Tuesday, December 22, 2009

Chlorine Fire

There is a brief article on KWTX.com describing a fire at a water treatment facility in Texas. The fire took place in a small building where two small chlorine cylinders were stored. The fire was detected by a patrolling deputy who saw the flames and a ‘yellow mushroom cloud’. Responding fire companies detected lethal concentrations of chlorine gas on site and an evacuation was ordered in a three-mile radius around the building. There were no reports of injury or death in the article, but there was certainly the potential for both in this incident. While this was apparently an accidental fire, it does point out that fire may be an excellent weapon to use in a terrorist attack on a facility holding toxic release COI. As such we should look at this incident for lessons that can be applied to defending chemical facilities against that type of attack. Fire Protection The first thing that we see is that the first detection of this release was made by a passing deputy. If circumstances had not brought that cruising deputy to that point at that particular time, the dispersal of the chlorine cloud might have certainly caused more problems for responders. Any facility that stores release COI of any type should be equipped with smoke and heat detecting fire alarms to give fire response personnel the earliest possible response. Needless to say, such alarms need to be tied directly to the local responders, unless the facility has an on-site fire company. The fact that there was a chlorine cloud noted by the deputy indicates that the fire had been burning for some time. The chlorine cylinders are pressure vessels. They are equipped with pressure relief devices that prevent pressure from building to the point where that the tank would ‘explode’ or catastrophically fail. The idea is that the controlled release of the material to the atmosphere reduces the potential danger area over that seen in a catastrophic release. The easiest way for that critical pressure to be reached is for there to be a fire with ‘direct flame impingement’ on the cylinder. This means that a commonsense protection mechanism for such cylinders is for there to be a water deluge system on the tanks to keep them cool enough that they don’t reach their ‘release’ pressure. Even a common sprinkler system would provide some protection and would delay any temperature related pressure release. Sprinklers would also reduce the amount of chlorine gas that reached the atmosphere by dissolving some of the released gas in the water. Evacuations One key mitigation measure for any toxic release COI is the evacuation of the potentially affected population. In this case, with less than 200 lbs of chlorine involved, the three-mile evacuation area was probably over kill. As the gas cloud moves away from the release point the cloud disperses and becomes less of a hazard. There are tools available that allow for easy predictions of the distance at which the cloud loses its hazardous characteristics. In this case, a fire actually reduces the potential area of concern. One of the problems with chlorine gas is that it is heavier than air and thus hugs the ground, flowing generally down hill. In this case the fire heats the gas making it rise. The more the cloud rises the more quickly it disperses. Unfortunately, it does spread it beyond the typical low-lying areas that authorities are normally concerned with. Evacuations are most effective when they are conducted before the actual release happens. This is another argument for fire alarms on the facility. It allows emergency responders to start evacuation procedures prior to the actual release of chlorine gas. The problem with post-release evacuations is that you can inadvertently direct people into the toxic cloud. This can result in needless casualties. Preventing these casualties requires knowledge of the actual concentrations of the toxic chemical in the evacuation area. This can be accomplished with fixed or mobile chemical detectors. The optimum situation is to have a network of fixed detectors and feeding that information into a computer program that maps the concentrations so that emergency personnel can easily tell who should be evacuated and who should shelter-in-place. The population that is most at risk in a toxic chemical release are those people that are living and/or working in the areas immediately adjacent to the facility. They have the shortest amount of time to properly respond to the release. They need specific training on appropriate actions to take and need the earliest warning. Depending on how close these people are to the fence line, it may be appropriate for them to be automatically notified at the same time as the emergency responders. Prior Planning The key to a well timed, effective response to a potential toxic chemical release is effective prior planning. Facility management, first responders, and emergency planners need to work together long before the release to be able to effectively respond in an emergency responders. The next key step is to inform the potentially affected public of the hazards and their appropriate responses in the event of an emergency. As with any emergency situation, proper prior planning prevents poor performance.

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