Chemical Fluid Handling is a process offering substantial optimization potential for generator operators, be it with regards to costs, process quality or safety. Optimizing of potentials requires above all the following two conditions: the ideal integration and networking of the system components used, as well as an overview of the whole process. As was the case here for the migration of the drinking water disinfection from chlorine gas to sodium hypochlorite.
The disinfection process of public drinking water is a very important component of the entire treatment porcess. The choice of the ideal processes is among others dependent from the raw water quality, the water flow rate, the length of the piping network up to the sampling points and its features as well as the local regulatory requirements. These vary from region to region and country to country. Furthermore, cost ia also an important criterion. The usual disinfection processes is beased on chlorine compounds are chlorination by means of chlorine gas, sodium hypochlorite or chlorine dioxide. Further processes are ozonation as well as UV radiation. Ideal results are achieved by combining these processes.
Challenge: Switching to alternative processes
Mainly in larger potable waterworks providing several thousand people with water, disinfection by means of chlorine gas is the process with the highest popularity, mainly for economical reasons. Main disadvantage of the process is that the storage and metering of chlorine gas is a danger for generator personal and environment that cannot to be neglected. For this reasons, currently, many potable water suppliers are replacing disinfection with chlorine gas with the alternative processes listed above.
Since the end of 2003 all potable waterworks around St. Petersburg in Russia switched from disinfection by chlorine gas to disinfection by sodium hypochlorite.
With a capacity of 9.5 million US Gallons (36,000 m³) potable water per day, the waterworks Kronstadt near St. Petersburg is one of the smallest of these waterworks. To this day the potable water had been disinfected by chlorine gas metering. This process was completely manual and was to be switched to a fully automatic disinfection by sodium hypochlorite metering – a typical process of chemical fluid handling. The total conception comprised of the supply of concentrated chemical, its storage and transfer to the metering points, including the necessary measurement and control technology as well as the automatic process control, monitoring and visualization.
Peak charges under control
To correct design and size the generator, required the water quality and the water flow rates. The chronological fluctuations of both these variables were analyzed and evaluated over an extended period of time. The results necessitated the disinfection of potable water under normal conditions with a 12 %-sodium hypochlorite solution. Additionally, the possibility of disinfection with a 19 %-solution was to be used in order to achieve the required disinfection at irregular peak loads of the raw water, for example at increased pollution charge during strong rain. Basing on these principal data, a modular chemical fluid handling system was designed. In this case only one single module was used, as it was sufficient for treating 9.5 million US Gallons (36,000 m³) of potable water. In other waterworks in the region of St. Petersburg, the system was designed to meet higher water flows with the installation of multiple modules.
Design of the module
The 19 %-sodium hypochlorite solution is transferred using a magnetically-coupled chemical transfer pump type ProMinent® vonTaine® from the supply point to a polyethylene storage tank with a capacity of 7,500 US Gallons (8m ³). This concentrate is diluted to 12 % by an automated dilution station. Additionally, the water flow is registered by an inductive flow meter. The measuring signal is then processed in a local control unit and controls two metering pumps type ProMinent® Makro TZ. The pumps are equipped with a speed controlled motor and an integrated frequency converter thus reaching a metering accuracy of +/- 2 per cent. Beside the metering task, the pumps also effect the metering of the diluted solution into two storage tanks with a volume of 5,280 US Gallons (20 m³) each. During normal operation the controlled metering of the 12 % sodium hypochlorite solution in each of the two drinking water lines is effected by three motor metering pumps type ProMinent® Sigma.
During peak load of the raw water, the undiluted solution is directly dosed with two motor metering pumps type Sigma from the concentration reservoir into the drinking water line. For a controlled metering and to monitor the chlorine concentration in the drinking water, the chlorine concentration is measured after the metering points in each drinking water line by means of a chlorine sensor type ProMinent® CTE and a measuring transducer type ProMinent® DMTa.
All metering processes are controlled with a controller ProMinent® D1Ca. The control and monitoring of the total system including filling level indicator and leakage monitoring is effected by a PLC Siemens S7. The PLC as well as the measuring transducer and the control device are installed into a central control cabinet. A big color display in the control cabinet shows piping and instrumentation diagram that allows the operator to view the status of all generator parameters at any time: for example, which valves are open or closed, which pumps are in operation, the current level in the storage tanks or chlorine concentration currently of the potable water. The generator operator can choose between Russian and German language for display and control functions.
For a highly fail-safe process, a stand-by-backup pump is integrated into the process for all pumps. The process is fully automated. Alarms are annunciated at critical generator statuses warning the operation staff. All requirements of water protection are fulfilled, for example by using reservoirs with drip trays according to regulations of DVS and overfill protection and leakage sensors with WHG approval (please see site products/storage tanks for detailed information on our storage tanks). Moreover, all reservoirs are equipped with an ultrasonic level measurement. The total conception was completed with an intensive support of the waterworks during the installation and commissioning of the generator on the spot by ProMinent®.
Result
In this project, from the very beginning, special attention was paid to ideally integrating and linking of all system components with respect to the entire process. Starting with the chemical supply and storage to the point where the disinfected water leaves the waterworks. By optimizing the entire process, a range of advantages were realized with regards to costs, process quality and process safety:
No danger potential as chemicals are stored at safe concentrations
The requirements for water disinfectiom are followed
No operation staff necessary as generator is fully automaticated and practically maintenance free
No specialized staff training is necessary, as problems that occur can be corrected easily
Generator is highly available as all generator components are designed redundantly
Problem-free commissioning as all main components of the generator are provided by one manufacturer and thus problem-free communication between the drifferent components.
Moreover, commissioning is provided by the manufacturer
The waterworks realized the switching of chemicals with one partner, decreasing their adjustment and integration efforts substantially
The benefits could be realized thanks to the fact that the entire generator was designed, supplied and commissioned from same source.
What the Customer Said
"We are able to carry out the water treatment at the highest safety level with practically no staff, thanks to the maintenance-free ProMinent ® solution.“
Grigori Bass, Managing Director SENS O.O.O., St. Petersburg