The detection of malfunction using a process computer

A process control computer has been used to detect malfunction in the instrumentation of control loops. A flow control loop is examined and a malfunction detection algorithm is developed which is based upon a comparison of the control valve position and flow. The technique assumes a relatively constant flow-pressure drop characteristic. It is postulated that a flow control loop has inherent measurement redundancy and a simple "static" or "tracking" state estimator is used to obtain an estimate of the flow from the valve position (or control demand) and flowmeter measurements. The check is based upon monitoring changes in the residuals generated by the estimator. The check technique does not require additional process instrumentation, uses little computer time or storage and can be performed while the control loop is operating under direct digital control. The method has been tested by extensive laboratory trials and some limited industrial application. [Continues.]