Control Notes

Control loop performance directly affects the operability and profitability of industrial plants. Considering the importance of control loops, one would expect that they always perform at their peak, but this is not the case. In fact, several studies have shown that roughly one third of industrial control loops perform poorly.

Poorly performing control loops can make a plant difficult to operate and may have several costly side-effects, including:

• Reduced production rate
• Increased emissions
• Lower efficiency
• Plant trips following process upsets
• Poor product quality
• Slower startup and transition times
• More off-spec product or rework
• Premature equipment wear

For these reasons, control loop performance should always be kept at the highest possible level. This is achieved through continuously monitoring loop performance and taking the appropriate corrective actions when sub-optimal performance is detected.

Loop Performance Monitoring

To effectively manage, improve, or sustain control loop performance, it is important to monitor how well loops perform. Loop performance monitoring can provide valuable feedback on the success of control optimization projects; it helps maintaining a high standard of loop performance in the long run; and it can be used to pinpoint offending control loops for corrective action.

Loop Performance Assessment

Loop performance should be evaluated from various perspectives. A control loop needs to be in its correct mode (mostly auto), stable and responsive, and must reduce process variability. Loop performance can be calculated in terms of these perspectives and expressed as a numerical value, or metric. The following metrics are essential for assessing the performance of the control loop:

• Percentage of time the controller is not in its correct mode
• Percentage of time the controller output is at its limits
• Standard deviation in error
• Tendency of loop to oscillate
• Controller responsiveness

Many other metrics can provide useful additional information on the performance of the control loop, control valve, and measurement device, for example:

• Process variable noise
• Cumulative control valve travel per day (can be used for predictive maintenance)
• Number of direction changes in control valve travel per day
• Mean value of controller output (can be used to indicate oversized and undersized valves or incorrectly ranged transmitters)
• Number of times the operator changed controller mode and/or output
• Number of tuning constant changes on the controller

In most cases it is sufficient to calculate loop performance metrics daily. These metrics should be averaged over a period of a week to obtain KPIs for average loop performance over the week.

Some metrics can be easily computed in the process historian, while others are best done by control loop performance monitoring software applications. The monitoring process should run autonomously and automatically by using control loop performance monitoring software, or by writing a custom application in the process historian (process information management system).

Overall Loop Health

Each metric can be compared to a threshold for proper loop performance. If one or more metrics exceed their threshold, the loop should be flagged as having poor performance and maintenance or engineering staff should attend to the problem.

Practical Matter

It is important to consider the operational state of the plant when evaluating loop performance. For example when a plant has been shut down, many loops will be in manual control mode and most of the remaining controllers will have their outputs saturated at 0 or 100 percent. These control loops should not be flagged as problem loops while the plant remains shut down.

Loop Monitoring Software

Several control performance monitoring software packages are available from many vendors including ABB, AspenTech, Control Arts, ControlSoft, Control Station, Emerson, ExperTune, Honeywell / Matrikon, PAS, and RoviSys. These packages can help to identify problem loops which can then be addressed to minimize the impact on production. Be advised though that the software requires a substantial amount of setup to work properly and accurately report loop performance.

Stay tuned!

Jacques Smuts – Author of the book Process Control for Practitioners