Though oscillations in control loops represent a significant challenge across the process industries at-large, they are especially challenging for those in the Oil & Gas sector. In short, if oscillations are allowed to persist, they lead to instability, reduce product quality, and increase operational costs.
Understanding the factors that contribute to oscillatory behavior is a critical first step for process control and automation engineers. Consider the following four (4) common factors:
- Aggressive Tuning: Tuning parameters with a high proportional gain and fast integral and derivative responses often lead to excessive correction. This overcompensation can cause a process to oscillate as it continuously overshoots and re-corrects. Ultimately, it destabilizes the process and creates persistent, unwanted fluctuations in Controller Output.
- Process Nonlinearities: Nonlinearities in a process, such as hysteresis or nonlinear gains, can cause the system’s response to deviate unpredictably from the control input. As the controller attempts to correct these deviations, it may create oscillations by continuously overcompensating or reacting inadequately to the nonlinear effects.
- Loop Interactions: Interactions between and among PID control loops can cause oscillations when their control actions conflict. If loops are not well-coordinated, one loop’s adjustments can counteract another’s. This contributes to instability and persistent oscillations as each loop tries to correct errors introduced by the others.
- Mechanical Issues: Oscillations in a process often arise from mechanical problems such as stiction and excessive Dead-Time. Stiction causes a control loop’s final control element to stick before responding, leading to delayed corrections. Similarly, excessive Dead-Time further delays responses, making the process prone to overshooting and oscillating as it attempts to stabilize.
- Unbalanced Tuning Parameters: You can achieve control with certain tuning parameters, but if the process changes then deviations between the model and the process can result in instability. Unstable behavior can result from an imbalance between the proportional and integral terms. Ensuring the controller tuning model is inherently stable is key the managing the variability in your process conditions.
Process modelling and PID controller tuning tools have been shown to help controls engineers to methodically address the challenge of oscillatory behavior. In particular, select software tools graphically simulate the behavior of a process with different tuning coefficients and they provide before-vs-after statistical analysis. These simple features allow users to more thoroughly assess a PID’s performance relative to Overshoot, Settling Time, among other attributes before implementing new tunings. Whether the issue stems from nonlinearities, loop interactions, or mechanical issues, these tools provide a more effective means of tuning controllers for effective control.
Control Station’s LOOP-PRO and PlantESP are proven solutions that help Oil & Gas manufacturers optimize their PID control loops, reduce oscillations, and maintain high levels of operational efficiency. For engineers looking to solve complex tuning and monitoring challenges, these tools provide the insights needed to keep processes running smoothly and profitably.
