Chlorine Shutdown System Improves Plant Safety
The Escambia County Utilities Authority (ECUA) supplies drinking water and wastewater treatment for Pensacola and Escambia County, Fla. Water supply from ground water is disinfected at each well site with chlorine. Chlorine gas is delivered directly to each unmanned well site. Initially the wells were remote from residential areas but with population growth, some of the sites now are near dwellings. The wastewater treatment plants also use chlorine gas for disinfection with sulfur dioxide for dechlorination. Both chemicals are supplied either from 150 pound cylinders or ton containers.
The ECUA conducted a safety analysis of its facilities following guidelines of OSHA?s PSM (Process Safety Management) and EPA?s RMP (Risk Management Plan) requirements. The report pointed out the quantities of chlorine and sulfur dioxide in use and storage and indicated a need for additional safety considerations. Chlorine and sulfur dioxide gas detector alarms located at all sites were connected to the ECUA SCADA system.
The proximity of the wastewater plant to downtown Pensacola was of particular concern. The ECUA wanted to provide additional safety protection and saw three viable options ? conversion to liquid chemicals (sodium hypochlorite and sulfite), the addition of a containment building with exhaust scrubbers or the installation of an automatic valve operator on each container in use. These alternatives were also considered for each well site.
The choice was made to use devices that would automatically shut off the chlorine and sulfur dioxide container valves. These automatic valve operators provide shut off action on receipt of an electrical signal. This signal could be from a chlorine gas detector, a fire alarm, an earthquake sensor or from a remote site manually or automatically via the SCADA system. The automatic valve operators only close the valve upon the receipt of a signal. The opening or reopening of the valve after any shutoff event must be done manually by the operator. After the event, the operator must visit the site, determine the cause and correct the problem that caused the closure.
Automatic valve operators (AVOs) are accepted as a viable containment method by model fire and building codes. Three AVOs were available ? two pneumatically operated and one electrically operated. The authority chose the electrically operated unit because of system simplicity. With the electrically operated AVO there was no need for a compressed gas supply from a cylinder or compressor as a pneumatic source.
The electrical unit had a battery power supply, continuously charged by 120 VAC. In the event of a power failure, the battery operates the microprocessor controller and actuator for several days. In addition, automatic operators always ?fail closed?, just what the ECUA wanted, and the electrical AVO design provides over torque protection for the valve through the electronic control system.
After a thorough evaluation and test, the authority purchased and installed 84 electrical valve operators from Halogen Valve Systems of Irvine, Calif. The automatic operators were installed on chlorine cylinders at all the well sites and on all ton containers at the wastewater treatment plants. The AVO design enabled mounting on either a vertically oriented valve (cylinders) or horizontally oriented valve (containers). Installation of all 84 valve operators was accomplished within 12 weeks by ECUA staff. The manufacturer provided training so that all operating and instrument personnel were knowledgeable in the equipment performance, maintenance and service.
Since installation in late 1998, the ECUA has had no incidents that caused the valves to be closed. The valve operators have responded as designed during preventive maintenance and operational checks of the system. The authority is confident that the operators are in position and ready to use should the situation warrant. The automatic valve operator has proven to be a cost effective procedure.
About the Authors:
Bernie Dahl is Scientific, Technical and Regulatory Administrator and Terry Platz is Instrument/Electrical Manager for Escambia County Utilities Authority, Pensacola, Florida.