Water Treatment Plant PLC Update


Operation of Water Treatment Plants (WTP) is often achieved by Programmable Logic Controllers (PLC) using a Human Machine Interface (HMI) to interact with the PLC. Like computers and other electronic devices, PLC technology advances rapidly leaving them no longer supportable by the manufacture or the modern laptop/ PC within only a few years.
Often the plant’s existing PLC or HMI system is no longer compatible with the current platform, making it difficult, if not impossible for end-users to update their existing PLC or HMI program when they update the plant or undertake equipment upgrades and replacements. The replacement of the PLC and HMI is a high risk operation with the potential of loss of production and the need to re-commission the system.


Ovivo offers a full package solution that takes into consideration the process, the software and the hardware of the plant. Ovivo’s knowledge of process control of water treatment plants combined with our house software team ensures that the PLC/ HMI replacement is carried out in an efficient manner with minimum down time. During this process, Ovivo will undertake a review of the:
• Existing system and recommend updates and improvements,
• Existing PLC/HMI and recommend to most appropriate upgrade,
• Existing instrumentation recommending modern replacement where necessary.
Ovivo will prepare or modify sequence charts, logic diagrams and system architecture diagrams, ensuring that the customer’s control requirements are is incorporated. The PLC and HMI code will be re-written to ensure that any changes to the operating regime and specifications are included. The system will be fully tested before being sent to site.
In addition, Ovivo will plan and undertake the installation of the new PLC and HMI system, planning the work for minimum down time and ensuring maximum production.

Case Study

Overview of the water treatment at Dow Corning plant

Overview of the water treatment at Dow Corning plant

The CHP plant at Dow Corning installed a Rockwell Software PLC 5 and an Interlution Fix 32 HMI in 1997 to control process operation. The PLC 5 has been withdrawn from Rockwell portfolio some time ago, and although spares are still available, they have become very expensive. The HMI ran on a Windows NT PC replacement of which was no longer possible. Windows NT does not support USB. This meant that the PC relied on the 3.5” floppy disk drive. The PC network used an Allen-Bradley proprietary network designed only to facilitate the exchange of data with the PLC. System upgrades were last made in 2000 when the (then) new Hero system, with a SLC500 controller was installed. Communication was through DH+ and RIO

Ovivo proposed to upgrade the existing PLC/HMI combo (PLC 5/Interlution Fix 32) to a more modern combination consisting of the Rockwell PLC software processor – the Control Logix L72 with the Genesis 64 HMI. The existing HERO would be controlled from the new PLC, with communication to the HERO via a redundant Ethernet ring, which would replace the DH+ and RIO. CHP accepted this proposal.

The final issue to be addressed was the actual installation of the new PLC/HMI system. The water treatment plant feeds a Combined Heat and Power (CHP) system which must provide a continual supply of de-ionised water to the customer. Tank sizing meant that the system could only be shut down for a maximum of 5 hours at a time. Ovivo agreed that the change could be carried out within a series of 5 hour windows. Included in the scope was the installation of a fibre optic system and the Ethernet ring, removal of the PLC5 and SLC500 PLC completely. Ovivo came up with a system were the PLC could be removed and replaced so that a number of 5 hour windows were available. The commissioning was carried out over a two week period with testing of the field interfaces and sequences done within the 5 hour window. The system was then put online full time with the old PLC being removed from the system. At no time did the customer lose the ability to produce de-ionised water and stream for their client.

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