This paper introduces the Fault Analysis Software Tool (FAST) which from a simplified description of any process is able to perform the structural analysis and systematically obtain the analytical redundancies to perform fault diagnosis, identifying the fault and the associated faulty component. The proposed tool can operate in different ways: stand alone in simulation mode, off-line; reading the measured values from data files, or on-line connected to the process through an OPC interface, enabling for connecting to a SCADA system for supervisory control. When running in on-line mode, in case of a fault, FAST is able to indicate the faulty device by means of updating the process database such that the SCADA can display and record the fault. Early countermeasures can be implemented, either by automatic reconfiguration of the system or by operator intervention. The fault diagnosis algorithms implemented in the tool are described and illustrated using the well-known two-tank system case study.