Electrochlorination Plant - Mesaieed Power Station
GE Fanuc 90-70 & Genius I/O
Electrochlorination is the process which produces the disinfectant compound Sodium Hypochlorite (NaOCl) from seawater. This is used to prevent biofouling and kill bacteria in the seawater intake system at the Mesaieed power plants. The Electrochlorination system is required to provide continuous dosing of the Hypochlorite required at Mesaieed Power Station, plus a 60 minute daily shock dose. The system takes in seawater through filters to provide two electrolyser streams which operate in duty/standby mode. The Sodium Hypochlorite is produced by controlling the DC current generated by a transformer rectifier.
We designed, implemented and commissioned a solution based on redundant GE Fanuc 90-70 PLCs with Genius I/O and an HMI based on a GE Quick Panel. Water is taken into the system via inlet valves the control of which are based on the flow in the stream. Each stream has three transformer/rectifiers which generate the current for their respective electrolysers. This current generates the sodium hypochlorite and also some hydrogen gas so the water stream then passes into a de-gassing unit to remove the hydrogen. From here the sodium hypochlorite solution passes to the dosing system. The dosing system operates both continually and periodically delivering a “shock” dose of Sodium Hypochlorite.
This is a demanding control project as there are a number of safety issues involved: keeping the hydrogen levels within safe limits, monitoring the flows, levels and temperatures in the system and ensuring safe shutdown in the event of any abnormal readings.
In addition the control system has to accommodate the periodic acid washing of the electrolyser streams to remove other contaminants and also monitor the pH & conductivity of the acid wash.