SUPPLEMENT FEATURE PLCS & INDUSTRIAL PCS Control from inlet to outlet
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 is controlled by a Mitsubishi Electric system redundant, Q series modular PLC solution
T
he 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. With so many people depending on a
continued supply of clean water the process for selecting a reliable automation control system for the entire works was particularly stringent. The majority of the subcontract supply
chain for this project has been sourced from the north Wales area, including the system integrator, Tycon Automation. Tycon has an extensive knowledge of the Mitsubishi product range, having used components to complete several large water control projects in the past. Investing in the development of a large library of field proven application specific software modules meant that Tycon was well positioned to provide the integration for what is a large and complex site control solution.
OPTIMISATION AT LLWYN ONN The water treatment process consists of dissolved air flotation (DAF) clarification, granular activated carbon (GAC) filtration followed by rapid gravity filtration (RGF) and a chlorine contact tank before being pumped to a storage tank - from where it is fed into the distribution network. Although this is not the conventional process order, it is an arrangement that has been designed to work in this area, which has the additional issue of manganese to deal with.
A crucial aspect to the control system
was the use of a full dual redundant PLC system. Complete with its own CPU, the secondary system is fully wired-in and tested, increasing the system reliability by mirroring the primary CPU so that in the unlikely event of a failure of the main CPU, power supply or base unit, the secondary system can take over within 21 milliseconds from the same control point. Andrew Robertson, technical director at Tycon, said, “There were elements in the contract specification which in our opinion meant it suited the Mitsubishi
S1 NOVEMBER 2014 | PLCS & INDUSTRIAL PCS 4
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.”
REALISING CONTROL WITH 1000+ I/O “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 a 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 - 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
Above: Dee Valley Water’s £17m flagship water treatment works at Llwyn Onn, North Wales, is controlled from inlet to outlet by a Mitsubishi Electric system redundant, Q series modular PLC solution
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. “The Mitsubishi GX Works II software is
a nice environment for us to work in, it is easy to find your way around and easy to use. We were able to use function blocks previously developed for water treatment applications by Tycon Automation, which made the job simpler to realise and enabled us to add value to the project. The flexibility of the programming
Above: 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
environment meant that the system architecture could be broken down into function blocks representing all parts of the plant. A remote access VPN link into the site means we can access the SCADA layer and therefore the PLC in real time and can easily interrogate the system to see what the raw code is doing and carry out any fault finding activities. It is important with such a large program to have a logical structure which makes it easy to interrogate.”
Mitsubishi Electric
gb3a.mitsubishielectric.com T: 01707 288 769
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