Fault diagnosis is the problem of finding out the root cause (the fault) of process malfunctions. In this work we present a new model based approach procedure for fault diagnosis in conventional chemical processes. The process model is a Signed Directed Graph (SDG). The SDG is used by a Qualitative Simulator. This stage allows us to know, for each potential fault, the possible process behaviour. This information is compiled into a set of IF-THEN rules. An Expert System evaluates them using information about the actual process state. Fuzzy Logic is used in this evaluation. By the way, the fault whose rule has the highest value of certainty should be the first one considered by the operator. Finally, the additional information provided by the Qualitative Simulation enables the Expert System to explain the diagnostic. Besides, this additional information can be used to improve the diagnostic.