Wastewater Management
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: “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, 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 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 4-20m A I/O (more than 1,000 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
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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 QnPRH offers a high level of
redundancy, which insures 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 water treatment process consists of dissolved air flotation clarification, granular activated carbon filtration followed by rapid gravity filtration and a chlorine contact tank before being pumped to a storage tank – from where it is fed into the distribution network.
A crucial aspect to the control system was the use of a full dual redundant PLC system.
The solution was developed specifically for use in applications where downtime cannot be tolerated for reasons of equipment damage, interruption of service, penalties or regulatory compliance. The dual redundant CPUs (control and standby) mean that any failure of the control CPU causes the immediate transfer of control to the back-up, which prevents any system failures or interruption. Synchronisation of up to 100,000 words of process data is possible between CPUs per scan. The switchover time is typically around 20-40ms, which ensures a ‘bumpless’ transfer. n
Fig. 2. The Mitsubishi QnPRH PLCs are designed to work in a redundant set-up. 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.
Mitsubishi Electric Europe, Factory Automation European Business Group has its European headquarters in Ratingen, near Dusseldorf, Germany.
www.mitsubishielectric.com
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