Donnacona is an industrial town just west of Quebec City. The town’s Water Treatment Plant was built in 1969 and has since undergone three major retrofits: in 1995 (new high pressure pumps & electrical upgrade), in 2000 (filters bottoms change), and in 2005 (river water intake upgrade). The plant includes conventional treatments for coagulation, sedimentation (pulsator), filtration (sand filters) and disinfection (Cl2). The raw water pumping station is 1.5km away from the plant and includes two single speed pumps.
To ensure compliance with the latest regulations about the quality of drinking water, the City decided in 2008 to modernize the plant. The treatment process was reviewed and it was determined the pulsators and filters needed to be dismantled and replaced with fine screening, a new coagulation chamber and a new ultra filtration membrane filtration step, which was to be the Zeeweed 1000.
Before undertaking the modernization, the potable water tank level was controlled by manipulating the raw water flow through a 14 inch butterfly control valve. Accurate level control of the potable water tank was not absolutely required, however the operators complained about the lack of controllability of the valve, especially at low flows. It eventually ended up being operated in manual mode. This poor controllability and the fact that the valve openings were too small, lead to wearing down of the internal components of the valve. This situation was tolerable given the dampening effect of the large volume of water contained in the Pulsators.
However, with the advent of the new water treatment process, water was to be fed directly from the contact chamber (around 12 min retention time) to the membranes. In addition, the fine screen washing and membrane back pulse cleaning would generate sudden raw water flow variations. Alain Gravel, Process Engineer with Dessau, the chosen Engineering Consulting firm for the job, said, “It was clear that a better level control solution was required just ahead of the new membranes.” Furthermore, the control philosophy recommended by GE Water & Process Technologies was based on the control of the water level in the distribution channel feeding the membranes. For cost reasons, installation of variable frequency drives on the raw water pumps was not an option.
The raw water flow varies broadly, between 1 200 and 100 USGPM which results in a large head difference delivered by the single speed pumps. Therefore, the pressure range available at the plant inlet varies between 10 and 58psi with a low back pressure available (lower than 10psi) for the valve. This translates into a pressure range drop across the valve of 2 to 53psi, and represents a tough job for most valves. The chosen valve had to feature a very low pressure drop when fully open and still be able to operate without cavitation when the pressure drop is high.
Simulations have shown that cavitation occurs below 1000 USGPM with butterfly valves. Two butterfly valves in a series is better, but still fails to properly drive the bottom half of the flows. For this reason, this type of valve was also not an option and so different configurations of globe valves were considered. Daniel Forest, Manager with Provan Control Associates, who supplied the valves said, “Our recommendation was to go with an arrangement of two 8 inch hydraulic globe valves in parallel with one of them having an anti-cavitation cage. For this, Singer Valve’s Model 106-PT-4SC-AC and 106-2SC-PCO were the best for the job.”
On low flows, there is enough inlet pressure to cause cavitation; however, on higher flows, the inlet pressure drops to a point that it is hard for the anti-cavitation valve to operate because of low differential pressure. For this application, Singer uses a dual-chambered solenoid control valve to account for the changes in upstream pressure.
The control panel is programmed to receive a flow setpoint from the central control system that controls the level in the distribution channel (Note: the panel can also receive the level setpoint if required). This level is controlled by manipulating the feed flow (cascade control). The operator sets the flow setpoint, and the controller adjusts the flow to ensure that the required level is maintained in the distribution channel.
On low flows, the controller controls the anti-cavitation valve (8” 106-PT-4SC-AC) depending on the setpoints of the level and flow controllers. When the anti-cavitation valve reaches a set position and the flow setpoint has not been achieved, the controller switches to the 8” 106-2SC-PCO and controls the valve. When the setpoint is lowered where the 106-2SC-PCO valve position is below the set position, the 106-2SC-PCO is fully closed, and control resumes on the anti-cavitation valve.
The valves were installed last fall and have been in operation since December, 2011. Given the staging of the work activities, the plant has operated the valves to maintain the level in the potable water tank. When the last membrane train is ready, the valves will control the level in the membrane distribution channel as planned. Alain Martel, Donnacona’s Director of Technical Services and Environmental Health said, “The installation and operation has been smooth and without any issues, so we are very pleased to have chosen Singer Valve’s solution.”