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Feb
11

Control Utilization

It can be shocking to examine control utilization in a plant where performance tools have not been installed and/or not used. For example, in the mid-1980s a control survey was conducted at a major pulp and paper plant where the control system had recently been updated to the latest distributed control system. At that time performance tools were not available to automatically evaluate control system performance. However, it was possible to manually evaluate control utilization using a snapshot of the plant operation. The results of this survey are documented in Chapter 3 of Advanced Control Foundation – Tools, Techniques, and Applications as shown below.

After management saw the results of this survey, an instrumentation team was formed to investigate loops that were not running in their normal (design) mode. This team was responsible for making sure measurement, control valve, and process problems were addressed in a timely fashion. The resulting reduction in variability led to significant improvements in plant throughput and product quality. Two years later the plant set a new production record.

Even today, similar problems can be found in process areas equipped with the latest control systems and field instrumentation. For example, a survey of seven areas in a petrochemical complex was recently conducted using performance monitoring tools that were embedded in the control system. A summary report of the control utilization for this plant is shown in below.

Once the plant management at this large petrochemical complex became aware of the low control utilization in key processing areas, manpower and funding were provided to investigate and correct the measurement, control, and process problems that were preventing the operator from using the control as designed. This led to significant operational improvements.  These examples demonstrate that the path to improving control performance should start with an assessment of control utilization. The automatic collection of control utilization statistics by the control system is of major benefit in identifying problems in measurement, control, or process design and operation.

In plants that do not have a control system that provides embedded control performance monitoring, this capability can be layered onto the existing control system. When a control loop is identified as not being fully utilized, the reason for poor utilization should be found and resolved. Once the issues of control utilization have been addressed it is possible to start examining the variability in control parameters, expressed by its standard deviation. Where the variation is large enough to impact plant production or product quality, the source of the variation should be investigated and steps taken to minimize it.

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