Time-constrained PLC integration project

PLC insights

• The existing control system at a water treatment works utilized a programmable logic controller (PLC), which provided abstraction, treatment, water quality and monitoring control.
• The solution integrated the two PLC control system via Ethernet/IP.

When Portsmouth Water’s monitoring program detected Cryptosporidium in the source water at its West Street Water Treatment Works, it was necessary to take the site out of service until a suitable Cryptosporidium inactivation treatment process was identified and installed. With summer demands fast approaching, returning the plant to full operation was key to ensuring sufficiency of customer supplies.

Trant Engineering, which had recently been awarded Portsmouth Water’s AMP7 and Non-Infrastructure Framework Contract, was asked to design, build, install and commission a suitable system and return the wastewater treatment plant to full operation within 13 weeks.

A multidisciplinary client and contractor project team was swiftly formed and identified ultraviolet (UV) disinfection as the preferred solution. Sourcing the required UV treatment process within the short time frame was a challenge, but UV treatment’s simple configuration naturally lends itself to containerization and Trant identified and purchased a portable containerized UV treatment unit, which had been in use at another site, and this enabled the project to stay on program.

Integrating the additional treatment process into the existing works operation and within the limited site space was a significant challenge. Control engineers from Te-Tech Process Solutions, part of the Trant Group, were able to help deliver the integration.

Integrating the system

The existing control system at the water treatment works utilized a PLC, which provided overall control of the abstraction, treatment, water quality and monitoring systems. The containerized UV system to be integrated into the existing work had its own local PLC and motor control center (MCC), controlling duty and standby UV treatment processes.

The containerized UV treatment process, had to meet the requirements of the Water Quality regulations before it could be returned to service. The works flow meant that the UV control system had to be changed from duty/standby to duty/assist configuration. This alteration enabled a single UV reactor to operate within a defined flow and water quality range, and if the single reactor failed the second reactor would resume control allowing the continuous compliant production of wholesome water. When the water treatment works operates at a high flow rate, or if water quality deteriorates, the system automatically reverts to duty/duty mode, allowing both UV reactors to operate in parallel, splitting the load equally between the two.

Integrating the signals from the UV treatment control system with that of the existing works was the next challenge. Autonomous operation, while ensuing sufficient protection measures, shutdown systems and local/remote monitoring functions were key objectives. Based on this scope, the integration of the network architecture of the two control systems could be designed, tested and proven. The solution integrated the two PLC control system via Ethernet/IP.

Portsmouth Water’s operational staff benefited from a modified human-machine interface (HMI), allowing key parameter and asset performance to be monitored. Operational flexibility was provided by allowing operators to log in and change some parameters – such as setpoints and alarm limits. However, changes to parameters outside of the designed work and operating window were prohibited, ensuring no water quality and treatment breaches.

The final integration package involved the retransmission of key operational and performance data to Portsmouth Water remote telemetry system. This activity utilized the same communications network, by the programming and configuration of a remote telemetry outstation and user terminals, via a Schneider GeoSCADA platform.

Te-Tech designed, installed and tested all the necessary cabling, integration hardware and ancillaries and its involvement in the project ensured that the implemented solution achieved the project’s objective, and Portsmouth Water was able to return the wastewater treatment plant to operation with a validated Cryptosporidium treatment process within the project period and before the increased summer demand.

– This originally appeared on Control Engineering Europe. Edited by Chris Vavra, web content manager, Control Engineering, CFE Media and Technology, cvavra@cfemedia.com.

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