Implementing a new distributed control system is one of the biggest and most complicated projects in a process control engineer’s career. Doing one successfully requires everything from a well-defined project document to good grounding practices. Here are recommendations for best practices and some pitfalls to avoid.
1. Standardize. Use of standard wiring throughout the system will make it for easier for others to understand and troubleshoot. Use standard, off-the-shelf components for ease of stocking and reordering. If possible, have two sources for the products being used or purchase interchangeable brands.
2. Remember the basics. It’s the little things that can trip you up. Make sure you use proper grounding, proper grouping of signals and proper termination of electrical signals. Make sure you understand the supplier’s grounding requirements for your DCS system. Grounding principles need to be clearly understood by all automation engineers, not just the electrical staff. International standards can be misinterpreted. Instruments and the control system need to be grounded separately. Double check the grounding before powering up any DCS system to avoid any short circuits, particularly during factory acceptance or site acceptance testing (FAT/SAT).
3. Is communication complete? While most automation suppliers have different software versions for communicating with the system, make sure they will transmit all the required information. Many systems only transmit the basic parameters, which means all diagnostic features will not be available. The introduction of the “Control in Field” concept, although not often used, has added some complications and needs to be thoroughly examined when implementing a DCS.
4. Structuring I/O. Since today’s electronics are available with high-temperature specs and may be G3 compliant (conforming coating), the I/O structures should be moved to the field, reducing the rack room footprint and cabling cost. Communication links should be used over fiber optic, in a ring configuration to provide some level of redundancy, to interconnect the field I/O structures. Extended I/O terminal blocks (three to four terminals per channel) should also be used to allow field wiring to be connected directly, avoiding marshaling terminal strips with the related space, additional cost, installation cost and the possibility of poor connections.
5. Dual purpose. The purpose of DCS is twofold. Centralized human control and interface to the plant as well as a centralized location for MIS info to the management network. DCS control should not include auto tuning of control loops other than simple on/off or start/stop functions. These should be the function of a local dedicated controller. Use the DCS to update the tuning parameters.
6. Good links. Distributed control systems are only as good as their communications links. Choose a very solid and reliable link between processing units.
7. FAT is where it’s at. Make sure you do a comprehensive and detailed factory acceptance test (FAT) before cutover. FAT involves experienced operations people interacting with engineering to validate graphics and verify that instruments in the configuration exist and will remain in service.
8. Use single server. Base the selection of a DCS system on its redundant capability. A single server system is preferred. Pay attention to the hardware license for client and server to avoid delays during a system or hard-disk crash. Care must also be taken in selecting appropriate layered switches for communication. Make sure you properly configure trends and history data for future analysis.
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