How Does Your Control System Measure Up?

A scoring system can identify improvement opportunities.

By George Buckbee, ExperTune, Inc.

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A plant’s process control system affects safety and environmental results, energy and operating costs, quality and production rate. Most sites have spent millions of dollars on control systems — an individual loop may cost upwards of $10,000 to implement when you consider expenses for engineering, sensor, wiring, controller, valve, configuration and programming. Yet, is this sizable investment paying off as well as it should? Do you really know if your control system is delivering results? Is it reducing variability and operating costs? Is it improving quality, safety and production?

If your plant is like most, you don’t have a good way to measure control system performance. So, here, we’ll discuss how to measure, track and boost performance of your process control system. We’ll introduce a scoring system that will help you evaluate potential for economic improvements.

When you measure and manage control system performance, you get better results from the process as a whole. Studies indicate that improved process control typically leads to reductions of between 2% and 6% in total operating cost [1].

Table 1 shows some control-system performance measures as well as results for typical and best-in-class plants. If you don’t know how well you’re doing, you’re hardly alone. Most plants aren’t monitoring even the most important of these metrics.

This situation isn’t surprising. Work on new control systems focuses heavily on infrastructure, connectivity and security, with little emphasis on applications and performance. Thus, many plants find themselves with shiny new control systems that run the same old control applications and strategies. Also, most Distributed Control Systems (DCS) don’t have tools to calculate and report basic information on performance.

So where can you get performance information? Most plants have a process data historian actively gathering thousands of pieces of data. Analyzing this information provides great insight into control system performance. Specialized “control loop monitoring” systems can extract the most meaningful data from the process historian (or directly from the DCS) and present these data in a useful prioritized fashion.

Let’s now look at a variety of parameters that can give you an indication of performance.

Simple Measures
Three specific factors provide a good starting point for assessing control system performance:

Percentage of loops in manual. When control loops are in manual, they aren’t performing their basic function — they’re not controlling. Yet it’s typical to find 30% of control loops running in manual. The loss of control poses safety and environmental risks and, unfortunately, has taken a deadly toll.

Furthermore, a loop running in manual usually indicates some other, underlying problem. If a sensor fails, a valve sticks, or controller tuning is unstable, the operator usually will place the loop in manual.

A control-loop-monitoring system can provide a complete list of loops in manual, pinpointing many opportunities to improve operation.

Valves at limit. When valves are fully open or fully closed, they’re not controlling. The process can wander without any ability to control it. When a valve is 100% open most of the time, it’s often a sign of an under-sized valve.

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