A QnPRH dual redundant PLC from Mitsubishi Electric UK was selected for a new drinking water plant as in event of a failure of the main CPU, the secondary system can take over within 21 milliseconds from the same control point.

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 new works provides DVW with the capacity to produce 47,000m3 of drinking water per day for Wrexham and the surrounding area.

Process optimisation

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 to remove naturally occurring manganese from the water.

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.

Realising control with 1000+ I/O

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, six carbon filters and six rapid gravity filters which give an inherent amount of redundancy in the process.

The pump motors are mostly controlled by variable speed drives (VSDs) which are connected using a Profibus network. Separate Profibus networks within the MCC and separate networks going out into the field were used to simplify design and increase robustness.

Using Mitsubishi GX Works II software as the programming 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 accesses 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.

Mitsubishi Electric QnPRH Series PLC architecture

The QnPRH is Mitsubishi’s most sophisticated control solution and represents the top end of the Q Series Automation Platform. It offers a high level of system redundancy which insures complete immunity to process interruptions caused by power or system failures. This is achieved by using a fully redundant architecture that duplicates processors and network links. Hot-swap capability provides an operational level of redundancy.

The dual redundant CPUs (control and standby) mean that any failure of the control CPU causes immediate transfer of control to the back-up, preventing system failure or interruption. Synchronization of up to 100,000 words of process data is possible between CPUs per scan. Switchover time is typically around 20-40ms, ensuring a “bumpless” transfer.

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