Detection and compensation of control valve stiction

Abstract

Presence of oscillation or large variability in control loops results in decreased economical advantage to process plants due to inferior quality products, larger rate of rejections, reduced average throughputs and overall increased energy requirements. Oscillation may arise due to physical and non-physical causes. Mechanical problem in control valves is a common physical cause. Among the physical problems of control valve which produces oscillations in the control loop, stiction is often referred as hidden culprit. This study presents a novel noninvasive stiction detection method which requires only routine operation data. It has been shown that the presence of stiction in control valve produces signals containing odd harmonics. The proposed method estimates frequencies, amplitudes and phases of control error signals and examines harmonic relations among them. The presence of odd harmonics indicates the presence of stiction in the control valve. The method has been validated by simulation and pilot plant experimentation. The proposed method has also been evaluated by using benchmark industrial data sets and found to perform better than currently available other stiction detection methods. Detection of control valve stiction and it's remedial action are two separate tasks. Oscillations of the process variables due to the presence of stiction in the control valve can't be stopped until valve maintenance. Normally, maintenance of sticky valves in the industry is usually carried out in a scheduled outage. If immediate actions can't be implemented after the detection of sticky valve, industry continues to incur economic losses from the harmful e ect of stiction such as early wear-o of the valve stem, reduced valve life and production of o -spec products. Thus, an e ective stiction compensation method to remove the harmful e ects of stiction on the process plant is a long felt desire of the industry. In this study, the performance of a self tuning adaptive controller in the presence of stiction was studied. PI and PID type adaptive controllers' e cacy has been studied as a compensator for valve stiction.