A number of basic criteria must be maintained before it’s possible to consider any process optimization project. The most critical is that basic regulatory control algorithms are all working properly, including loop tuning and alarm management. Then we must remember that basic regulatory control is only as good as the field devices. Fortunately, with smart or intelligent field devices it’s now possible to know how the field devices that form the lowest level of the process control pyramid (Figure 1) are doing in real time, and to know that modern alarm management/HMI systems are able to manage and route/display effectively that information to the right people to act on the information, thus increasing overall system reliability and performance.
One organization that has made a significant contribution to the ability to measure sensor effectiveness is NAMUR, for Normenarbeitsgemeinschaft für Mess- und Regeltechnik in der Chemischen Industrie, or in English, the User Association of Automation Technology in Process Industries. NAMUR has prepared two documents related to the effective use of intelligent field devices (“NE43 – Standardization of the Signal Level for the Failure Information of Digital Transmitters” and “NE107 – Self-Monitoring and Diagnosis of Field Devices”) that are the basis for much of the work on communicating device health to control systems in a meaningful, standard way. If you’re interested in reading these documents, free copies are available at www.namur.net/en/recommendations-and-worksheets/order-of-free-copies.html.
Because the recommendations in these documents are being widely accepted by field device manufacturers, there’s now a standard way to classify device health to the control system and incorporate it into the basic regulatory control loops, so that they only control on good signals.
Foundation fieldbus uses this with its “Status” parameter transmitted with every PV update. Similarly, wireless networks incorporate status information in their messages that could be used the same way. Incorporating this information into the HMI so that, at a glance, an operator can understand the status of the process is the next logical step.
The operator’s role is to optimize the way the facility is run based on the best knowledge we can present on the status of all parts of the operation; hence, the reason for all the work being done with HMI development to give operators the time to do this and, in the event something does happen, return to steady state as quickly and safely as possible.
“Fancy” algorithms such as those used for multivariate control and real-time optimization are certainly useful tools to make a facility more efficient, especially as the facilities continue to get more complex, and it is necessary to manage all the process measurements now available courtesy of modern smart and wireless sensors. Despite all this, the basic truths of automating a plant remain true. You can only control what you can measure (PVs) and manipulate (OPs). All the stuff in between is only math.