Abstract: For each process unit in a multi-process unit plant, a required input and output of a fluid (eg water), in terms of both purity and flow-rate of the fluid, is determined. All of the inputs and outputs of all of the process units are represented in a graphical or tabular form, in terms of both purity and flow-rate of the fluid. The efficiency of use of the fluid is then indicated by the graphical or tabular representation. Connections between the process units may be derived from the graphical or tabular representation, to increase re-use or re-generation of the fluid between the process units. Efficiency of use of the fluid in the multi-process unit plant may then be increased by effecting those connections between the process units.