Simplified PLC replacement minimises downtime

18 September 2023

Ageing technology that increasingly fails and/or parts becoming hard to source is the usual driver for updating a control system and this was why systems integration company, Sigma was tasked with updating the control system of a treatment line at the Peschiera Borromeo municipal wastewater treatment plant.

Ageing technology that increasingly fails and/or parts becoming hard to source is the usual driver for updating a control system and this was why systems integration company, Sigma was tasked by the CAP Group to update the control system of the second treatment line at the Peschiera Borromeo municipal wastewater treatment plant, which has the capacity to treat wastewater from a 250,000 population equivalent.

 However, but the biological filtration and oxygenated reactor technology used requires extensive automation and created greater control system complexity.

Riccardo Azzoni, automation engineer at Sigma Italia, takes up the story: “The electrical and automation system for the treatment line is distributed over two buildings, each with two control cabinets. Each control cabinet had a GE Fanuc 90-30 PLC installed, with a GE Fanuc proprietary serial bus interfacing to a remote I/O unit. Some of the controllers had a PROFIBUS DP master interface for connecting to specific equipment. The PLCs were segregated from the I/O, to allow installation in convenient locations. The CPUs exchanged data over the Ethernet network via a GE Fanuc proprietary protocol and an HMI SCADA system enabled operators to monitor the system from the control room.

“System redundancy was provided by an extra CPU in each panel that had an identical configuration to the primary CPU. The additional CPU was also connected to the remote I/O, enabled by the bus, which supports multi-master redundancy. The redundant CPU could be put into operation if the primary failed. However, this required a failure to be recognised early, and manual intervention by plant personnel. Operators would also need to ensure the CPUs were not running simultaneously, in order to avoid IP address duplication.”

The need for a revamp of the automation system arose because the PLCs were being discontinued. In addition, the CAP Group was keen to implement a modern, plant-wide Ethernet-based data communications network, because the proprietary bus was unable to provide the functionality needed to support its new system requirements. Moreover, the twin CPU configuration made it difficult to keep track of changes to the control logic and created additional complexity.

The project had to be reconciled with the need to avoid a long plant shutdown and to minimise the project timescale and budget, the new technology needed to be compatible with the existing equipment and processes. 

“Sigma was tasked with providing a solution and proposed to only replace the controllers and communication-related components, retaining the field I/O wiring and structure, while adding true redundancy,” said Ricardo. “Firstly, the GE Fanuc 90-30 PLCs were replaced with Emerson’s PACSystems RX3i controllers. The controllers provide modern functionality to help improve operational performance but can also be quickly and easily implemented in GE Fanuc 90-30 applications due to the hardware being fully backwardly compatible in terms of form and fit.” This compatibility enabled gradual replacement of existing PLCs, rather than having to perform one major replacement of all devices at the same time. 

“The 90-30 racks, equipped with CPU and controller were replaced with two rackless CPUs in hot-standby redundancy configuration,” continued Ricardo. “Where PROFIBUS DP was utilised, a new VersaMax node, equipped with a PROFIBUS DP master gateway was installed. The existing I/O scanners – which only had a single power supply – were also replaced by VersaMax PROFINET IO devices, which have an independent dual power supply and a dual Ethernet interface that is able to support a ring network topology. This has increased network reliability and resiliency. 

The new automation solution provides hot-standby redundancy that is automatically managed by the controllers. Both CPUs are switched on, but only one is in command. Should there be a failure, the redundant device immediately takes control. This allows operators to focus on tasks directly related to the treatment process. The new CPUs provide both a real-time operating system and a Linux environment, which provides a new opportunity to implement web HMI functionality, edge-based control, and speed up the convergence with management and IT systems. OPC-UA Server functionality is also available. 


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