Reliability assessment represent a key issue in any advanced technology application where the systems must satisfy high level of reliability in order to guarantee ambient, personnel and system safety. This paper is focused on Reliability Block Diagram (RBD) technique with the aim of providing a reliability prediction for complex systems. In particular the case study is focused on gas turbine auxiliary systems. These systems are made up of hydraulic items (valves, pumps, filters, etc.) and electronic ones (sensors, instruments, control logic); these complex structures can be decomposed to a collection of blocks and interconnections in order to achieve reliability performance of the whole system. The aim of this paper is to introduce a new approach to assess standby redundant architectures widely used many field of applications and allow project engineers to focus system design on reliability assessment: in this way project engineers can reduce time-delivery, reduce time for improvements, achieve reliability targets and guarantee availability performance to the costumers. Furthermore a dedicated tool is shown, RBDesigner, that semi-automatically generates a RBD starting from the sketch of thermal-hydraulic systems and provides the most important reliability parameters. The strengths of the developed tools were provided by a comparison with other commercial software solutions.