Waterworks modernisation uses bumpless control changeover
Waterworks modernisation uses bumpless control changeover
A water treatment works (WTW) in South Wales has had its control system completely upgraded, without disruption of supply to the population of around 70,000 people it serves.
The Nantybwch WTW, owned and operated by DCWW, has been supplying potable water to the towns of Tredegar, Ebbw Vale and Bargoed in South Wales since the works was first built in 1993. The original control system needed upgrading and local system integrator Oasis Software Solutions was brought in to help with design, installation and commissioning. Mitsubishi Electric dual redundant ‘bumpless’ PLCs were selectedsupplier
Nantybwch WTW usually supplies around 20 megalitres of potable water per day, but has a maximum design capacity of 30 megalitres per day. Its catchment area is the mountainous eastern side of the Brecon Beacons National Park. As the quality of the water held in the reservoir is good, a straightforward process is used. The primary treatment is based on dissolved air flotation (DAF), followed by two stages of rapid gravity filters.
Dual redundant PLC control
The architecture of the final design provided a system that comprises of two pairs of redundant Mitsubishi Electric Q25 programmable logic controllers (PLCs) in the main works and a Q06 PLC in the pumping station. Each redundant system has two central processor units (CPUs) and a mix of local and remote I/O. The remote I/O is connected in a number of different networks, some using fibre optic MELSECNet/ H and others using Profibus.
“The mixture of technologies used for the remote I/O solutions demonstrates the flexibility and versatility of the Mitsubishi Electric Q Series product. Where the system required low density I/O and a Profibus network already in place to interface a number of drives, a slice I/O solution was deployed.
For critical areas of the process, a redundant MELSECNET/H solution was provided with the capability of utilising high density I/O modules, connected to a high speed, high integrity, fibre optic network.
A fibre optic Ethernet ring was installed to provide peer to peer PLC communications and SCADA networking, offering a modern, future proof interface adhering to the DCWW AS427 Industrial Network Communications standard.”, explains Neil MacDonald, Managing Director of Oasis.
Mitsubishi Electric Q Series PLCs provide a high functionality platform for automation. Of particular interest to the water industry, they can offer a high level of system redundancy, which ensures complete immunity to process interruptions caused by power or system failures. This is achieved by using a fully redundant architecture, which duplicates processors and network links. Hot-swap capability provides an operational level of redundancy.
Bumpless transfer
The dual redundant CPUs (control and standby) mean that any failure of the control CPU causes immediate transfer of control to the backup, preventing system failure or interruption. Synchronisation 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. Because the CPUs reside on physically separate racks, the control CPU can be replaced while the backup maintains system operation. Most parts are interchangeable with standard Q Series systems, which helps to control both purchase cost and total cost of ownership.
Energy efficiency can be significant in the Water and Wastewater Industry as plant is operating 24/7. Energy tariffs can change on a thirty- minute basis, so energy consumption is constantly monitored and even small savings can add up. Savings can be made by trimming motor speed to optimum levels and there are often options for switching some motors from running constantly to intermittent operation.
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