Tuesday, December 24, 2019

Accidental-Mixture Chlorine Release Incident – 12-23-19

An industrial mixing accident earlier this week in Berkeley County, WV resulted in a chlorine gas release. No injuries were reported, but a large portion of downtown Martinsburg were evacuated overnight because of the incident. Local news reports of the incident can be found here, here, here, here, here, here and here.

The Incident

Apparently, a truck containing a ferric chloride (FeCl3) solution was inadvertently unloaded into a sodium hypochlorite tank. The resulting chemical reaction released chlorine gas into the atmosphere. It was this chlorine gas cloud that was the reason for the evacuations.

Ferric chloride is used in water treatment facilities as a coagulant to help remove solid particles from the water in a pre-treatment filtration step for drinking water. The sodium hypochlorite (industrial strength bleach) is used for disinfecting drinking water. The bleach used in a water treatment facility is typically 10 to 13 wt% sodium hypochlorite where household bleach is usually about 6%.

Since this is a fairly standard acid-base reaction, in addition to the chlorine gas released, a significant amount of heat was also produced. Because of the large amounts of water in both solutions, this heat was not dangerous from a facility safety perspective, but it probably resulted in a steam cloud being released along with the chlorine gas which would make the event a bit more visibly impressive.


It has been a while since I have talked about this type of incident, but that is not because it is too infrequent. Adding chemicals to the wrong tank happens way too often, particularly in smaller operations where the truck driver is the one responsible for hooking up the unloading connections. Where facilities have dedicated bulk-unloading personnel, these incidents are not as common.

Facilities that handle ferric chloride or sodium hypochlorite solutions are not required to conduct a process hazard analysis (PHA) for unloading or handling these chemicals; neither the EPA nor OSHA regulations provide oversight of reactive chemical hazards. Still any facility handling chemicals should conduct a safety review before bulk handling of any chemicals takes place.

An important part of any such safety review would be to identify chemicals used at the facility that would have dangerous chemical interactions. When such a review identifies a potential chemical reaction that produces chlorine gas, for instance, prudence dictates that steps are taken to ensure that an unintended mixture of the two chemicals does not result. Steps could include physical isolation of the tanks and unloading connections; warning signs at connections, requiring the presence of someone from facility when unloading, or separately keyed locks on each unloading valve. Increased safety would come from combining two or more of the above listed techniques.

Finally, this is one of those types of incidents that local emergency response personnel should plan for whenever there is a facility in their jurisdiction that has bulk storage of sodium hypochlorite. While chlorine gas releases typically require significant evacuation zone (the ½ mile used in this incident is typical) the amount of chlorine gas that is released in this type incident may not actually require evacuations; shelter-in-place is probably more than sufficient for all but the closest structures to the release. Making that type of decision in advance (and notifying potentially affected personnel what it means) is a decision that is easier to make in advance of an incident.

Emergency response planning for this type of incident also means determining what type of chemical detection equipment is needed to evaluate the all-clear call at the end of the incident. One news report indicated that the responders used ‘test strips’ to test for chlorine gas. I am unaware of any test strips that are effective on low concentrations of chlorine vapors; they are designed to test for chlorine concentration in water. A slightly more sophisticated gas detection/measurement system is required. Again, this is best determined before an incident occurs.

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