Fieldbus instruments support a larger vision

InTech, Jul 2004 by Thomas, Laura, Kalinowski, Jay, Cook, Curtis, Verduzco, Lou

Intelligence moves plant from preventive maintenance to predictive maintenance.

Applying fieldbus technology to a water treatment plant requires a shift from traditional instrumentation and electrical design methods.

Orange County Water District (OCWD) is creating a new, advanced water treatment facility (AWTF) to replenish the groundwater in the lower Santa Ana River basin with reclaimed wastewater effluent from the neighboring Orange County Sanitation District treatment plant.

This project develops a new, cost-effective source of reliable, high-quality potable water to refill the Orange County groundwater basin and expand the existing seawater intrusion barrier. However, the groundwater replenishment (GWR) system will significantly increase the complexity of managing, operating, and maintaining OCWD water production facilities.

Orange County Water District had the opportunity and the challenge of developing a control system that would enable it to change its approach to operations and maintenance. OCWD's vision of a control system architecture that would enhance its asset management program led to the decision to use fieldbus technology and distributed control. The preliminary design phase of the project relied heavily on input from the manufacturers of the process equipment. The process included microfiltration (MF), reverse osmosis, and ultraviolet disinfection. The traditional controls approach with these technologies was to allow the manufacturer to provide all control system programming on a programmable logic controller (PLC) system architecture using ladder logic, and then turn over the whole system to the owner. OCWD had to weigh the desire to have a fieldbus-based distributed control system (DCS) against the major equipment manufacturer's lack of standard products using this type of system.

OCWD made the decision to preselect a DCS, develop software and hardware standards, and have all systems developed using those standards. Before preselecting a DCS, OCWD decided to use Foundation fieldbus for instrumentation and DeviceNet for motor control centers and variable frequency drives. Classic I/O would serve as necessary. After a preselection process, OCWD selected Emerson Process Management's DeltaV system.

The organization had to develop system design standards, software design standards, and hardware design standards to ensure uniformity and, ultimately, maintainability of the system. In addition, OCWD developed a "pilot" project called the initial microfiltration facility (IMF) to use DeltaV on the MF process to allow operations staff to get familiar with the new control system on a much smaller scale than the final plant. The IMF is in the final stages of start-up.

This article will show design considerations such as selection and application of the fieldbus, the process and instrumentation diagram (P&ID) presentation, segment diagram development, and instrument and equipment specifications. Here is an analysis of the costs, benefits, and challenges associated with using fieldbus technology. The benefits include reduced wiring costs, greater access to diagnostic data, and the ability to support predictive maintenance procedures. The challenges of using a new technology include limited availability of experienced designers and integrators, limited fieldbus products, and limited manufacturer support of fieldbus products.

Design considerations

Applying fieldbus technology to a water treatment plant requires a shift from traditional instrumentation and electrical design methods. P&IDs, loop diagrams, conduit routing, and equipment and instrument specifications end up significantly different through the use of fieldbus technology. One must develop fieldbus design standards early and strictly adhere to those standards. In addition, the work flow process becomes more challenging, and a higher level of coordination between the instrumentation and electrical disciplines is required.

Selection and application of fieldbuses: The most critical design decision is the selection of specific fieldbuses and their application in the design. For the GWR system project, bus selection depended on the buses' technical strengths and prevalent application in process plants. This ensured maximum performance and the greatest number of available, compatible field devices. As questions arose concerning the use of these buses, it was helpful to have developed some "big picture" constraints in advance:

* Foundation fieldbus would handle transmitters, analyzers, and control valves. Its application would favor analog signal processing, with the exception of open/close valves.

* Foundation fieldbus control logic would reside in the process controllers (not in field devices), but devices that interact to form a traditional transmitter and control valve proportional, integral, derivative (PID) algorithm loop would be assigned to the same segment for possible future migration of PID control to the field device.

* DeviceNet was to handle smart motor starters, variable frequency drives (WDs), power monitors, and remote clusters of hardwired signals. Its application would favor discrete signal processing, with the exception of VFDs.