlet of the relief valves was piped to a disposal header collection system that discharged into a blowdown drum fitted with a vent stack that discharged to the atmosphere.
The disposal header collection system received liquid and/or vapor hydrocarbons from venting relief and blowdown valves from equipment in the ISOM unit and discharged them to the blowdown drum. The blowdown drum and stack were designed to accept mixed liquid and/or vapor hydrocarbons from venting relief and blowdown valves during unit upsets or following a unit shutdown. In normal operation, light hydrocarbon vapors disengage from liquids, rise through a series of baffles, and disperse out the top of the stack into the atmosphere. Any liquids or heavy hydrocarbon vapors released into the drum either fall, or condense and then fall, to the bottom of the drum where they collect. The approximate liquid full volume of the blowdown drum and stack was 22,800 gallons. Liquid would then be discharged from the base of the blowdown drum into the ISOM unit sewer system because a 6-inch manual block valve was chained open. This practice of discharging to the sewer was unsafe; industry safety guidelines recommend against discharging flammable liquids that evaporate readily into a sewer.