Process graphs (P-graphs) have been proven to be useful in identifying optimal structures of process systems and business processes. The provision of redundant critical units can significantly reduce operational risk. Redundant units and subsystems can be modelled in P-graphs by adding nodes that represent logical conditions of the operation of the units. It is revealed in this paper that P-graphs extended by logical condition units can be transformed into reliability block diagrams and based on the cut sets and path sets of the graph a polynomial risk model can be extracted. Since the exponents of the polynomial represent the number of redundant units, the cost function of the reliability – redundancy allocation problem as a nonlinear integer programming model can be formalised, where the cost function handles the costs associated with consequences of equipment failure and repair times. The applicability of this approach is illustrated in a case study related to the asset-intensive chemical, oil, gas and energy sector. The results show that the proposed algorithm is useful for risk-based priority resource allocation in a reforming reaction system.