Novel Model-Free Approach for Stiction Compensation in Control Valves

Stiction in control valves is a frequent cause of loop performance deterioration. The solution is maintenance, but since plant shutdown to perform this task can take a long time, loop performance improvement through stiction compensation affords increased efficiency and profitability by reducing the necessity for unscheduled plant shutdowns. Non-model-based methods add compensating pulses to the control signal, thereby reducing process variability. However, the current practice of this strategy transfers variability to the control valve stem, degrading this actuator prematurely. In this paper, such methods are analyzed and a new method for reducing both process variability and valve stem actuation is proposed. The essence of this new method is to add the pulses only when they are required for reduction of process error. The compensation algorithm presented can handle set point changes and perturbations as is illustrated via simulation and through application to a pilot plant flow loop. The proposed method's performance is measured using integral absolute error and valve reversal indexes. It produces better results than similar methods found in the literature.