The Dee Valley Water (DVW) project to build a new water treatment works at its flagship site, Llwyn Onn in North Wales, has now been completed. The £17M project was officially opened in March 2014 by the Minister for Natural Resources and Food, Alun Davies and is controlled by a Mitsubishi Electric system redundant, Q series modular PLC solution.
The new works provides DVW with the capacity to produce 47,000m3 of drinking water per day for Wrexham and the surrounding area. In addition to the standard treatment processes, Llwyn Onn also includes an automated processing system for removing Manganese, a mineral that occurs naturally in the surface water of the DVW area.
Andrew Robertson, Technical Director at Tycon, ‘There were elements in the contract specification which in our opinion meant it suited the Mitsubishi hardware. Historically I believe this was a Rockwell site but Dee Valley Water were flexible in their approach and were looking for a fully redundant system. They could see that we had achieved this before with a very reliable fully redundant architecture on a larger system installed on the Isle of Man, which we arranged for them to visit, it meant that we could meet both the performance targets and the price point set for this project.
‘We spoke to Dee Valley Water at length about their preferences and reliability was of paramount concern. The Mitsubishi QnPRH PLCs are designed to work in a redundant set-up and therefore met the specification from the outset. The system was designed with two processor racks and three I/O racks in the main MCC, with 1/3 of the plant on each I/O rack. The system lent itself to being designed this way as the process contains three DAF lanes which give an inherent amount of redundancy in the process, we matched with the control system design to provide the most robust engineering solution.
‘The pump motors are mostly controlled by variable speed drives (VSDs) which are connected using a Profibus network. We used separate Profibus networks within the MCC, with separate networks going out into the field to simplify design and increase robustness. We employed Mitsubishi Slice I/O with Profibus interfaces to manage the I/O locally in the field, which reduces cabling and installation costs and works well with the Mitsubishi PLC which will accept most fieldbus network protocols with a plug-in comms module.
‘Speed wise, and because there is a lot of digital and 4-20mA I/O (over a thousand I/O points) including flow meters, level instruments and quality instruments we segmented the Profibus networks to provide maximum redundancy and system resilience. The response time of the PLC however is far beyond what we would need for this application. PID control loops for flow control valves etc are well within the processing capability of the CPU, with the control loops for the chemical dosing system being the most critical. For this application we are talking seconds, rather than milliseconds, which we often work with in other more dynamic higher-speed applications.
Ongoing system support
The support agreement for the site has been revised to take into account the new plant on a 24hr and 7 day per week basis. Tycon Automation Ltd are always available for advice over the telephone, with remote interrogation of the control system being the next step. The relative transparency of the system means that networks and communications issues right down to the I/O point for any component can be identified and any issues addressed quickly; making sure service support is both fast and cost effective.