One of the most significant factors when planning and executing a control system upgrade is the amount of time that the system can be down during the upgrade. In manufacturing environments, one can often plan for a period that a production process or packaging line can be taken offline. This period could come from alternate process paths or lines being utilized, material inventory buildup or a planned annual maintenance shutdown period. During a downtime period, the entire control panel(s) may be replaced and the system restarted and commissioned with all the new equipment.
In the case of municipal water or wastewater treatment, the allowable process downtime is near zero. The time is often measured in hours for water where gravity fed storage is utilized and minutes for wastewater systems where flows are continuous. Even with redundancies built into the equipment, the control system is a critical component for the entire facility and can’t be taken offline.
In the typical case where an obsolete 20 or 30+ year old control system is to be upgraded to a modern system, the project plan must provide for near continuous operation while changing over multiple components such as SCADA/HMI operator interface display hardware and software, communication networks, programmable logic controllers (PLCs), physical input and output (IO) hardware and IO wire terminations. Often, the process consists of multiple physical areas where controls are distributed such as influent pump stations, wet wells, filter units, chemical rooms, aeration tanks, clarifier tanks and effluent pump stations. Data from one area may be required for control of another area. As an example, residual chlorine may be measured at the effluent and used to trim the chlorine injection that is primarily controlled as a ratio of flow measured at the influent. Therefore, equipment controlled in mid-process is dependent on data provided by instrumentation / PLC IO at the beginning and end of the process. This exchange of data must be considered when staging a complete control system upgrade.
The following scenario is an example of a plan to perform a complete control system upgrade with near zero downtime.
The key elements of this plan are:
During the wiring migration, the new AB PLC code will utilize IO mapping from both TI and AB IO to the controller IO tags. This allows the PLC IO tag to link to either TI505 IO or AB IO and is separate from the control logic. To accomplish this task, the mapping code is developed as follows:
Utilizing the mapping code, the equipment signal is lost only for the time it takes to move the wire from the old IO terminal to the new IO terminal. Downtime meets the near zero requirement.
Fallback plan – if a problem arises that can’t be resolved in allowed time, restore the TI 545 PLC control and reconnect the old SCADA system.
Fallback plan – if a problem arises that can’t be resolved in allowed time, restore the IO point to the original IO rack.
With this method the panel work can be completed in partial steps within normal working hours for plant personnel. Scheduling is simplified and migration can take as long as necessary to prove each point. The migration process is less harried so it can be completed with reduced stress and increased confidence. A rack can be partially migrated with the plant operating when a workday is over.
The four buildings / areas / control panels are indicated as CNTRL, ADMIN, CSO and GRIT
The migration plan has described upgrading a TI545 PLC & TIWAY Network. This plan can be similarly applied to other legacy PLCs and networks such as old Modicon or SCADAPack PLCs using Modbus or DNP3 networks. The protocol gateway device described has options that connect AB Ethernet IP to many different protocols. Gateways can be purchased from vendors such as Prosoft and HMS Anybus.
In the migration plan described above, status and control data are shared between IO racks during migration because the single PLC CPU can communicate with all the existing and all the new IO racks simultaneously. A similar migration plan can be applied to the more complicated case of distributed PLC control. In this scenario, multiple protocol gateway devices may be utilized. All the new AB PLC CPUs and Gateways would be installed in the first step. All the existing PLCs would become IO racks for the new AB PLC CPUs to start the migration process. To make the existing PLCs IO for the new PLCs, all the existing PLC programs are replaced with a program that simply maps the communications data to/from IO points. As each panel is migrated, the new AB IO is installed along with the old IO. Operation continues as wires are moved. After an entire rack is migrated to the new PLC IO, the old PLC rack and gateway device can be removed from the panel. Fallback Plan: If there is a problem, re-install the original Modbus PLC programs and reconnect existing SCADA until the problem is resolved.
For a distributed PLC system, the steps of migration would be similar with the remote IO scenario.
When working in live panels, always utilize appropriate personal protective equipment (PPE).
About the author
Allan Stier isa Lead Controls Engineer for Hallam-ICS in our South Burlington, Vermont office.
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About Hallam-ICS
Hallam-ICS is an engineering and automation company that designs MEP systems for facilities and plants, engineers control and automation solutions, and ensures safety and regulatory compliance through arc flash studies, commissioning, and validation. Our offices are located in Massachusetts, Connecticut, New York, Vermont and North Carolina, Texas and Florida and our projects take us world-wide.