In preliminary findings, investigators from the United States Chemical Safety and Hazard Investigation Board (CSB) say that November 2003's chlorine release at the DPC Enterprises Glendale AZ facility resulted from failure to shut off a chlorine transfer line when safety alarms sounded.
Alarms indicated the near-depletion of an essential chemical in a safety device called a scrubber, but the CSB found it was common to allow chlorine to flow even after the alarms sounded, in violation of DPC's own written procedures.
The chlorine release began while operators were transferring liquid chlorine from a railroad tank car to a tanker truck.
As the tanker truck was filled with liquid chlorine, chlorine vapors were directed into the scrubber to prevent them from being vented into the atmosphere. The scrubber works by a chemical process where chlorine vapors are absorbed by a water solution with up to 20% caustic soda (sodium hydroxide), forming liquid bleach, a saleable by-product.
CSB investigators said the company regularly ran the concentration of caustic soda in the scrubber down to less than 0.5% — a level “that left a limited safety margin.” The CSB found that on the day of the accident, Nov 17, 2003, the caustic concentration was allowed to drop to zero. The depleted solution could no longer absorb chlorine vapors, which then vented to the atmosphere. Furthermore, shutting off the transfer operation did not stop the release, as the overchlorinated bleach solution broke down in a series of chemical reactions, generating chlorine gas.
Up to 3,500 pounds of chlorine were released in the incident. Authorities instructed more than 4,000 people to evacuate from the immediate area in Glendale and bordering Phoenix.