Fieldbus instruments support a larger vision

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

With a fieldbus-based plant design, I&C and electrical engineers must work together to group devices on segments based on a number of constraints, including device communication limitations (in the case of the GWR system, four valves, 10 transmitters, or some combination thereof), bus communication limitations (some longer distance DeviceNet buses communicate at slower rates), separation of parallel equipment to prevent a power loss from disabling multiple equipment trains, and in some cases, the cost of bus interfaces when in close proximity to the controller cabinet.

Clearly defining fieldbus segments must happen before conduit and wire design begins. This adds a level of complexity to the work flow process and requires more interaction between disciplines. Also, because of the tight linkage between segment diagrams, I/O lists, and electrical design, the impact of a change during the final design is greater. Good planning and consistent adherence to design standards can offset these additional constraints.

Cost versus benefit of fieldbus

The early decision to use fieldbus instruments supported a larger vision of using the intelligence available from microprocessor-based instruments to move OCWD from a preventive maintenance mode to a predictive maintenance mode. The DeltaV system includes asset management software that allows the user to call up the status of any fieldbus device. The ultimate success of OCWD's asset management program will be the successful integration of a computerized maintenance management system with its new financial information system and the DeltaV controls, thus enabling OCWD to realize the full potential for savings using an intelligent plant. However, some immediate savings of using fieldbus are:

Device configuration: Configuration of field devices is easier and simpler using the new system. OCWD estimates the configuration time required for a device using a PLC-based system is thirty minutes (fifteen minutes to configure the I/O channels and fifteen minutes to document the changes). With DeltaV it is about five minutes, because the system automatically senses a fieldbus device when it is connected. Also, the configuration of a device documents automatically within the device, which eliminates documentation time altogether. Given that OCWD plans to install 1,200 fieldbus devices, the savings potential during commissioning is approximately $50,000.

Device calibration documentation: OCWD determined that the largest savings in device calibration labor using a fieldbus system versus a PLC-bascd system is in the documentation. Defining a calibration procedure would take about the same amount of time with a PLC-based system or a fieldbus-based system. The time to locate a test procedure would halve from ten minutes to five minutes using fieldbus, because the procedures are readily available. Also, the time to locate an instrument in the control system is less, because the historical information for each instrument is readily available via the HMI. Documentation of configuration changes occurs automatically with the new system, so that time goes to zero minutes. Total time to document a device calibration was twenty-five minutes less. If 65% of devices are affected, then there is a potential savings of $75,000.