An automated water treatment system was introduced to the Magalies Water Vaalkop water treatment works to increase the existing 120 ml/day plant capacity by 20 ml/d.
This state-of-the-art control and monitoring system (with the exception of the new filter system), as well as the Ethernet network was designed, developed and commissioned by Supervisory and Control Engineering. A large component of the existing plant was upgraded and integrated with the new control and monitoring system. The control system comprises of networked PLCs, intelligent instrumentation, HMI panels, and a scada system for monitoring and operator intervention purposes.
Control system architecture
The 11 networked main PLCs, three remote PLCs and remote I/O units were modbus plus linked using a self-healing fibre ring, with copper branches. The latest Modicon quantum PLCs were used for the main PLCs, and the Modicon momentum PLCs were used for the remote PLCs to interface with third party devices. Modicon momentum remote I/O (input/output) was also utilised for distributed areas. The PLCs are all programmed with the ConceptXL IEC1131 software-programming tool. A total of 2500 field I/Os are monitored and controlled.
The main areas of the plant are controlled by the 11 main PLCs and the remote I/O option was used to access distributed I/O using a modbus-plus network connection. The main PLCs are networked on a modbus-plus fibre-optic backbone, where the fibre-optic terminations were done directly to the main PLC. This self-healing ring network on modbus-plus, enables the control system to function even if there is a breakage in the network. A central point of programming and monitoring has been installed in the technician workshop. From this node, all the main PLCs on the plant can be monitored and configured.
The self-healing network is very important, as critical software interlocking and data are communicated between the different PLCs for the different plant processes. Intelligent instrumentation used on the plant is linked with the modbus protocol to Modicon momentum PLCs. The information is then transferred to the main PLCs via modbus- plus connections to make this information available on the main network. Typical intelligent third party devices connected in this way are various variable speed drives, turbidity meters, network analysers and a power quality meter.
Monitor system architecture
An Adroit scada system is used to monitor the plant and enable operator control from a central control room. In addition to the plant being monitored on the scada via PLCs, the distribution system is also monitored via several telemetry stations in the distribution network. The scada is integrated into the PLC network via a modbus-plus interface modem.
An access database is used as an archive for information available regarding chemical storage levels, dosing rates, water quality, water levels etc. Reporting can be viewed over the Ethernet LAN from anywhere on the plant. A daily trip reporting utility gives maintenance personnel the necessary information to investigate problems on the plant. Device and process failures get alarmed for operator attention on the scada. Critical alarms and process status are also relayed to supervisors' cellphones via SMS messages, giving the supervisors the ability to do high level monitoring of the plant from anywhere.
A view node facility via Ethernet to the administration, allows managers to monitor the plant from their offices. This solution ensures that the control network is still separated from the administration network.
Control philosophy and functioning
The majority of the controllable devices on the plant can be controlled automatically by one of two modes. Firstly, by operator control from the scada system where total control for the device is in the hands of the operator. Secondly (normal state), by process control where the PLC, and the operator do the decision-making only needs to supply the set points values. All processes are interlinked via PLC communication ensuring that overall process control is achieved. In this way provision is made for safety factors, eg no chemical dosing will be done if raw water supply is off.
Chemical dosing is fully automated. This includes ferric, polymer and ammoniac dosing via variable speed drives according to the raw water flow. A lime dosing system was fully automated from silo transfer to variable speed dosing to control the pH via PLC loop control. All filter control and wash sequences are fully automated. Desludging of the sedimentation tanks is also fully automated.
High lift pumps are PLC controlled and protected. High lift pumps and raw water pumps at the extraction plant are controlled and monitored locally by Modicon Magelis HMI panels. A total of 10 Magelis panels were installed on the plant. The starting of the high lift pumps is only possible from these local panels for safety reasons.
The implementation of an automated control and monitor system at the Vaalkop water purification works ensured a major improvement in the monitoring abilities of the operators and supervisors. It also ensured a central point from where control functions can be executed, monitored and coordinated.
Supervisory and Control Engineering
(016) 982 3700
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version