NT-based HMI System Upgrades Water Treatment Plant

By Ken Legal

In 1999, the Downingtown Municipal Water Authority (DMWA) in Downingtown, Pa., was under pressure to review the status of its plant control system. The Y2K issue was approaching, the system's 386-based PCs were causing maintenance problems, and the EPA was on the verge of requiring additional monitoring and reporting.

The existing system had done an excellent job of maintaining water quality since its installation in 1994. Unfortunately, the UNIX-based HMI (Human Machine Interface) was too difficult for plant personnel to reprogram to meet all the new requirements. The water authority had a good relationship with a local control equipment distributor, Applied Controls, Malvern, PA, so the decision was made to upgrade to a more modern Windows NT-based HMI system, Siemens WinCC.

Matching HMIs

The original Siemens TISTAR control system at DMWA included two redundant Model 20 HMIs and six Simatic 545 PLCs, all connected by two separate TIWAY local area networks (LANs) that spanned a distance of less than one mile. The system controlled the operation of a water treatment plant that can produce 3 MGD, operating around the clock.

The Model 20s were based on 386 PCs operating under UNIX. Changes to operator screens and control schemes were not easy to make, and the 386 hardware was becoming unreliable. The PLCs had been programmed using Siemens' APT engineering tool, and no major changes were required in the control code.

Downingtown considered simply modifying the old system to address both the Y2K concerns and the new EPA requirements for electronic recording and reporting of turbidity data, but determined that since the same HMI hardware would remain, upgrading was the only reasonable solution.

As long as the system had to be upgraded, the DMWA wanted something that was Windows-based, more user-friendly, and familiar to the operators. When the first control system was installed, very few of the operators had PCs at home, so compatibility and familiarity were not issues. Today, almost everyone in the plant has a PC at home, so a Windows-based system made sense.

KenMyr Controls (KMC) was commissioned to program and configure the system for a changeover by Jan. 1, 2000. John Lichman, Executive Director of the DWMA, had a major directive for KMC: "I want to have all the old operator screens in place. The only difference the operators will see is a new computer and a different color monitor, but as they go from screen to screen, it should have the same look and feel."

The existing system had more than 100 process screens, loop displays, data trend pages, and pop-ups.

KMC accomplished this by replacing one of the Model 20s with a new WinCC system and leaving one old system in place so they could be operated side-by-side in the control room. At first, the new system was a carbon copy of the old, so the operators were comfortable using it. Later, as they became more familiar with the system, they made suggestions and recommendations on how to improve the displays. They caught on quickly and suggested many changes that have significantly improved operations, saved money, and improved quality.

The first system was up before January 2000, with the second becoming operational in February 2000, after the suggested improvements were installed and tested.

Converting TISTARs

When Siemens purchased the Texas Instruments PLC Division nearly 10 years ago it inherited a sizeable installed base of TISTAR batch and process control systems. Because these systems contain PLCs, they are highly reliable and last a long time. The only real problem with aging TISTAR systems is with the human-machine interfaces, which cannot keep up with modern demands for information and communications. So Siemens developed the WinCC HMI to provide a way of upgrading existing TISTARs such as the DMWA system.

WinCC runs on an industrial PC under Windows NT. It comes equipped with hardware and software interfaces to the PLCs and the LANs used in TISTAR systems, making it easy to upgrade the legacy systems. It also has modern communications interfaces, such as Ethernet and Profibus, for connecting to higher-level control and data systems.

In addition, it is compatible with the APT engineering tools that are used to program PLCs, as in the DMWA system. This preserves the investment in legacy system software and makes it possible to upgrade HMI and PLC systems to newer hardware when necessary. Its HMI/SCADA software easily can import the existing APT tags directly into the tag manager, significantly reducing overall project effort. This capability is vital in a system like Downingtown's, which has more than 2,000 individual tags.

Going On Line

Because DMWA's existing system functionality was simply duplicated in Phase One, the conversion was easy. KenMyr replicated the existing screens and functionality on a one-for-one basis, so the new system looked and worked exactly like the old one.

The new system is much easier to work with because operators can edit online; DMWA can make changes and immediately observe the results. Modifications could be made only in the offline mode with the old UNIX system. WinCC simplifies the work by providing instant feedback to the operators.

Phase Two of the project involved adding more advanced control and display functions. In many cases, these were requested by operators. For example, the plant has some very large pumps that have had a history of cavitating on startup, potentially burning up their large electric motors. In the past, although flow and pressure information was available to the control system, operators had no easy way of using that data to be aware of this particular problem.

The Windows-based WinCC acquires flow and pressure data, determines when the pump is cavitating and alerts the operator on two different screens to take action. The advanced alarming functions have allowed operators to take corrective actions on this and other conditions.

Input was solicited from all plant operators, and as many suggested improvements were implemented as possible. For example, operators asked for a display that summarized run times and other performance data of various filters in the plant so they could determine when it was necessary to perform a backwash. Such data never had been available before. These and many other changes were made to improve plant operation and efficiency.

The plant has been online with WinCC for only a few months, but Lichman reports that the authority is already saving money. For example, providing information to operators in a faster, more user-friendly manner enables them to better optimize the use of chemicals-which saves money and produces higher quality water at the same time.

The system also lets staff better manage tank levels and plan for upcoming treatments. Because the plant uses surface water instead of wells, its intake can be affected by rain and dirty water. With the old system, the operators could see only one tank level, so they made their judgment regarding how much water needed to be processed based on incomplete information. Now, the HMIs display total water inventory for the entire system and operators can better determine how much water needs to be processed and whether they can delay processing dirty water after a rainstorm.

Future plans called for adding turbidity monitoring and reporting functions required by EPA and giving staff the ability to monitor the control system from a home PC. The turbidity function (along with other reports and data gathering) have been installed and debugged, so EPA reports will be available when they are required. And when an operator calls Lichman in the middle of the night, the WinCC system will soon allow him to sign on to the Internet, use his Web browser at home, and see what's going on— all without having to go into the plant.

About the Author: Ken Legal is owner of the consulting firm KenMyr Controls. Prior to forming the company, Legal worked in the research arm of a large petroleum company, a major PLC vendor, and a systems integrator. He has a Bachelor of Engineering degree (major in Chemical Engineering) from Stevens Institute of Technology, and more than 20 years of experience in control and automation.