This article addresses a new aspect of the problem of sensor network design, namely the concept of flexibility of sensor network. Algorithms based on graph theoretical concepts and MINLP methods are developed for analyzing the flexibility of a given sensor network, designing a flexible sensor network, and upgrading a sensor network to improve its flexibility. Several examples are reported to illustrate the presented algorithms. Using the proposed approach, one can obtain a flexible sensor network, which is able to ensure the observability of all the key variables even under some cases in which the original flowsheet changes.