Safeguarding Wellington’s water supplies

The reliable provision of clean water is essential for any large population. It is now commonplace for water treatment plants to run autonomously and, for this to be effective, a reliable process control system is needed to supervise plant activities. Through the 1980s, distributed control system (DCS) architectures were widely used to manage these processes. However, many of these systems are now reaching the end of their design lives. Consequently they suffer from declining availability of spares and waning access to technical assistance.

This was the situation faced by the Te Marua Water Treatment Plant in New Zealand. Functioning in parallel with two additional water treatment facilities, Te Marua supplies up to 50% of Wellington city’s annual water consumption. The three plants are owned and operated by the Greater Wellington Regional Council (GWRC) and were all facing increasing problems sourcing spare parts and expertise for their legacy operating systems. In addressing the issue, GWRC decided to upgrade the 22-year-old DCS system at Te Marua to an Allen-Bradley ControlLogix programmable automation controller (PAC) from Rockwell Automation.

The GWRC was also looking for a solution that would be compatible with the existing human machine interface (HMI), which provides a common graphics package system-wide across the entire water distribution network. “We had reached a dead end with the old system. We could not expand or develop the functionality as we were unable to efficiently write new code for the system,” says GWRC Control Systems Development Technician Philip Wilkie. “With three plants each running a different control system, our goal was to bring these into line on a common process automation platform.”

The Allen-Bradley ControlLogix solution proved an obvious choice. “It was the only option to meet all requirements for the three different plants,” says Wilkie.

The solution implemented is based on a redundant pair of ControlLogix processors, which are linked to the HMI/data network via ethernet says Rockwell Automation Industry Manager for New Zealand Prasad Nory.

As the Te Marua treatment works cannot be taken offline for more than 12 hours without impacting Wellington’s water supply, it was essential to keep the existing plant running while the new control system was installed. “We decided to install the ControlLogix platform between the legacy DCS and the I/O it was controlling,” Wilkie explains. “This means that when the DCS directed a pump to run, for example, it actually instructed the ControlLogix system to carry out the function. The beauty of this set-up was that it allowed us to transition control one element at a time, and test each function of the new system as we progressed.”

Once the control functions for the valves, motors and pumps had been transferred to ControlLogix, the procedural functions - such as the dosing, filtering, sequencing and backwashing - were also brought across from the DCS, one element at a time. The last stage was to transfer control of the main site inlet valve and a range of chlorine instruments. This final cutover involved a plant shutdown of just four hours. In total, the transition of plant automation control from DCS to ControlLogix was accomplished in six months.

“I’ve been involved in a number of similar upgrades and this has run the smoothest,” says Wilkie. “ControlLogix is extremely intuitive to use and program, and this enabled us to complete the whole project in house, for a small fraction of the cost that it would have been if we had employed a third-party systems integrator.”

The plant is now easier and more intuitive to operate and provides a platform from which improvements can be made more readily. Energy and chemical costs on site have already been reduced by an estimated 15%, and greater familiarity by site personnel has allowed more functionality and performance to be leveraged from the system. For example, backwash time of the filters has been reduced from four to two hours and maximum production capacity has potentially increased from 100 to 120 ML per day.

As part of GWRC’s ‘water information system’, FactoryTalk Historian data collection software and FactoryTalk VantagePoint data reporting software from Rockwell Automation are currently being installed on site. These will improve the delivery of documentation required by the council - especially Drinking Water Standard reports.

The move to a ControlLogix platform has also opened the doors for incorporating advanced process control into the plant’s operations - especially with chemical dosing. “We are currently using an online spectrophotometer to monitor raw water quality and provide real-time prediction of the exact amount of alum required based on the measured concentration and type of organics in the raw water,” Wilkie says. “Traditional feedback methodology often results in under - over-dosing. We anticipate usage reductions of alum - our biggest chemical cost on site - by as much as 20% as a result of this new technology. The advanced ControlLogix platform we are now running will allow us to take full advantage of this information to ensure we are eliminating organics in the treated water to exactly the level desired.”