The alarms indicated the near-depletion of an essential chemical in a safety device called a scrubber, but the CSB found it was common practice to allow chlorine to flow even after the alarms sounded, in violation of the company'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 percent caustic soda (sodium hydroxide), forming liquid bleach, a saleable byproduct.
CSB investigators said the company regularly ran the concentration of caustic soda in the scrubber down to less than 0.5 percent 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 over-chlorinated 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, using a reverse-911 call system. Fourteen people, including 10 police officers, suffered chlorine inhalation symptoms and required evaluation at a hospital.
CSB Board Member John Bresland, who accompanied investigators to the accident site, told attendees at the community meeting, "Companies which manage or produce chlorine have the responsibility of handling it safely. Chlorine gas is highly toxic by inhalation, and the CSB takes accidents that involve the release of chlorine very seriously. Fortunately in this case, no one was critically injured."
CSB lead investigator John Murphy pointed out that DPC had written procedures requiring transfer operations to be shut down when alarms indicated that the scrubber solution was in danger of becoming too weak to absorb chlorine. With the transfer halted, the procedures directed operators to use a chemical test to determine the exact caustic concentration in the scrubber solution. "However," he noted, "we found that it was common practice for plant operators to continue transferring chlorine after the safety alarms sounded while they conducted their analyses. On the day of the accident the narrow margin of safety ran out."
CSB investigators said the large multi-agency emergency response, with the Glendale Fire Department in command, was largely effective.