Product flow paths are the routes that products take while flowing through a plant. The development of systems that ensure and monitor the correct and safe transport of material is usually plant-specific and based on informal knowledge, and is time-consuming and error-prone. In this work we seek a synthesis solution for the task of monitoring the safety of product flow paths in processing plants. A formal model of the plant is used, which defines a simplified plant representation that considers the possibility of flow through its elements. Based on this model, we present a formalisation of the safety of a product flow path at a given plant state. This formulation may be used as a guideline for automating the construction of systems which perform safety monitoring of product flow paths. An outline for the design of such a system following a decentralisation scheme is also presented.