Grey box fault detection of heat exchangers

Abstract

A grey-box model-based method for fault diagnosis is proposed in this paper. The method is based on a first principle model of the process unit, i.e. a heat exchanger, and on a grey-box model of the fault, i.e. the deterioration of the heat transfer surface by aging. During normal operating conditions the heat transfer coefficient is constant or slowly decreasing due to material settling on the heat transfer surface. In old heat exchangers big pieces of settled material can break off causing damage. When this happens, the heat transfer coefficients will rise sharply. In the proposed method a recursive least-squares estimator with forgetting factor is used to track the heat transfer coefficients. The settled material breakage fault is detected via detection of abrupt positive jump in the estimated heat transfer coefficients using a cumulative sum (CUSUM) test. The capability to detect faults in any industrial equipment is heavily dependent on the availability of suitable measurements. For heat exchangers the variables related to the in- and outflows of the equipment (flowrates and temperatures) are usually measured, but measurements along the equipment length are rarely available. Therefore, the possibilities of fault location in space are rather limited. However, simplified models can be used for fault detection in this case. Moreover, a fault detection method is proposed with the possibility of spatial fault location when measurements along the cold side are available. The proposed method is illustrated on simulated examples with different measurement situations.