Compensating the effect of control valve stiction

Abstract

Valve stiction is the hidden culprit of the process control loop. The presence of stiction in a control valve limits the control loop performance. It is the most commonly found problem in pneumatic control valves. It decreases the control loop efficiency and causes oscillations in process variables. Repair and maintenance is the only definitive solution to fix a sticky valve. However, this fact implies to stop the operation of the control loop, which is only possible during plant shutdown. Compensation of its effect is beneficial before the sticky valve can be sent for maintenance. In this study, a new stiction compensation method has been developed by adding an extra pulse for a certain period of time to the detuned controller action. A method for estimating the parameters required for the proposed compensator has also been developed. The performance of the proposed stiction compensation scheme is evaluated using MATLAB Simulink software. The developed compensator has the desired capability of reducing the process variability with a minimum number of valve reversals. It has the capability of good set-point tracking and disturbance rejection. The performance of the proposed compensator has been compared with other compensators available in the literature. The proposed compensator outperforms all of other compensators. The proposed compensator has been implemented to a level control loop in a pilot plant experimental set-up. It has been found to be successful in removing valve stiction induced oscillations from process variables. Moreover, the proposed compensation scheme is simple. It requires minimum process knowledge. It would not be difficult to implement it in real process plants.