(605d) Further Evaluation On A Valve Stiction Detection Method – Its Capabilities And Limitations

Many control loops in process plants perform poorly due to valve stiction as one of the most common equipment problems [1-4]. Valve stiction may cause oscillation in control loops which increases variability in product quality, accelerates equipment wear, or leads to control system instability and other issues that potentially disrupt the operation. Due to its importance, methods have been developed to detect valve stiction during the last decade (see, e.g., [5-12]).

In our previous work [12], a stiction detection method was proposed to fit two different functions, triangular wave and sinusoidal wave, to the measured output signal of the first integrating component after the valve, i.e., controller output for self-regulating processes or process output for integrating processes. A better fit to a triangular wave indicates valve stiction, a better fit to a sinusoidal wave non-stiction. Because the mechanism behind a sticky valve and effect of an integrator were taken into account, the proposed method is effective in valve stiction detection as demonstrated by both simulations and industrial data sets. Advantages of the proposed method lies in its industrial practicability, including straightforward methodology, fully automatic execution with no user interaction, robustness to noise, flexibility in handling asymmetric, or damped oscillations, and applicability to both self-regulating and integrating processes.

In this work, the capabilities as well as the limitations of the proposed method are further investigated.

First, the proposed method is applied to a first order plus time delay (FOPTD) process to identify its gray area where a decision can not be made. The gray area associated with the proposed method is compared to other published valve stiction methods.

Second, control loop oscillations can also be caused by process nonlinearities other than valve stiction. This aspect has not been addressed by other valve stiction detection methods. In this work, the proposed method is applied to limit cycle oscillations caused by various hard nonlinearities including valve stiction, saturation, etc. to test its capability in differentiating valve stiction from other hard nonlinearities.

Keywords: valve stiction; process nonlinearity; oscillation diagnosis; fault detection

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