Abstract:
Valve stiction is the hidden culprit of the process control loop. Among all nonlinearities of control valve, stiction has been being one of the major valve problems since the past two decades. Over the last two decades, several well established methods have been developed for detection and quantification of stiction. But methods for stiction compensation are still in developing stage. Among the existing two methods of stiction compensation, the ‘Knocker’ method is the most popular one. The other method known ‘two move’ method is yet to be applied in the process industries. Inverse stiction method in presence of saturation is another method which will require pilot plant study. However though PID controllers are inherent part of modern process industries, no noticeable work is yet done to compensate valve stiction through PID controller tuning. In this study, a novel approach to compensate stiction by using a PID controller was studied. A Second Order Process with Time Delay (SOPTD) was chosen for the study. Stiction was simulated by using the two parameter stiction model. It was confirmed that the Integral part of a controller has adverse effect on stiction. It was noteworthy that the derivative part of a controller has no effect on stiction. An empirical correlation was developed to relate the Proportional gain with the process parameters through extensive simulation for varying process parameters and controller gains. This empirical relation was evaluated for a large number of SOPTD processes. These results reveal that the correlation works well for process gain 2 to 5. It also works up to a gain of 10, if time delay is small. It is inspiring that stiction can be handled with a proportional only controller with the cost of some offset.
More investigations on proportional only controller has revealed that a low fixed value proportional controller can handle SOPTD with high gains. However, to remove the offset problem, using a very high integral time gives satisfactory results. A guideline for efficient tuning of the PID controller to handle stiction was also proposed which might act as the starting point of tuning a PID controller in presence of a sticky valve in the control loop.
Inverse stiction method was developed with the help of the two parameter stiction method in the later part of the study, which works both in parallel and series combination with the sticky valve. Both combinations nullify the oscillation caused by the presence of stiction. But application of the parallel inverse stiction method has practical limitations. Series inverse stiction has got the importance for further evaluation through pilot plants and process plant tests. Application of a dither signal to compensate stiction was also re-investigated and it was found that for pneumatic valve, adding dither signal with the controller signal is not practically possible due to its faster dynamics. Applying physical dither by creating continuous vibration in the valve packing region can be an option for future work.
