Llwyn Onn WTW gets Mitsubishi DR treatment 17 July 2014

The £17 million Dee Valley Water's (DVW) Llwyn Onn, North Wales, water treatment works, which opened in March 2014, is controlled by a Mitsubishi Electric redundant Q series PLC, chosen for its reliability stats.

In addition to the standard treatment processes, Llwyn Onn includes an automated processing system for removing manganese, a mineral that occurs naturally in the surface water of the DVW area.

Main project contractor Black & Veatch selected the Mitsubishi solution, through North Wales system integrator Tycon Automation.

The water treatment process is unconventional, consisting of dissolved air flotation (DAF) clarification, granular activated carbon (GAC) filtration, followed by rapid gravity filtration (RGF) and a chlorine contact tank before pumping to a storage tank and on to the distribution network.

"There were elements in the contract specification that, in our opinion, meant it suited the Mitsubishi hardware," comments Andrew Robertson, technical director at Tycon.

"Historically, this was a Rockwell site but Dee Valley Water were flexible in their approach and were looking for a fully redundant system," he continues.

"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."

The Mitsubishi QnPRH PLC system was designed with two processor and three I/O racks in the main motor control centre, with one third of the plant on each I/O rack.

"The system lent itself to being designed this way, as the process contains three DAF lanes, six carbon filters and six rapid gravity filters, which give an inherent amount of redundancy in the process," explains Robertson.

"We matched with the control system design to provide the most robust engineering solution."

As for the rest, the pump motors are mostly controlled by variable speed drives (VSDs), connected using a Profibus network.

"We used separate Profibus networks within the MCC, with networks going out into the field to simplify design and increase robustness," says Robertson.

"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 (more than one thousand I/O points), including flowmeters, level instruments and quality instruments, Tycon segmented the Profibus networks to mximise redundancy and system resilience at the I/O level too.

"The response time of the PLC 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," comments Robertson.

"For this application we are talking seconds, rather than milliseconds, which we often work with in other more dynamic higher-speed applications."

Brian Tinham

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