Multi-stage flash desalination plants consist of many complex parts and equipment. In general, the process of producing water passes through many steps until the final product, which is distillate water, is obtained. At the present, troubleshooting in MSF plants relies on expert operators in the plant or on knowledge available in the plant manual. In this paper, a fuzzy logic-based expert system designed to give real time troubleshooting advice is presented. The paper, in particular, deals with the brine heater of the MSF plant. The development of the fuzzy expert system was carried out by selecting the input variables associated with the brine heater operation, building the fuzzy membership functions for input and output variables, and developing knowledge based rule structures. The system was proven to be capable of performing troubleshooting tasks and can be used either on-site or off-line for operator training purposes.