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Case Study: Membrane and RO Plant Control
The Project:
Control system for a new water treatment plant at Ratcliffe Power Station to produce a high quality feed for the existing ion exchange units to replace the original clarifiers and rapid gravity filters.
The new water treatment plant utilises the process of ultra filtration by means of submerged membranes and reverse osmosis (‘RO’). The submerged membranes are fed directly from the River Soar by the existing water treatment plant feed pumps. The submerged membrane plant consists of 4 self-contained compartments, each containing 2 membrane cassettes, which are treated as independent streams.
The submerged membranes are periodically regenerated with chemicals to remove foulants. The chemicals are either dosed into the backpulse line during a maintenance clean, or the membranes are soaked in chemicals for a prolonged period during a recovery clean.
The 3 reverse osmosis skids, operating as duty/assist/assist, are fed via variable speed centrifugal pumps that boost the water pressure to the RO to approximately 17 bar. Variable speed pumps are necessary to compensate for the flux variance brought about by changes in feed water temperature.
The reverse osmosis feed pumps are controlled by the water level in the existing filtered water storage tank. These drive the membrane permeate through the RO membranes and into the degasser. From there the flow gravitates into the existing filtered water storage tank prior to further treatment at the existing ion exchange plant.
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Our Solution:
The control system was designed to monitor and control two Motor Control Centres.
The PLC consisted of a ControlLogix Processor and associated I/O racks and cards. The second MCC was controlled and monitored by two remote I/O racks communicating on ControlNet.
An Allen Bradley PanelView 1000, colour operator interface, with ControlNet communications was installed in each MCC to provide a local operator interface facility at each MCC. The two HMI units allow control and monitoring of the whole plant from either location.
The control system was integrated with the existing Water Treatment Plant SCADA system, which is a standalone Adroit system. The method of interface is via Data Highway Plus, using copper and fibre optic media and converters.
The control system was extensively tested using simulation techniques to ensure the commissioning period was kept to a minimum. The system was operational and producing feed water with only a two week commissioning period.
