By flexibility we mean the capability that a design has of having feasible steady state operation for a range of uncertain conditions that may be encountered during plant operation. Clearly, there are other aspects to the operability of a plant, such as controllability, safety, and reliability, which are equally important. However, flexibility is the first step that must be considered for the operability of a design.

Flexibility analysis can be used to quantify the extent to which the uncertainty or changes in parameters are tolerated by a particular process design. In other words, if a process remains feasible over a predefined range of potential deviations in process parameters, it could be concluded that the process is flexible in that range. In the feasible operation, all the constraints (heat and material balance, capacity, and equipment specifications) must be satisfied.