Patent ID: 9429938
Filing Date: 2016-08-30
CPC Classification: G05B

Claim Text:
1. A method of qualifying performance of a conventional control valve that does not have a self-provided feedback mechanism that allows a user to qualify valve performance in a process plant, the valve being controlled by a controller, the method comprising: sensing, with a flow rate sensor, a flow rate at a location in the process plant, the flow rate being controlled by the conventional control valve; storing in a database of a server of the process plant the flow rate sensed by the sensor, a set-point being a pre-determined desired value for the flow rate, and a manipulated variable being a value sent from the controller to the conventional control valve for adjusting the flow rate to reach the set-point; a processor obtaining data samples from the database stored on the server of the process plant, each data sample comprising the flow rate sensed by the flow rate sensor, the set-point, and the manipulated variable; the processor computing a non-linearity index from the data samples and determining if the non-linearity index is greater than a threshold value; if the non-linearity index is greater than the threshold value the processor charting a plot of the process variable against the manipulated variable and determining if the plot has an elliptical or rectangular fit; if the plot has an elliptical fit, the processor determining if a percentage of the total number of data samples lying within a theoretical ellipse encompassed within the elliptical fit is less than or equal to a preset percentage; if the plot has an elliptical or rectangular fit and the percentage is less than or equal to the preset percentage, the processor computing a model fitness stiction index of the plot relative to a standard valve stiction model, and determining if the model fitness stiction index is greater than a pre-set value known to arise due to valve stiction; the processor determining whether valve stiction is associated with the conventional control valve based on whether the model fitness stiction index is greater than the pre-set value; and adjusting, using the controller, the flow rate at the location in the process plant to reach the set-point by sending the value corresponding to the manipulated variable to the conventional control valve.