Mason City Water Department - Executive Summary
Chlorine is utilized at the Mason City Water Treatment Plant (WTP) to purify water for drinking. Chlorine equipment and related procedures are designed to maintain the safe handling and use of chlorine. In the Mason City WTP five-year accident history, no off-site accidental releases of chlorine have occurred.
Chlorine is stored in quantities up to 8,000 pounds in a Program 3 process. The Mason City WTP operates with two chlorine containers connected to the process manifold with additional containers stored in the chlorine room. The chlorine process, as defined under RMP requirements, includes vessels that are interconnected and co-located.
The worst-case scenario assumes that 2000 pounds of chlorine are released from a ton container as a gas in a ten minute period. However, the release rate of chlorine from the chlorine building is estimated to be 55% of the release rate from the container due to the passive mitigation of the chlorine building. The released plume is estimated
to travel 0.9 miles before no longer posing a hazard to the public.
The alternative scenario assumes the release of chlorine from a ton container due to a valve failure, bad flexible tubing connection, or flexible tubing failure. Any of these events could release chlorine gas through the 5/16 inch diameter valve body opening. Over a 60 minute period, 318 pounds of chlorine are estimated to be released. This release is predicted to travel 0.31 miles before no longer posing a hazard to the public.
The Mason City WTP uses a variety of equipment to prevent accidental releases. The process is a vacuum system for the gas feed of chlorine. Among this equipment are monitors to detect releases, vacuum regulators on the chlorine flow from the chlorine containers, the chlorine room with its fire prevention design, and chlorine containers in conformance with ASME codes. Procedures will include the safe storage of chlorine containers, periodic inspections and maintenance of the chlorine equ
ipment, maintenance that requires regular part replacement, the review of proposed chlorine process changes to preserve the safe operation of the chlorine process, the periodic review of process hazards, and steps to investigate releases that do occur in order to prevent reoccurrence. These procedures and more are developed through the OSHA PSM and RMP Program 3 requirements.
To prepare for an accidental release, the Mason City WTP has coordinated response activites with off-site responders. Mason City WTP employees will evacuate the site and notify these responders according to the Mason City WTP emergency action plan.