Nowadays, system reliability performance represents a key issue in any advanced technology application in order to guarantee ambient, personnel, and system safety. The core of this paper is the reliability block diagram (RBD) generation with the aim of providing project engineers a reliability prediction in the early stages of industrial product development. This study is focused on gas turbine auxiliary systems; these systems include both mechanical items (in particular hydraulic devices such as valves, pumps, and filters) and electronic ones (e.g., sensors, instruments, control logic). Such complex structures are decomposed in single blocks and interconnections to establish their mutual relationship and achieve reliability performance of the whole system. The case study is one of the most important gas turbine auxiliary systems, the mineral lube oil console. The aim of this paper is to introduce a new approach to assess system reliability prediction in presence of redundant and stand-by architectures; redundancy is widely used in industrial applications since it is one of the best techniques to achieve fault tolerance. The proposed method led to the development of a dedicated software tool RBDesigner that semiautomatically generates a RBD starting from the sketch of thermal-hydraulic systems and provides the most important reliability parameters. The use of the proposed tool allows project engineers to reduce time delivery, reduce time for improvements, achieve reliability targets, and guarantee availability performance to the customers. The strengths of RBDesigner were finally validated by a comparison with other commercial software solutions.