by Alasdair MacLean and Cliff Thomson
The Town of Gilbert, AZ, has been one of the fastest growing communities in the US for some time. It had a population of just over 5,700 in 1980, and now has over 180,000 residents. Numerous capital improvement projects have been completed to keep pace with the population growth, including several expansions of the water treatment systems. The town used the most recent expansion as a trigger to upgrade automation and implement current technology for its Supervisory Control and Data Acquisition (SCADA) system.
Chemical feed area of the Gilbert WTP.
Prior to the upgrade, many systems were operated manually and even those systems that worked on individual timers required an operator to schedule each part of the process separately and in such a manner that the proper sequence was maintained. Also, as a result of multiple expansions, there were several different copies of process code and differing means of accomplishing the same functions for otherwise comparable devices (e.g. valves).
The major goals of this project included standardizing the code controlling each type of device and standardizing the higher level code controlling processes such that future expansions would be easier and more of the system could be operated in automatic mode.
Originally the control system consisted of three PLCs with local Inputs/Outputs (I/O) and a total of 17 remote I/O racks handling about 1,600 I/O between them. The scope of the project was to replace all this equipment with Allen-Bradley ControlLogix PLCs and I/O, and to implement new logic in the PLCs to replace and improve the existing operating logic. The new logic had to be compatible with existing Wonderware InTouch Human Machine Interfaces (HMIs) that included 440 windows and extensive scripting.
The hardware replacement was scheduled to be completed during a 4-week annual shutdown. Programming was to be performed onsite. At the end of the shutdown, critical operations in the plant would operate under PLC control, with some supporting operations permitted to remain in manual while the programming was finished over the following weeks.
Program layers were assigned to obtain an appropriate separation of programs for Process (layer 1), Device (layer 2), and I/O (layers 3 & 4). Redundancy between layers 3 and 4 was selected so that I/O check on layer 4 could be executed even while logic was being changed in layers 1 and 2 simply by un-scheduling layer 3; during development the installation team was able to switch rapidly between I/O check and process testing for maximum efficiency of these two activities within the compressed development schedule.
Gilbert WTP filter deck with remote I/O cabinets.
A key objective was to generate the lowest layers of the application architecture from existing system information in order to minimize errors and rapidly get to the point where the team could start work on the process layer, even while the I/O was still being checked out. In order to do this, reports and database definitions were extracted from the existing controllers and HMI; these were copied into Excel and manipulated using macros that were developed specifically for this conversion project. These macros:
- replaced all the addresses in the InTouch database with the corresponding tag used in the PLC and generated the device tags for ControlLogix,
- generated code for the Hard-Map and scaled analog points using range values from InTouch, and
- generated code for the Soft-Map connecting each device to its corresponding I/O tags.
The Gilbert plant includes a 16 million gallon underground reservoir.
During the five weeks before plant shutdown, code was developed for the devices and the macros were developed and used to generate both Hard- and Soft-Maps for the smallest PLC in the system. At the same time, the process layer for the small PLC was developed, and that code was started up during the week preceding shutdown.
At the start of shutdown, electricians began swapping out the hardware and programmers prepared layers 4, 3, and 2 of the software so that I/O and device testing could begin as soon as the electricians completed each remote I/O cabinet. Based on the team’s experience with the first PLC, minor revisions were made to the device code and the device check was completed. All devices were tested by directly manipulating data in the device arrays while changes were made to the HMI so that a single, faceplate used indirect tags to control all the devices of each type.
DLEC employee (Bruce Mayhook) finishing up the connections.
Custom process programs were written for unique pieces of equipment; however the 18 filters and 12 sedimentation collectors were controlled by two programs that took advantage of the Device arrays to indirectly address the valves and motors. As this approach took shape, aliases were re-arranged in the device array to maximize the efficiency of the programs; since both the Soft-map and the other process programs addressed devices by their alias, this change did not affect any other programs in the system.
A direct result of the approach taken to perform this retrofit, the program organization is uniform across all three PLCs, naming conventions are enforced by the UDTs and the Town of Gilbert has an extremely consistent system that should prove much easier to maintain and expand than the system that was replaced.
To conclude, the upgrade of the SCADA system for the Town of Gilbert’s water system has resulted in several benefits: the operations have been simplified for operations and maintenance, scheduling of the process sequences has been simplified, and processes that were previously manual have been automated. Expansion will be facilitated because the PLC code is standardized and HMI screens are homogenous throughout the process.
About the Authors:
Alasdair MacLean, Ph.D., is a senior automation design engineer and project manager with considerable experience in utilities projects, as well as regulated industries such as pharmaceuticals and biotech. His experience includes design, implementation and validation of automation systems and coordination, development and installation of systems involving multiple vendors. Cliff Thomson, P.E., is the Director of Engineering for DL Engineering & Controls Inc., and has over 20 years of domestic and international engineering design experience in process controls. DL Engineering & Controls specializes in industrial facility electrical design, construction management services, and integration and support services, with a major focus in the water and wastewater industry. Headquartered in Phoenix, with offices in California and Nevada, DLEC provides solutions ranging from brief letter reports to multi-million dollar SCADA system design and integration projects for municipalities, private water companies, developers, civil engineering firms and general contractors throughout the Southwestern U.S.
Control of the water system involved eleven different process steps, namely:
- Intake flow control
- Alum feed and rapid mixers
- Four pre-sedimentation basins
- Low-lift pump station
- Chemical feed, rapid mixers and 3-stage flocculation
- Twelve final sedimentation basins
- Filter aid polymer and rapid mixers
- Eighteen filters
- Backwash recovery and sludge handling
- Two multi-stage booster stations