This paper presents an application of systems analysis in handling impurities in a complex plant for operation diagnosis or for revamping. Tracing impurities necessitates careful calibration of the plant material balance and the study of the recycle loops dynamics. Thus may be predicted operation difficulties originating mainly from interactions between recycles rather than from insufficient equipment performance. Steady-state flowsheeting combined with controllability tools enables to detect recycles interactions, to evaluate plantwide control properties of flowsheet alternatives and to suggest design improvements. Then, dynamic simulation on a reduced flowsheet may be used to asses robust flowsheet alternatives. A case study of a VCM plant illustrates the methodology.