Troubleshooting Plantwide Oscillations using Nonlinearity Information

Abstract

Plant-wide oscillations are common in many processes. Their effects propagate to many units and may
impact the overall process performance. It is important to detect and diagnose the cause of such
oscillations in order to rectify the situation and maintain the proper profitability of the plant. This paper
proposes a new procedure to detect and diagnose plant-wide oscillations using routine operating data. The
method has been developed based on the nonlinearity information in the process data. A new Total
Nonlinearity Index (TNLI) has been defined to quantify nonlinearities. The method is based on the
assumption that the nonlinearity is highest near the source and decreases as one moves away from the
source. This assumption is true because chemical processes have the nature of low-pass filters and they
filter out gradually higher order harmonics of the signals. Signals with higher order harmonics are
generally more nonlinear. The proposed diagnostic method has already been successfully applied for
troubleshooting many industrial plant-wide oscillation problems. Two of such case studies have been
presented in this paper.


Journal of Chemical Engineering Vol.ChE 24 2006 50-60