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.
