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Watching over the Controls
Building a server-based control system requires that the real-time process control portion be extremely reliable and able to carry on without supervision when the link to servers goes down. Or, as Stauffer puts it, “This approach is only feasible if the DCS equipment has a high level of built-in diagnostics and intelligence that allows it to report faults or potential problems to the operations team.”
“With the advent of intelligent devices, information and diagnostics can be used to predict maintenance behavior,” adds Jensen. “Predictive diagnostics, for example, let a valve tell maintenance personnel that it should have maintenance in the next number of days. The application of the diagnostic engine and the algorithms and methods of prediction are just being developed. This is in its infancy.”
Or maybe you should step up and handle things. "I think it's time the process control department took back fieldbus from the instrument shop,” says Rezabek. “There are many instances where distributed intelligence, control and single loop integrity contribute more to process availability than diagnostics alone."
You may need to fix some problems remotely, says Pat Kennedy, president of OSIsoft. “To be truly remoted, we need remote maintenance of these systems, which means secure access to the configuration information and tools for maintaining the configurations,” he says. “We also need exchange of not just data but also structures, graphics, configurations, and many other kinds of information. For example, if a control system does not share which entities comprise a loop, graphic, advanced control or report, then trying to remotely use the embedded information is not possible.”
“Running software remotely is not a problem nor is viewing it provided that you have a good network and the software is designed either for remote operation or supports tools like Remote Desktop Connection [RDC],” adds Kennedy. “However, to maintain that software you have to have good knowledge of its current state, security issues, potential problems, and configuration changes, plus the ability to fix issues without traveling to the site. It will be a long time -- if ever -- before this is multi-vendor, so look for products that are designed to run remotely with appropriate attention to back up operation, redundancy, fault avoidance for network problems, configuration management, and so on.”
Finally, field equipment with smart diagnostics might overwhelm an operator, points out Lane Desborough, marketing manager at Honeywell. “Consider that today an analog point in a control system can have on the order of ten configured alarms, such as high, low, rate of change, etc.,” he explains. "Abnormal Situation Management consortium research shows that operators are overwhelmed with the volume of alarms which happen during an abnormal situation. Now picture a plant where each device has two hundred alarmable parameters and events. Imagine the amount of additional load this is going to place on an already overtaxed operator. Impending signs of catastrophic events are going to be buried in a sea of spurious alerts from NAGs - nuisance alarm generators.”
Desborough continues, “If the distribution of control actions, alarms and events to thousands of ‘smart’ field devices results in the operator becoming the first line focal point for coordinating activities among these thousands of smart devices, then the operator is going to be a very busy person. Likewise, if the smart devices are automatically sending work orders to the maintenance system, where is the maintenance planner getting the process knowledge and business insight to prioritize these actions for maximum business benefit?”
There is no doubt that remote, unattended control systems pose maintenance problems far different than those in a staffed plant. However, modern software makes it possible to run all device and system alarms through alarm management programs that sift out the spurious from the critical. Also, one feature of a server-based remote control system is that maintenance and operations can be staffed around the clock by experienced engineers and technicians at a central location who can bring vast knowledge and resources to bear on a problem.
As Jensen says, diagnostics of remote systems is in its infancy, and managing the maintenance of unmanned sites may be the biggest problem of all. As we note in this month’s Control Report, maintaining a modern distributed control system, with its myriad embedded processors, can be a nightmare.
The Importance of Being Open
While we’ve concentrated on describing remote, unattended systems, very few control systems operate in true standalone mode. Almost all need to be supervised, monitored, analyzed and fine tuned. As Desborough puts it, “What happens if the business objective suddenly changes from ‘maximize throughput’ to ‘minimize energy consumption’ and, as a result, all the control loops have a sudden change in performance and each take independent action to notify the operator or maintenance guy? How is the business context, the goal, communicated down to the individual controllers in order to suppress alerts?”
Enterprise-level software makes it possible to analyze the workings of a plant and coordinate supervisory changes when a situation like Desborough posed occurs. Such software includes loop analysis, maintenance management, asset management, MES, ERP, SCADA, historians, and a host of specialty software for various industries.
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